GNU Coreutils

This manual documents version 9.1 of the GNU core utilities, including the standard programs for text and file manipulation.

Copyright © 1994–2022 Free Software Foundation, Inc.

Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.3 or any later version published by the Free Software Foundation; with no Invariant Sections, with no Front-Cover Texts, and with no Back-Cover Texts. A copy of the license is included in the section entitled “GNU Free Documentation License”.

Table of Contents

Short Table of Contents


1 Introduction

This manual is a work in progress: many sections make no attempt to explain basic concepts in a way suitable for novices. Thus, if you are interested, please get involved in improving this manual. The entire GNU community will benefit.

The GNU utilities documented here are mostly compatible with the POSIX standard.

Please report bugs to . Include the version number, machine architecture, input files, and any other information needed to reproduce the bug: your input, what you expected, what you got, and why it is wrong.

If you have a problem with sort or date, try using the --debug option, as it can often help find and fix problems without having to wait for an answer to a bug report. If the debug output does not suffice to fix the problem on your own, please compress and attach it to the rest of your bug report.

Although diffs are welcome, please include a description of the problem as well, since this is sometimes difficult to infer. See Bugs in Using and Porting GNU CC.

This manual was originally derived from the Unix man pages in the distributions, which were written by David MacKenzie and updated by Jim Meyering. What you are reading now is the authoritative documentation for these utilities; the man pages are no longer being maintained. The original fmt man page was written by Ross Paterson. François Pinard did the initial conversion to Texinfo format. Karl Berry did the indexing, some reorganization, and editing of the results. Brian Youmans of the Free Software Foundation office staff combined the manuals for textutils, fileutils, and sh-utils to produce the present omnibus manual. Richard Stallman contributed his usual invaluable insights to the overall process.


2 Common options

Certain options are available in all of these programs. Rather than writing identical descriptions for each of the programs, they are described here. (In fact, every GNU program accepts (or should accept) these options.)

Normally options and operands can appear in any order, and programs act as if all the options appear before any operands. For example, ‘sort -r passwd -t :’ acts like ‘sort -r -t : passwd’, since ‘:’ is an option-argument of -t. However, if the POSIXLY_CORRECT environment variable is set, options must appear before operands, unless otherwise specified for a particular command.

A few programs can usefully have trailing operands with leading ‘-’. With such a program, options must precede operands even if POSIXLY_CORRECT is not set, and this fact is noted in the program description. For example, the env command’s options must appear before its operands, since in some cases the operands specify a command that itself contains options.

Most programs that accept long options recognize unambiguous abbreviations of those options. For example, ‘rmdir --ignore-fail-on-non-empty’ can be invoked as ‘rmdir --ignore-fail’ or even ‘rmdir --i’. Ambiguous options, such as ‘ls --h’, are identified as such.

Some of these programs recognize the --help and --version options only when one of them is the sole command line argument. For these programs, abbreviations of the long options are not always recognized.

--help

Print a usage message listing all available options, then exit successfully.

--version

Print the version number, then exit successfully.

--

Delimit the option list. Later arguments, if any, are treated as operands even if they begin with ‘-’. For example, ‘sort -- -r’ reads from the file named -r.

A single ‘-’ operand is not really an option, though it looks like one. It stands for a file operand, and some tools treat it as standard input, or as standard output if that is clear from the context. For example, ‘sort -’ reads from standard input, and is equivalent to plain ‘sort’. Unless otherwise specified, a ‘-’ can appear as any operand that requires a file name.


2.1 Exit status

Nearly every command invocation yields an integral exit status that can be used to change how other commands work. For the vast majority of commands, an exit status of zero indicates success. Failure is indicated by a nonzero value—typically ‘1’, though it may differ on unusual platforms as POSIX requires only that it be nonzero.

However, some of the programs documented here do produce other exit status values and a few associate different meanings with the values ‘0’ and ‘1’. Here are some of the exceptions: chroot, env, expr, nice, nohup, numfmt, printenv, sort, stdbuf, test, timeout, tty.


2.2 Backup options

Some GNU programs (at least cp, install, ln, and mv) optionally make backups of files before writing new versions. These options control the details of these backups. The options are also briefly mentioned in the descriptions of the particular programs.

-b
--backup[=method]

Make a backup of each file that would otherwise be overwritten or removed. Without this option, the original versions are destroyed. Use method to determine the type of backups to make. When this option is used but method is not specified, then the value of the VERSION_CONTROL environment variable is used. And if VERSION_CONTROL is not set, the default backup type is ‘existing’.

Note that the short form of this option, -b does not accept any argument. Using -b is equivalent to using --backup=existing.

This option corresponds to the Emacs variable ‘version-control’; the values for method are the same as those used in Emacs. This option also accepts more descriptive names. The valid methods are (unique abbreviations are accepted):

none
off

Never make backups.

numbered
t

Always make numbered backups.

existing
nil

Make numbered backups of files that already have them, simple backups of the others.

simple
never

Always make simple backups. Please note ‘never’ is not to be confused with ‘none’.

-S suffix
--suffix=suffix

Append suffix to each backup file made with -b. If this option is not specified, the value of the SIMPLE_BACKUP_SUFFIX environment variable is used. And if SIMPLE_BACKUP_SUFFIX is not set, the default is ‘~’, just as in Emacs.


2.3 Block size

Some GNU programs (at least df, du, and ls) display sizes in “blocks”. You can adjust the block size and method of display to make sizes easier to read. The block size used for display is independent of any file system block size. Fractional block counts are rounded up to the nearest integer.

The default block size is chosen by examining the following environment variables in turn; the first one that is set determines the block size.

DF_BLOCK_SIZE

This specifies the default block size for the df command. Similarly, DU_BLOCK_SIZE specifies the default for du and LS_BLOCK_SIZE for ls.

BLOCK_SIZE

This specifies the default block size for all three commands, if the above command-specific environment variables are not set.

BLOCKSIZE

This specifies the default block size for all values that are normally printed as blocks, if neither BLOCK_SIZE nor the above command-specific environment variables are set. Unlike the other environment variables, BLOCKSIZE does not affect values that are normally printed as byte counts, e.g., the file sizes contained in ls -l output.

POSIXLY_CORRECT

If neither command_BLOCK_SIZE, nor BLOCK_SIZE, nor BLOCKSIZE is set, but this variable is set, the block size defaults to 512.

If none of the above environment variables are set, the block size currently defaults to 1024 bytes in most contexts, but this number may change in the future. For ls file sizes, the block size defaults to 1 byte.

A block size specification can be a positive integer specifying the number of bytes per block, or it can be human-readable or si to select a human-readable format. Integers may be followed by suffixes that are upward compatible with the SI prefixes for decimal multiples and with the ISO/IEC 80000-13 (formerly IEC 60027-2) prefixes for binary multiples.

With human-readable formats, output sizes are followed by a size letter such as ‘M’ for megabytes. BLOCK_SIZE=human-readable uses powers of 1024; ‘M’ stands for 1,048,576 bytes. BLOCK_SIZE=si is similar, but uses powers of 1000 and appends ‘B’; ‘MB’ stands for 1,000,000 bytes.

A block size specification preceded by ‘'’ causes output sizes to be displayed with thousands separators. The LC_NUMERIC locale specifies the thousands separator and grouping. For example, in an American English locale, ‘--block-size="'1kB"’ would cause a size of 1234000 bytes to be displayed as ‘1,234’. In the default C locale, there is no thousands separator so a leading ‘'’ has no effect.

An integer block size can be followed by a suffix to specify a multiple of that size. A bare size letter, or one followed by ‘iB’, specifies a multiple using powers of 1024. A size letter followed by ‘B’ specifies powers of 1000 instead. For example, ‘1M’ and ‘1MiB’ are equivalent to ‘1048576’, whereas ‘1MB’ is equivalent to ‘1000000’.

A plain suffix without a preceding integer acts as if ‘1’ were prepended, except that it causes a size indication to be appended to the output. For example, ‘--block-size="kB"’ displays 3000 as ‘3kB’.

The following suffixes are defined. Large sizes like 1Y may be rejected by your computer due to limitations of its arithmetic.

kB

kilobyte: 10^3 = 1000.

k
K
KiB

kibibyte: 2^{10} = 1024. ‘K’ is special: the SI prefix is ‘k’ and the ISO/IEC 80000-13 prefix is ‘Ki’, but tradition and POSIX use ‘k’ to mean ‘KiB’.

MB

megabyte: 10^6 = 1,000,000.

M
MiB

mebibyte: 2^{20} = 1,048,576.

GB

gigabyte: 10^9 = 1,000,000,000.

G
GiB

gibibyte: 2^{30} = 1,073,741,824.

TB

terabyte: 10^{12} = 1,000,000,000,000.

T
TiB

tebibyte: 2^{40} = 1,099,511,627,776.

PB

petabyte: 10^{15} = 1,000,000,000,000,000.

P
PiB

pebibyte: 2^{50} = 1,125,899,906,842,624.

EB

exabyte: 10^{18} = 1,000,000,000,000,000,000.

E
EiB

exbibyte: 2^{60} = 1,152,921,504,606,846,976.

ZB

zettabyte: 10^{21} = 1,000,000,000,000,000,000,000

Z
ZiB

2^{70} = 1,180,591,620,717,411,303,424.

YB

yottabyte: 10^{24} = 1,000,000,000,000,000,000,000,000.

Y
YiB

2^{80} = 1,208,925,819,614,629,174,706,176.

Block size defaults can be overridden by an explicit --block-size=size option. The -k option is equivalent to --block-size=1K, which is the default unless the POSIXLY_CORRECT environment variable is set. The -h or --human-readable option is equivalent to --block-size=human-readable. The --si option is equivalent to --block-size=si. Note for ls the -k option does not control the display of the apparent file sizes, whereas the --block-size option does.


2.4 Floating point numbers

Commands that accept or produce floating point numbers employ the floating point representation of the underlying system, and suffer from rounding error, overflow, and similar floating-point issues. Almost all modern systems use IEEE-754 floating point, and it is typically portable to assume IEEE-754 behavior these days. IEEE-754 has positive and negative infinity, distinguishes positive from negative zero, and uses special values called NaNs to represent invalid computations such as dividing zero by itself. For more information, please see David Goldberg’s paper What Every Computer Scientist Should Know About Floating-Point Arithmetic.

Commands that accept floating point numbers as options, operands or input use the standard C functions strtod and strtold to convert from text to floating point numbers. These floating point numbers therefore can use scientific notation like 1.0e-34 and -10e100. Commands that parse floating point also understand case-insensitive inf, infinity, and NaN, although whether such values are useful depends on the command in question. Modern C implementations also accept hexadecimal floating point numbers such as -0x.ep-3, which stands for −14/16 times 2^-3, which equals −0.109375. See Parsing of Floats in The GNU C Library Reference Manual.

Normally the LC_NUMERIC locale determines the decimal-point character. However, some commands’ descriptions specify that they accept numbers in either the current or the C locale; for example, they treat ‘3.14’ like ‘3,14’ if the current locale uses comma as a decimal point.


2.5 Signal specifications

A signal may be a signal name like ‘HUP’, or a signal number like ‘1’, or an exit status of a process terminated by the signal. A signal name can be given in canonical form or prefixed by ‘SIG’. The case of the letters is ignored. The following signal names and numbers are supported on all POSIX compliant systems:

HUP

1. Hangup.

INT

2. Terminal interrupt.

QUIT

3. Terminal quit.

ABRT

6. Process abort.

KILL

9. Kill (cannot be caught or ignored).

ALRM

14. Alarm Clock.

TERM

15. Termination.

Other supported signal names have system-dependent corresponding numbers. All systems conforming to POSIX 1003.1-2001 also support the following signals:

BUS

Access to an undefined portion of a memory object.

CHLD

Child process terminated, stopped, or continued.

CONT

Continue executing, if stopped.

FPE

Erroneous arithmetic operation.

ILL

Illegal Instruction.

PIPE

Write on a pipe with no one to read it.

SEGV

Invalid memory reference.

STOP

Stop executing (cannot be caught or ignored).

TSTP

Terminal stop.

TTIN

Background process attempting read.

TTOU

Background process attempting write.

URG

High bandwidth data is available at a socket.

USR1

User-defined signal 1.

USR2

User-defined signal 2.

POSIX 1003.1-2001 systems that support the XSI extension also support the following signals:

POLL

Pollable event.

PROF

Profiling timer expired.

SYS

Bad system call.

TRAP

Trace/breakpoint trap.

VTALRM

Virtual timer expired.

XCPU

CPU time limit exceeded.

XFSZ

File size limit exceeded.

POSIX 1003.1-2001 systems that support the XRT extension also support at least eight real-time signals called ‘RTMIN’, ‘RTMIN+1’, …, ‘RTMAX-1’, ‘RTMAX’.


2.6 chown, chgrp, chroot, id: Disambiguating user names and IDs

Since the user and group arguments to these commands may be specified as names or numeric IDs, there is an apparent ambiguity. What if a user or group name is a string of digits? 1 Should the command interpret it as a user name or as an ID? POSIX requires that these commands first attempt to resolve the specified string as a name, and only once that fails, then try to interpret it as an ID. This is troublesome when you want to specify a numeric ID, say 42, and it must work even in a pathological situation where ‘42’ is a user name that maps to some other user ID, say 1000. Simply invoking chown 42 F, will set Fs owner ID to 1000—not what you intended.

GNU chown, chgrp, chroot, and id provide a way to work around this, that at the same time may result in a significant performance improvement by eliminating a database look-up. Simply precede each numeric user ID and/or group ID with a ‘+’, in order to force its interpretation as an integer:

chown +42 F
chgrp +$numeric_group_id another-file
chown +0:+0 /

The name look-up process is skipped for each ‘+’-prefixed string, because a string containing ‘+’ is never a valid user or group name. This syntax is accepted on most common Unix systems, but not on Solaris 10.


2.7 Sources of random data

The shuf, shred, and sort commands sometimes need random data to do their work. For example, ‘sort -R’ must choose a hash function at random, and it needs random data to make this selection.

By default these commands use an internal pseudo-random generator initialized by a small amount of entropy, but can be directed to use an external source with the --random-source=file option. An error is reported if file does not contain enough bytes.

For example, the device file /dev/urandom could be used as the source of random data. Typically, this device gathers environmental noise from device drivers and other sources into an entropy pool, and uses the pool to generate random bits. If the pool is short of data, the device reuses the internal pool to produce more bits, using a cryptographically secure pseudo-random number generator. But be aware that this device is not designed for bulk random data generation and is relatively slow.

/dev/urandom suffices for most practical uses, but applications requiring high-value or long-term protection of private data may require an alternate data source like /dev/random or /dev/arandom. The set of available sources depends on your operating system.

To reproduce the results of an earlier invocation of a command, you can save some random data into a file and then use that file as the random source in earlier and later invocations of the command. Rather than depending on a file, one can generate a reproducible arbitrary amount of pseudo-random data given a seed value, using for example:

get_seeded_random()
{
  seed="$1"
  openssl enc -aes-256-ctr -pass pass:"$seed" -nosalt \
    </dev/zero 2>/dev/null
}

shuf -i1-100 --random-source=<(get_seeded_random 42)

2.8 Target directory

The cp, install, ln, and mv commands normally treat the last operand specially when it is a directory or a symbolic link to a directory. For example, ‘cp source dest’ is equivalent to ‘cp source dest/source’ if dest is a directory. Sometimes this behavior is not exactly what is wanted, so these commands support the following options to allow more fine-grained control:

-T
--no-target-directory

Do not treat the last operand specially when it is a directory or a symbolic link to a directory. This can help avoid race conditions in programs that operate in a shared area. For example, when the command ‘mv /tmp/source /tmp/dest’ succeeds, there is no guarantee that /tmp/source was renamed to /tmp/dest: it could have been renamed to /tmp/dest/source instead, if some other process created /tmp/dest as a directory. However, if mv -T /tmp/source /tmp/dest succeeds, there is no question that /tmp/source was renamed to /tmp/dest.

In the opposite situation, where you want the last operand to be treated as a directory and want a diagnostic otherwise, you can use the --target-directory (-t) option.

-t directory
--target-directory=directory

Use directory as the directory component of each destination file name.

The interface for most programs is that after processing options and a finite (possibly zero) number of fixed-position arguments, the remaining argument list is either expected to be empty, or is a list of items (usually files) that will all be handled identically. The xargs program is designed to work well with this convention.

The commands in the mv-family are unusual in that they take a variable number of arguments with a special case at the end (namely, the target directory). This makes it nontrivial to perform some operations, e.g., “move all files from here to ../d/”, because mv * ../d/ might exhaust the argument space, and ls | xargs ... doesn’t have a clean way to specify an extra final argument for each invocation of the subject command. (It can be done by going through a shell command, but that requires more human labor and brain power than it should.)

The --target-directory (-t) option allows the cp, install, ln, and mv programs to be used conveniently with xargs. For example, you can move the files from the current directory to a sibling directory, d like this:

ls | xargs mv -t ../d --

However, this doesn’t move files whose names begin with ‘.’. If you use the GNU find program, you can move those files too, with this command:

find . -mindepth 1 -maxdepth 1 \
  | xargs mv -t ../d

But both of the above approaches fail if there are no files in the current directory, or if any file has a name containing a blank or some other special characters. The following example removes those limitations and requires both GNU find and GNU xargs:

find . -mindepth 1 -maxdepth 1 -print0 \
  | xargs --null --no-run-if-empty \
      mv -t ../d

The --target-directory (-t) and --no-target-directory (-T) options cannot be combined.


2.9 Trailing slashes

Some GNU programs (at least cp and mv) allow you to remove any trailing slashes from each source argument before operating on it. The --strip-trailing-slashes option enables this behavior.

This is useful when a source argument may have a trailing slash and specify a symbolic link to a directory. This scenario is in fact rather common because some shells can automatically append a trailing slash when performing file name completion on such symbolic links. Without this option, mv, for example, (via the system’s rename function) must interpret a trailing slash as a request to dereference the symbolic link and so must rename the indirectly referenced directory and not the symbolic link. Although it may seem surprising that such behavior be the default, it is required by POSIX and is consistent with other parts of that standard.


2.11 Treating / specially

Certain commands can operate destructively on entire hierarchies. For example, if a user with appropriate privileges mistakenly runs ‘rm -rf / tmp/junk’, that may remove all files on the entire system. Since there are so few legitimate uses for such a command, GNU rm normally declines to operate on any directory that resolves to /. If you really want to try to remove all the files on your system, you can use the --no-preserve-root option, but the default behavior, specified by the --preserve-root option, is safer for most purposes.

The commands chgrp, chmod and chown can also operate destructively on entire hierarchies, so they too support these options. Although, unlike rm, they don’t actually unlink files, these commands are arguably more dangerous when operating recursively on /, since they often work much more quickly, and hence damage more files before an alert user can interrupt them. Tradition and POSIX require these commands to operate recursively on /, so they default to --no-preserve-root, but using the --preserve-root option makes them safer for most purposes. For convenience you can specify --preserve-root in an alias or in a shell function.

Note that the --preserve-root option also ensures that chgrp and chown do not modify / even when dereferencing a symlink pointing to /.


2.12 Special built-in utilities

Some programs like nice can invoke other programs; for example, the command ‘nice cat file’ invokes the program cat by executing the command ‘cat file’. However, special built-in utilities like exit cannot be invoked this way. For example, the command ‘nice exit’ does not have a well-defined behavior: it may generate an error message instead of exiting.

Here is a list of the special built-in utilities that are standardized by POSIX 1003.1-2004.

. : break continue eval exec exit export readonly return set shift times trap unset

For example, because ‘.’, ‘:’, and ‘exec’ are special, the commands ‘nice . foo.sh’, ‘nice :’, and ‘nice exec pwd’ do not work as you might expect.

Many shells extend this list. For example, Bash has several extra special built-in utilities like history, and suspend, and with Bash the command ‘nice suspend’ generates an error message instead of suspending.


2.13 Standards conformance

In a few cases, the GNU utilities’ default behavior is incompatible with the POSIX standard. To suppress these incompatibilities, define the POSIXLY_CORRECT environment variable. Unless you are checking for POSIX conformance, you probably do not need to define POSIXLY_CORRECT.

Newer versions of POSIX are occasionally incompatible with older versions. For example, older versions of POSIX required the command ‘sort +1’ to sort based on the second and succeeding fields in each input line, but in POSIX 1003.1-2001 the same command is required to sort the file named +1, and you must instead use the command ‘sort -k 2’ to get the field-based sort. To complicate things further, POSIX 1003.1-2008 allows an implementation to have either the old or the new behavior.

The GNU utilities normally conform to the version of POSIX that is standard for your system. To cause them to conform to a different version of POSIX, define the _POSIX2_VERSION environment variable to a value of the form yyyymm specifying the year and month the standard was adopted. Three values are currently supported for _POSIX2_VERSION: ‘199209’ stands for POSIX 1003.2-1992, ‘200112’ stands for POSIX 1003.1-2001, and ‘200809’ stands for POSIX 1003.1-2008. For example, if you have a POSIX 1003.1-2001 system but are running software containing traditional usage like ‘sort +1’ or ‘tail +10’, you can work around the compatibility problems by setting ‘_POSIX2_VERSION=200809’ in your environment.


2.14 coreutils: Multi-call program

The coreutils command invokes an individual utility, either implicitly selected by the last component of the name used to invoke coreutils, or explicitly with the --coreutils-prog option. Synopsis:

coreutils --coreutils-prog=PROGRAM

The coreutils command is not installed by default, so portable scripts should not rely on its existence.


3 Output of entire files

These commands read and write entire files, possibly transforming them in some way.


3.1 cat: Concatenate and write files

cat copies each file (‘-’ means standard input), or standard input if none are given, to standard output. Synopsis:

cat [option] [file]…

The program accepts the following options. Also see Common options.

-A
--show-all

Equivalent to -vET.

-b
--number-nonblank

Number all nonempty output lines, starting with 1.

-e

Equivalent to -vE.

-E
--show-ends

Display a ‘$’ after the end of each line. The \r\n combination is shown as ‘^M$’.

-n
--number

Number all output lines, starting with 1. This option is ignored if -b is in effect.

-s
--squeeze-blank

Suppress repeated adjacent blank lines; output just one empty line instead of several.

-t

Equivalent to -vT.

-T
--show-tabs

Display TAB characters as ‘^I’.

-u

Ignored; for POSIX compatibility.

-v
--show-nonprinting

Display control characters except for LFD and TAB using ‘^’ notation and precede characters that have the high bit set with ‘M-’.

On systems like MS-DOS that distinguish between text and binary files, cat normally reads and writes in binary mode. However, cat reads in text mode if one of the options -bensAE is used or if cat is reading from standard input and standard input is a terminal. Similarly, cat writes in text mode if one of the options -bensAE is used or if standard output is a terminal.

An exit status of zero indicates success, and a nonzero value indicates failure.

Examples:

# Output f's contents, then standard input, then g's contents.
cat f - g

# Copy standard input to standard output.
cat

3.2 tac: Concatenate and write files in reverse

tac copies each file (‘-’ means standard input), or standard input if none are given, to standard output, reversing the records (lines by default) in each separately. Synopsis:

tac [option]… [file]…

Records are separated by instances of a string (newline by default). By default, this separator string is attached to the end of the record that it follows in the file.

The program accepts the following options. Also see Common options.

-b
--before

The separator is attached to the beginning of the record that it precedes in the file.

-r
--regex

Treat the separator string as a regular expression.

-s separator
--separator=separator

Use separator as the record separator, instead of newline. Note an empty separator is treated as a zero byte. I.e., input and output items are delimited with ASCII NUL.

On systems like MS-DOS that distinguish between text and binary files, tac reads and writes in binary mode.

An exit status of zero indicates success, and a nonzero value indicates failure.

Example:

# Reverse a file character by character.
tac -r -s 'x\|[^x]'

3.3 nl: Number lines and write files

nl writes each file (‘-’ means standard input), or standard input if none are given, to standard output, with line numbers added to some or all of the lines. Synopsis:

nl [option]… [file]…

nl decomposes its input into (logical) page sections; by default, the line number is reset to 1 at each logical page section. nl treats all of the input files as a single document; it does not reset line numbers or logical pages between files.

A logical page consists of three sections: header, body, and footer. Any of the sections can be empty. Each can be numbered in a different style from the others.

The beginnings of the sections of logical pages are indicated in the input file by a line containing exactly one of these delimiter strings:

\:\:\:

start of header;

\:\:

start of body;

\:

start of footer.

The characters from which these strings are made can be changed from ‘\’ and ‘:’ via options (see below), but the pattern of each string cannot be changed.

A section delimiter is replaced by an empty line on output. Any text that comes before the first section delimiter string in the input file is considered to be part of a body section, so nl treats a file that contains no section delimiters as a single body section.

The program accepts the following options. Also see Common options.

-b style
--body-numbering=style

Select the numbering style for lines in the body section of each logical page. When a line is not numbered, the current line number is not incremented, but the line number separator character is still prepended to the line. The styles are:

a

number all lines,

t

number only nonempty lines (default for body),

n

do not number lines (default for header and footer),

pbre

number only lines that contain a match for the basic regular expression bre. See Regular Expressions in The GNU Grep Manual.

-d cd
--section-delimiter=cd

Set the section delimiter characters to cd; default is ‘\:’. If only c is given, the second remains ‘:’. As a GNU extension more than two characters can be specified, and also if cd is empty (-d ''), then section matching is disabled. (Remember to protect ‘\’ or other metacharacters from shell expansion with quotes or extra backslashes.)

-f style
--footer-numbering=style

Analogous to --body-numbering.

-h style
--header-numbering=style

Analogous to --body-numbering.

-i number
--line-increment=number

Increment line numbers by number (default 1). number can be negative to decrement.

-l number
--join-blank-lines=number

Consider number (default 1) consecutive empty lines to be one logical line for numbering, and only number the last one. Where fewer than number consecutive empty lines occur, do not number them. An empty line is one that contains no characters, not even spaces or tabs.

-n format
--number-format=format

Select the line numbering format (default is rn):

ln

left justified, no leading zeros;

rn

right justified, no leading zeros;

rz

right justified, leading zeros.

-p
--no-renumber

Do not reset the line number at the start of a logical page.

-s string
--number-separator=string

Separate the line number from the text line in the output with string (default is the TAB character).

-v number
--starting-line-number=number

Set the initial line number on each logical page to number (default 1). The starting number can be negative.

-w number
--number-width=number

Use number characters for line numbers (default 6).

An exit status of zero indicates success, and a nonzero value indicates failure.


3.4 od: Write files in octal or other formats

od writes an unambiguous representation of each file (‘-’ means standard input), or standard input if none are given. Synopses:

od [option]… [file]…
od [-abcdfilosx]… [file] [[+]offset[.][b]]
od [option]… --traditional [file] [[+]offset[.][b] [[+]label[.][b]]]

Each line of output consists of the offset in the input, followed by groups of data from the file. By default, od prints the offset in octal, and each group of file data is a C short int’s worth of input printed as a single octal number.

If offset is given, it specifies how many input bytes to skip before formatting and writing. By default, it is interpreted as an octal number, but the optional trailing decimal point causes it to be interpreted as decimal. If no decimal is specified and the offset begins with ‘0x’ or ‘0X’ it is interpreted as a hexadecimal number. If there is a trailing ‘b’, the number of bytes skipped will be offset multiplied by 512.

If a command is of both the first and second forms, the second form is assumed if the last operand begins with ‘+’ or (if there are two operands) a digit. For example, in ‘od foo 10’ and ‘od +10’ the ‘10’ is an offset, whereas in ‘od 10’ the ‘10’ is a file name.

The program accepts the following options. Also see Common options.

-A radix
--address-radix=radix

Select the base in which file offsets are printed. radix can be one of the following:

d

decimal;

o

octal;

x

hexadecimal;

n

none (do not print offsets).

The default is octal.

--endian=order

Reorder input bytes, to handle inputs with differing byte orders, or to provide consistent output independent of the endian convention of the current system. Swapping is performed according to the specified --type size and endian order, which can be ‘little’ or ‘big’.

-j bytes
--skip-bytes=bytes

Skip bytes input bytes before formatting and writing. If bytes begins with ‘0x’ or ‘0X’, it is interpreted in hexadecimal; otherwise, if it begins with ‘0’, in octal; otherwise, in decimal. bytes may be, or may be an integer optionally followed by, one of the following multiplicative suffixes:

b’  =>            512 ("blocks")
‘KB’ =>           1000 (KiloBytes)
‘K’  =>           1024 (KibiBytes)
‘MB’ =>      1000*1000 (MegaBytes)
‘M’  =>      1024*1024 (MebiBytes)
‘GB’ => 1000*1000*1000 (GigaBytes)
‘G’  => 1024*1024*1024 (GibiBytes)

and so on for ‘T’, ‘P’, ‘E’, ‘Z’, and ‘Y’. Binary prefixes can be used, too: ‘KiB’=‘K’, ‘MiB’=‘M’, and so on.

-N bytes
--read-bytes=bytes

Output at most bytes bytes of the input. Prefixes and suffixes on bytes are interpreted as for the -j option.

-S bytes
--strings[=bytes]

Instead of the normal output, output only string constants: at least bytes consecutive ASCII graphic characters, followed by a zero byte (ASCII NUL). Prefixes and suffixes on bytes are interpreted as for the -j option.

If bytes is omitted with --strings, the default is 3.

-t type
--format=type

Select the format in which to output the file data. type is a string of one or more of the below type indicator characters. If you include more than one type indicator character in a single type string, or use this option more than once, od writes one copy of each output line using each of the data types that you specified, in the order that you specified.

Adding a trailing “z” to any type specification appends a display of the single byte character representation of the printable characters to the output line generated by the type specification.

a

named character, ignoring high-order bit

c

printable single byte character, C backslash escape or a 3 digit octal sequence

d

signed decimal

f

floating point (see Floating point numbers)

o

octal

u

unsigned decimal

x

hexadecimal

The type a outputs things like ‘sp’ for space, ‘nl’ for newline, and ‘nul’ for a zero byte. Only the least significant seven bits of each byte is used; the high-order bit is ignored. Type c outputs ‘ ’, ‘\n’, and \0, respectively.

Except for types ‘a’ and ‘c’, you can specify the number of bytes to use in interpreting each number in the given data type by following the type indicator character with a decimal integer. Alternately, you can specify the size of one of the C compiler’s built-in data types by following the type indicator character with one of the following characters. For integers (‘d’, ‘o’, ‘u’, ‘x’):

C

char

S

short

I

int

L

long

For floating point (f):

F

float

D

double

L

long double

-v
--output-duplicates

Output consecutive lines that are identical. By default, when two or more consecutive output lines would be identical, od outputs only the first line, and puts just an asterisk on the following line to indicate the elision.

-w[n]
--width[=n]

Dump n input bytes per output line. This must be a multiple of the least common multiple of the sizes associated with the specified output types.

If this option is not given at all, the default is 16. If n is omitted, the default is 32.

The next several options are shorthands for format specifications. GNU od accepts any combination of shorthands and format specification options. These options accumulate.

-a

Output as named characters. Equivalent to ‘-t a’.

-b

Output as octal bytes. Equivalent to ‘-t o1’.

-c

Output as printable single byte characters, C backslash escapes or 3 digit octal sequences. Equivalent to ‘-t c’.

-d

Output as unsigned decimal two-byte units. Equivalent to ‘-t u2’.

-f

Output as floats. Equivalent to ‘-t fF’.

-i

Output as decimal ints. Equivalent to ‘-t dI’.

-l

Output as decimal long ints. Equivalent to ‘-t dL’.

-o

Output as octal two-byte units. Equivalent to -t o2.

-s

Output as decimal two-byte units. Equivalent to -t d2.

-x

Output as hexadecimal two-byte units. Equivalent to ‘-t x2’.

--traditional

Recognize the non-option label argument that traditional od accepted. The following syntax:

od --traditional [file] [[+]offset[.][b] [[+]label[.][b]]]

can be used to specify at most one file and optional arguments specifying an offset and a pseudo-start address, label. The label argument is interpreted just like offset, but it specifies an initial pseudo-address. The pseudo-addresses are displayed in parentheses following any normal address.

An exit status of zero indicates success, and a nonzero value indicates failure.


3.5 base32: Transform data into printable data

base32 transforms data read from a file, or standard input, into (or from) base32 encoded form. The base32 encoded form uses printable ASCII characters to represent binary data. The usage and options of this command are precisely the same as for base64. See base64: Transform data into printable data.


3.6 base64: Transform data into printable data

base64 transforms data read from a file, or standard input, into (or from) base64 encoded form. The base64 encoded form uses printable ASCII characters to represent binary data. Synopses:

base64 [option]… [file]
base64 --decode [option]… [file]

The base64 encoding expands data to roughly 133% of the original. The base32 encoding expands data to roughly 160% of the original. The format conforms to RFC 4648.

The program accepts the following options. Also see Common options.

-w cols
--wrap=cols

During encoding, wrap lines after cols characters. This must be a positive number.

The default is to wrap after 76 characters. Use the value 0 to disable line wrapping altogether.

-d
--decode

Change the mode of operation, from the default of encoding data, to decoding data. Input is expected to be base64 encoded data, and the output will be the original data.

-i
--ignore-garbage

When decoding, newlines are always accepted. During decoding, ignore unrecognized bytes, to permit distorted data to be decoded.

An exit status of zero indicates success, and a nonzero value indicates failure.


3.7 basenc: Transform data into printable data

basenc transforms data read from a file, or standard input, into (or from) various common encoding forms. The encoded form uses printable ASCII characters to represent binary data.

Synopses:

basenc encoding [option]… [file]
basenc encoding --decode [option]… [file]

The encoding argument is required. If file is omitted, basenc reads from standard input. The -w/--wrap,-i/--ignore-garbage, -d/--decode options of this command are precisely the same as for base64. See base64: Transform data into printable data.

Supported encodings are:

--base64

Encode into (or decode from with -d/--decode) base64 form. The format conforms to RFC 4648#4. Equivalent to the base64 command.

--base64url

Encode into (or decode from with -d/--decode) file-and-url-safe base64 form (using ‘_’ and ‘-’ instead of ‘+’ and ‘/’). The format conforms to RFC 4648#5.

--base32

Encode into (or decode from with -d/--decode) base32 form. The encoded data uses the ‘ABCDEFGHIJKLMNOPQRSTUVWXYZ234567=’ characters. The format conforms to RFC 4648#6. Equivalent to the base32 command.

--base32hex

Encode into (or decode from with -d/--decode) Extended Hex Alphabet base32 form. The encoded data uses the ‘0123456789ABCDEFGHIJKLMNOPQRSTUV=’ characters. The format conforms to RFC 4648#7.

--base16

Encode into (or decode from with -d/--decode) base16 (hexadecimal) form. The encoded data uses the ‘0123456789ABCDEF’ characters. The format conforms to RFC 4648#8.

--base2lsbf

Encode into (or decode from with -d/--decode) binary string form (‘0’ and ‘1’) with the least significant bit of every byte first.

--base2msbf

Encode into (or decode from with -d/--decode) binary string form (‘0’ and ‘1’) with the most significant bit of every byte first.

--z85

Encode into (or decode from with -d/--decode) Z85 form (a modified Ascii85 form). The encoded data uses the ‘0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTU VWXYZ.-:+=^!/*?&<>()[]{}@%$#’. characters. The format conforms to ZeroMQ spec:32/Z85.

When encoding with --z85, input length must be a multiple of 4; when decoding with --z85, input length must be a multiple of 5.

Encoding/decoding examples:

$ printf '\376\117\202' | basenc --base64
/k+C

$ printf '\376\117\202' | basenc --base64url
_k-C

$ printf '\376\117\202' | basenc --base32
7ZHYE===

$ printf '\376\117\202' | basenc --base32hex
VP7O4===

$ printf '\376\117\202' | basenc --base16
FE4F82

$ printf '\376\117\202' | basenc --base2lsbf
011111111111001001000001

$ printf '\376\117\202' | basenc --base2msbf
111111100100111110000010

$ printf '\376\117\202\000' | basenc --z85
@.FaC

$ printf 01010100 | basenc --base2msbf --decode
T

$ printf 01010100 | basenc --base2lsbf --decode
*

4 Formatting file contents

These commands reformat the contents of files.


4.1 fmt: Reformat paragraph text

fmt fills and joins lines to produce output lines of (at most) a given number of characters (75 by default). Synopsis:

fmt [option]… [file]…

fmt reads from the specified file arguments (or standard input if none are given), and writes to standard output.

By default, blank lines, spaces between words, and indentation are preserved in the output; successive input lines with different indentation are not joined; tabs are expanded on input and introduced on output.

fmt prefers breaking lines at the end of a sentence, and tries to avoid line breaks after the first word of a sentence or before the last word of a sentence. A sentence break is defined as either the end of a paragraph or a word ending in any of ‘.?!’, followed by two spaces or end of line, ignoring any intervening parentheses or quotes. Like TeX, fmt reads entire “paragraphs” before choosing line breaks; the algorithm is a variant of that given by Donald E. Knuth and Michael F. Plass in “Breaking Paragraphs Into Lines”, Software—Practice & Experience 11, 11 (November 1981), 1119–1184.

The program accepts the following options. Also see Common options.

-c
--crown-margin

Crown margin mode: preserve the indentation of the first two lines within a paragraph, and align the left margin of each subsequent line with that of the second line.

-t
--tagged-paragraph

Tagged paragraph mode: like crown margin mode, except that if indentation of the first line of a paragraph is the same as the indentation of the second, the first line is treated as a one-line paragraph.

-s
--split-only

Split lines only. Do not join short lines to form longer ones. This prevents sample lines of code, and other such “formatted” text from being unduly combined.

-u
--uniform-spacing

Uniform spacing. Reduce spacing between words to one space, and spacing between sentences to two spaces.

-width
-w width
--width=width

Fill output lines up to width characters (default 75 or goal plus 10, if goal is provided).

-g goal
--goal=goal

fmt initially tries to make lines goal characters wide. By default, this is 7% shorter than width.

-p prefix
--prefix=prefix

Only lines beginning with prefix (possibly preceded by whitespace) are subject to formatting. The prefix and any preceding whitespace are stripped for the formatting and then re-attached to each formatted output line. One use is to format certain kinds of program comments, while leaving the code unchanged.

An exit status of zero indicates success, and a nonzero value indicates failure.


4.2 pr: Paginate or columnate files for printing

pr writes each file (‘-’ means standard input), or standard input if none are given, to standard output, paginating and optionally outputting in multicolumn format; optionally merges all files, printing all in parallel, one per column. Synopsis:

pr [option]… [file]…

By default, a 5-line header is printed at each page: two blank lines; a line with the date, the file name, and the page count; and two more blank lines. A footer of five blank lines is also printed. The default page_length is 66 lines. The default number of text lines is therefore 56. The text line of the header takes the form ‘date string page’, with spaces inserted around string so that the line takes up the full page_width. Here, date is the date (see the -D or --date-format option for details), string is the centered header string, and page identifies the page number. The LC_MESSAGES locale category affects the spelling of page; in the default C locale, it is ‘Page number’ where number is the decimal page number.

Form feeds in the input cause page breaks in the output. Multiple form feeds produce empty pages.

Columns are of equal width, separated by an optional string (default is ‘space’). For multicolumn output, lines will always be truncated to page_width (default 72), unless you use the -J option. For single column output no line truncation occurs by default. Use -W option to truncate lines in that case.

The program accepts the following options. Also see Common options.

+first_page[:last_page]
--pages=first_page[:last_page]

Begin printing with page first_page and stop with last_page. Missing ‘:last_page’ implies end of file. While estimating the number of skipped pages each form feed in the input file results in a new page. Page counting with and without ‘+first_page’ is identical. By default, counting starts with the first page of input file (not first page printed). Line numbering may be altered by -N option.

-column
--columns=column

With each single file, produce column columns of output (default is 1) and print columns down, unless -a is used. The column width is automatically decreased as column increases; unless you use the -W/-w option to increase page_width as well. This option might well cause some lines to be truncated. The number of lines in the columns on each page are balanced. The options -e and -i are on for multiple text-column output. Together with -J option column alignment and line truncation is turned off. Lines of full length are joined in a free field format and -S option may set field separators. -column may not be used with -m option.

-a
--across

With each single file, print columns across rather than down. The -column option must be given with column greater than one. If a line is too long to fit in a column, it is truncated.

-c
--show-control-chars

Print control characters using hat notation (e.g., ‘^G’); print other nonprinting characters in octal backslash notation. By default, nonprinting characters are not changed.

-d
--double-space

Double space the output.

-D format
--date-format=format

Format header dates using format, using the same conventions as for the command ‘date +format’. See date: Print or set system date and time. Except for directives, which start with ‘%’, characters in format are printed unchanged. You can use this option to specify an arbitrary string in place of the header date, e.g., --date-format="Monday morning".

The default date format is ‘%Y-%m-%d %H:%M’ (for example, ‘2020-07-09 23:59’); but if the POSIXLY_CORRECT environment variable is set and the LC_TIME locale category specifies the POSIX locale, the default is ‘%b %e %H:%M %Y’ (for example, ‘Jul  9 23:59 2020’.

Timestamps are listed according to the time zone rules specified by the TZ environment variable, or by the system default rules if TZ is not set. See Specifying the Time Zone with TZ in The GNU C Library Reference Manual.

-e[in-tabchar[in-tabwidth]]
--expand-tabs[=in-tabchar[in-tabwidth]]

Expand tabs to spaces on input. Optional argument in-tabchar is the input tab character (default is the TAB character). Second optional argument in-tabwidth is the input tab character’s width (default is 8).

-f
-F
--form-feed

Use a form feed instead of newlines to separate output pages. This does not alter the default page length of 66 lines.

-h header
--header=header

Replace the file name in the header with the centered string header. When using the shell, header should be quoted and should be separated from -h by a space.

-i[out-tabchar[out-tabwidth]]
--output-tabs[=out-tabchar[out-tabwidth]]

Replace spaces with tabs on output. Optional argument out-tabchar is the output tab character (default is the TAB character). Second optional argument out-tabwidth is the output tab character’s width (default is 8).

-J
--join-lines

Merge lines of full length. Used together with the column options -column, -a -column or -m. Turns off -W/-w line truncation; no column alignment used; may be used with --sep-string[=string]. -J has been introduced (together with -W and --sep-string) to disentangle the old (POSIX-compliant) options -w and -s along with the three column options.

-l page_length
--length=page_length

Set the page length to page_length (default 66) lines, including the lines of the header [and the footer]. If page_length is less than or equal to 10, the header and footer are omitted, as if the -t option had been given.

-m
--merge

Merge and print all files in parallel, one in each column. If a line is too long to fit in a column, it is truncated, unless the -J option is used. --sep-string[=string] may be used. Empty pages in some files (form feeds set) produce empty columns, still marked by string. The result is a continuous line numbering and column marking throughout the whole merged file. Completely empty merged pages show no separators or line numbers. The default header becomes ‘date page’ with spaces inserted in the middle; this may be used with the -h or --header option to fill up the middle blank part.

-n[number-separator[digits]]
--number-lines[=number-separator[digits]]

Provide digits digit line numbering (default for digits is 5). With multicolumn output the number occupies the first digits column positions of each text column or only each line of -m output. With single column output the number precedes each line just as -m does. Default counting of the line numbers starts with the first line of the input file (not the first line printed, compare the --page option and -N option). Optional argument number-separator is the character appended to the line number to separate it from the text followed. The default separator is the TAB character. In a strict sense a TAB is always printed with single column output only. The TAB width varies with the TAB position, e.g., with the left margin specified by -o option. With multicolumn output priority is given to ‘equal width of output columns’ (a POSIX specification). The TAB width is fixed to the value of the first column and does not change with different values of left margin. That means a fixed number of spaces is always printed in the place of the number-separator TAB. The tabification depends upon the output position.

-N line_number
--first-line-number=line_number

Start line counting with the number line_number at first line of first page printed (in most cases not the first line of the input file).

-o margin
--indent=margin

Indent each line with a margin margin spaces wide (default is zero). The total page width is the size of the margin plus the page_width set with the -W/-w option. A limited overflow may occur with numbered single column output (compare -n option).

-r
--no-file-warnings

Do not print a warning message when an argument file cannot be opened. (The exit status will still be nonzero, however.)

-s[char]
--separator[=char]

Separate columns by a single character char. The default for char is the TAB character without -w and ‘no character’ with -w. Without -s the default separator ‘space’ is set. -s[char] turns off line truncation of all three column options (-COLUMN|-a -COLUMN|-m) unless -w is set. This is a POSIX-compliant formulation.

-S[string]
--sep-string[=string]

Use string to separate output columns. The -S option doesn’t affect the -W/-w option, unlike the -s option which does. It does not affect line truncation or column alignment. Without -S, and with -J, pr uses the default output separator, TAB. Without -S or -J, pr uses a ‘space’ (same as -S" "). If no ‘string’ argument is specified, ‘""’ is assumed.

-t
--omit-header

Do not print the usual header [and footer] on each page, and do not fill out the bottom of pages (with blank lines or a form feed). No page structure is produced, but form feeds set in the input files are retained. The predefined pagination is not changed. -t or -T may be useful together with other options; e.g.: -t -e4, expand TAB characters in the input file to 4 spaces but don’t make any other changes. Use of -t overrides -h.

-T
--omit-pagination

Do not print header [and footer]. In addition eliminate all form feeds set in the input files.

-v
--show-nonprinting

Print nonprinting characters in octal backslash notation.

-w page_width
--width=page_width

Set page width to page_width characters for multiple text-column output only (default for page_width is 72). The specified page_width is rounded down so that columns have equal width. -s[CHAR] turns off the default page width and any line truncation and column alignment. Lines of full length are merged, regardless of the column options set. No page_width setting is possible with single column output. A POSIX-compliant formulation.

-W page_width
--page_width=page_width

Set the page width to page_width characters, honored with and without a column option. With a column option, the specified page_width is rounded down so that columns have equal width. Text lines are truncated, unless -J is used. Together with one of the three column options (-column, -a -column or -m) column alignment is always used. The separator options -S or -s don’t disable the -W option. Default is 72 characters. Without -W page_width and without any of the column options NO line truncation is used (defined to keep downward compatibility and to meet most frequent tasks). That’s equivalent to -W 72 -J. The header line is never truncated.

An exit status of zero indicates success, and a nonzero value indicates failure.


4.3 fold: Wrap input lines to fit in specified width

fold writes each file (- means standard input), or standard input if none are given, to standard output, breaking long lines. Synopsis:

fold [option]… [file]…

By default, fold breaks lines wider than 80 columns. The output is split into as many lines as necessary.

fold counts screen columns by default; thus, a tab may count more than one column, backspace decreases the column count, and carriage return sets the column to zero.

The program accepts the following options. Also see Common options.

-b
--bytes

Count bytes rather than columns, so that tabs, backspaces, and carriage returns are each counted as taking up one column, just like other characters.

-s
--spaces

Break at word boundaries: the line is broken after the last blank before the maximum line length. If the line contains no such blanks, the line is broken at the maximum line length as usual.

-w width
--width=width

Use a maximum line length of width columns instead of 80.

For compatibility fold supports an obsolete option syntax -width. New scripts should use -w width instead.

An exit status of zero indicates success, and a nonzero value indicates failure.


5 Output of parts of files

These commands output pieces of the input.


5.1 head: Output the first part of files

head prints the first part (10 lines by default) of each file; it reads from standard input if no files are given or when given a file of -. Synopsis:

head [option]… [file]…

If more than one file is specified, head prints a one-line header consisting of:

==> file name <==

before the output for each file.

The program accepts the following options. Also see Common options.

-c [-]num
--bytes=[-]num

Print the first num bytes, instead of initial lines. However, if num is prefixed with a ‘-’, print all but the last num bytes of each file. num may be, or may be an integer optionally followed by, one of the following multiplicative suffixes:

b’  =>            512 ("blocks")
‘KB’ =>           1000 (KiloBytes)
‘K’  =>           1024 (KibiBytes)
‘MB’ =>      1000*1000 (MegaBytes)
‘M’  =>      1024*1024 (MebiBytes)
‘GB’ => 1000*1000*1000 (GigaBytes)
‘G’  => 1024*1024*1024 (GibiBytes)

and so on for ‘T’, ‘P’, ‘E’, ‘Z’, and ‘Y’. Binary prefixes can be used, too: ‘KiB’=‘K’, ‘MiB’=‘M’, and so on.

-n [-]num
--lines=[-]num

Output the first num lines. However, if num is prefixed with a ‘-’, print all but the last num lines of each file. Size multiplier suffixes are the same as with the -c option.

-q
--quiet
--silent

Never print file name headers.

-v
--verbose

Always print file name headers.

-z
--zero-terminated

Delimit items with a zero byte rather than a newline (ASCII LF). I.e., treat input as items separated by ASCII NUL and terminate output items with ASCII NUL. This option can be useful in conjunction with ‘perl -0’ or ‘find -print0’ and ‘xargs -0’ which do the same in order to reliably handle arbitrary file names (even those containing blanks or other special characters).

For compatibility head also supports an obsolete option syntax -[num][bkm][cqv], which is recognized only if it is specified first. num is a decimal number optionally followed by a size letter (‘b’, ‘k’, ‘m’) as in -c, or ‘l’ to mean count by lines, or other option letters (‘cqv’). Scripts intended for standard hosts should use -c num or -n num instead. If your script must also run on hosts that support only the obsolete syntax, it is usually simpler to avoid head, e.g., by using ‘sed 5q’ instead of ‘head -5’.

An exit status of zero indicates success, and a nonzero value indicates failure.


5.2 tail: Output the last part of files

tail prints the last part (10 lines by default) of each file; it reads from standard input if no files are given or when given a file of ‘-’. Synopsis:

tail [option]… [file]…

If more than one file is specified, tail prints a one-line header before the output for each file, consisting of:

==> file name <==

For further processing of tail output, it can be useful to convert the file headers to line prefixes, which can be done like:

tail … |
awk '
  /^==> .* <==$/ {prefix=substr($0,5,length-8)":"; next}
  {print prefix$0}
' | …

GNU tail can output any amount of data (some other versions of tail cannot). It also has no -r option (print in reverse), since reversing a file is really a different job from printing the end of a file; BSD tail (which is the one with -r) can only reverse files that are at most as large as its buffer, which is typically 32 KiB. A more reliable and versatile way to reverse files is the GNU tac command.

The program accepts the following options. Also see Common options.

-c [+]num
--bytes=[+]num

Output the last num bytes, instead of final lines. However, if num is prefixed with a ‘+’, start printing with byte num from the start of each file, instead of from the end. num may be, or may be an integer optionally followed by, one of the following multiplicative suffixes:

b’  =>            512 ("blocks")
‘KB’ =>           1000 (KiloBytes)
‘K’  =>           1024 (KibiBytes)
‘MB’ =>      1000*1000 (MegaBytes)
‘M’  =>      1024*1024 (MebiBytes)
‘GB’ => 1000*1000*1000 (GigaBytes)
‘G’  => 1024*1024*1024 (GibiBytes)

and so on for ‘T’, ‘P’, ‘E’, ‘Z’, and ‘Y’. Binary prefixes can be used, too: ‘KiB’=‘K’, ‘MiB’=‘M’, and so on.

-f
--follow[=how]

Loop forever trying to read more characters at the end of the file, presumably because the file is growing. If more than one file is given, tail prints a header whenever it gets output from a different file, to indicate which file that output is from.

There are two ways to specify how you’d like to track files with this option, but that difference is noticeable only when a followed file is removed or renamed. If you’d like to continue to track the end of a growing file even after it has been unlinked, use --follow=descriptor. This is the default behavior, but it is not useful if you’re tracking a log file that may be rotated (removed or renamed, then reopened). In that case, use --follow=name to track the named file, perhaps by reopening it periodically to see if it has been removed and recreated by some other program. Note that the inotify-based implementation handles this case without the need for any periodic reopening.

No matter which method you use, if the tracked file is determined to have shrunk, tail prints a message saying the file has been truncated and resumes tracking from the start of the file, assuming it has been truncated to 0, which is the usual truncation operation for log files.

When a file is removed, tail’s behavior depends on whether it is following the name or the descriptor. When following by name, tail can detect that a file has been removed and gives a message to that effect, and if --retry has been specified it will continue checking periodically to see if the file reappears. When following a descriptor, tail does not detect that the file has been unlinked or renamed and issues no message; even though the file may no longer be accessible via its original name, it may still be growing.

The option values ‘descriptor’ and ‘name’ may be specified only with the long form of the option, not with -f.

The -f option is ignored if no file operand is specified and standard input is a FIFO or a pipe. Likewise, the -f option has no effect for any operand specified as ‘-’, when standard input is a FIFO or a pipe.

With kernel inotify support, output is triggered by file changes and is generally very prompt. Otherwise, tail sleeps for one second between checks— use --sleep-interval=n to change that default—which can make the output appear slightly less responsive or bursty. When using tail without inotify support, you can make it more responsive by using a sub-second sleep interval, e.g., via an alias like this:

alias tail='tail -s.1'
-F

This option is the same as --follow=name --retry. That is, tail will attempt to reopen a file when it is removed. Should this fail, tail will keep trying until it becomes accessible again.

--max-unchanged-stats=n

When tailing a file by name, if there have been n (default n=5) consecutive iterations for which the file has not changed, then open/fstat the file to determine if that file name is still associated with the same device/inode-number pair as before. When following a log file that is rotated, this is approximately the number of seconds between when tail prints the last pre-rotation lines and when it prints the lines that have accumulated in the new log file. This option is meaningful only when polling (i.e., without inotify) and when following by name.

-n [+]num
--lines=[+]

Output the last num lines. However, if num is prefixed with a ‘+’, start printing with line num from the start of each file, instead of from the end. Size multiplier suffixes are the same as with the -c option.

--pid=pid

When following by name or by descriptor, you may specify the process ID, pid, of the sole writer of all file arguments. Then, shortly after that process terminates, tail will also terminate. This will work properly only if the writer and the tailing process are running on the same machine. For example, to save the output of a build in a file and to watch the file grow, if you invoke make and tail like this then the tail process will stop when your build completes. Without this option, you would have had to kill the tail -f process yourself.

$ make >& makerr & tail --pid=$! -f makerr

If you specify a pid that is not in use or that does not correspond to the process that is writing to the tailed files, then tail may terminate long before any files stop growing or it may not terminate until long after the real writer has terminated. Note that --pid cannot be supported on some systems; tail will print a warning if this is the case.

-q
--quiet
--silent

Never print file name headers.

--retry

Indefinitely try to open the specified file. This option is useful mainly when following (and otherwise issues a warning).

When following by file descriptor (i.e., with --follow=descriptor), this option only affects the initial open of the file, as after a successful open, tail will start following the file descriptor.

When following by name (i.e., with --follow=name), tail infinitely retries to re-open the given files until killed.

Without this option, when tail encounters a file that doesn’t exist or is otherwise inaccessible, it reports that fact and never checks it again.

-s number
--sleep-interval=number

Change the number of seconds to wait between iterations (the default is 1.0). During one iteration, every specified file is checked to see if it has changed size. When tail uses inotify, this polling-related option is usually ignored. However, if you also specify --pid=p, tail checks whether process p is alive at least every number seconds. The number must be non-negative and can be a floating-point number in either the current or the C locale. See Floating point numbers.

-v
--verbose

Always print file name headers.

-z
--zero-terminated

Delimit items with a zero byte rather than a newline (ASCII LF). I.e., treat input as items separated by ASCII NUL and terminate output items with ASCII NUL. This option can be useful in conjunction with ‘perl -0’ or ‘find -print0’ and ‘xargs -0’ which do the same in order to reliably handle arbitrary file names (even those containing blanks or other special characters).

For compatibility tail also supports an obsolete usage ‘tail -[num][bcl][f] [file]’, which is recognized only if it does not conflict with the usage described above. This obsolete form uses exactly one option and at most one file. In the option, num is an optional decimal number optionally followed by a size letter (‘b’, ‘c’, ‘l’) to mean count by 512-byte blocks, bytes, or lines, optionally followed by ‘f’ which has the same meaning as -f.

On systems not conforming to POSIX 1003.1-2001, the leading ‘-’ can be replaced by ‘+’ in the traditional option syntax with the same meaning as in counts, and on obsolete systems predating POSIX 1003.1-2001 traditional usage overrides normal usage when the two conflict. This behavior can be controlled with the _POSIX2_VERSION environment variable (see Standards conformance).

Scripts intended for use on standard hosts should avoid traditional syntax and should use -c num[b], -n num, and/or -f instead. If your script must also run on hosts that support only the traditional syntax, you can often rewrite it to avoid problematic usages, e.g., by using ‘sed -n '$p'’ rather than ‘tail -1’. If that’s not possible, the script can use a test like ‘if tail -c +1 </dev/null >/dev/null 2>&1; then …’ to decide which syntax to use.

Even if your script assumes the standard behavior, you should still beware usages whose behaviors differ depending on the POSIX version. For example, avoid ‘tail - main.c’, since it might be interpreted as either ‘tail main.c’ or as ‘tail -- - main.c’; avoid ‘tail -c 4’, since it might mean either ‘tail -c4’ or ‘tail -c 10 4’; and avoid ‘tail +4’, since it might mean either ‘tail ./+4’ or ‘tail -n +4’.

An exit status of zero indicates success, and a nonzero value indicates failure.


5.3 split: Split a file into pieces.

split creates output files containing consecutive or interleaved sections of input (standard input if none is given or input is ‘-’). Synopsis:

split [option] [input [prefix]]

By default, split puts 1000 lines of input (or whatever is left over for the last section), into each output file.

The output files’ names consist of prefix (‘x’ by default) followed by a group of characters (‘aa’, ‘ab’, … by default), such that concatenating the output files in traditional sorted order by file name produces the original input file (except -nr/n). By default split will initially create files with two generated suffix characters, and will increase this width by two when the next most significant position reaches the last character. (‘yz’, ‘zaaa’, ‘zaab’, …). In this way an arbitrary number of output files are supported, which sort as described above, even in the presence of an --additional-suffix option. If the -a option is specified and the output file names are exhausted, split reports an error without deleting the output files that it did create.

The program accepts the following options. Also see Common options.

-l lines
--lines=lines

Put lines lines of input into each output file. If --separator is specified, then lines determines the number of records.

For compatibility split also supports an obsolete option syntax -lines. New scripts should use -l lines instead.

-b size
--bytes=size

Put size bytes of input into each output file. size may be, or may be an integer optionally followed by, one of the following multiplicative suffixes:

b’  =>            512 ("blocks")
‘KB’ =>           1000 (KiloBytes)
‘K’  =>           1024 (KibiBytes)
‘MB’ =>      1000*1000 (MegaBytes)
‘M’  =>      1024*1024 (MebiBytes)
‘GB’ => 1000*1000*1000 (GigaBytes)
‘G’  => 1024*1024*1024 (GibiBytes)

and so on for ‘T’, ‘P’, ‘E’, ‘Z’, and ‘Y’. Binary prefixes can be used, too: ‘KiB’=‘K’, ‘MiB’=‘M’, and so on.

-C size
--line-bytes=size

Put into each output file as many complete lines of input as possible without exceeding size bytes. Individual lines or records longer than size bytes are broken into multiple files. size has the same format as for the --bytes option. If --separator is specified, then lines determines the number of records.

--filter=command

With this option, rather than simply writing to each output file, write through a pipe to the specified shell command for each output file. command should use the $FILE environment variable, which is set to a different output file name for each invocation of the command. For example, imagine that you have a 1TiB compressed file that, if uncompressed, would be too large to reside on secondary storage, yet you must split it into individually-compressed pieces of a more manageable size. To do that, you might run this command:

xz -dc BIG.xz | split -b200G --filter='xz > $FILE.xz' - big-

Assuming a 10:1 compression ratio, that would create about fifty 20GiB files with names big-aa.xz, big-ab.xz, big-ac.xz, etc.

-n chunks
--number=chunks

Split input to chunks output files where chunks may be:

n      generate n files based on current size of input
k/n    output only kth of n to standard output
l/n    generate n files without splitting lines or records
l/k/n  likewise but output only kth of n to stdout
r/n    like ‘l’ but use round robin distribution
r/k/n  likewise but output only kth of n to stdout

Any excess bytes remaining after dividing the input into n chunks, are assigned to the last chunk. Any excess bytes appearing after the initial calculation are discarded (except when using ‘r’ mode).

All n files are created even if there are fewer than n lines, or the input is truncated.

For ‘l’ mode, chunks are approximately input size / n. The input is partitioned into n equal sized portions, with the last assigned any excess. If a line starts within a partition it is written completely to the corresponding file. Since lines or records are not split even if they overlap a partition, the files written can be larger or smaller than the partition size, and even empty if a line/record is so long as to completely overlap the partition.

For ‘r’ mode, the size of input is irrelevant, and so can be a pipe for example.

-a length
--suffix-length=length

Use suffixes of length length. If a length of 0 is specified, this is the same as if (any previous) -a was not specified, and thus enables the default behavior, which starts the suffix length at 2, and unless -n or --numeric-suffixes=from is specified, will auto increase the length by 2 as required.

-d
--numeric-suffixes[=from]

Use digits in suffixes rather than lower-case letters. The numerical suffix counts from from if specified, 0 otherwise.

from is supported with the long form option, and is used to either set the initial suffix for a single run, or to set the suffix offset for independently split inputs, and consequently the auto suffix length expansion described above is disabled. Therefore you may also want to use option -a to allow suffixes beyond ‘99’. Note if option --number is specified and the number of files is less than from, a single run is assumed and the minimum suffix length required is automatically determined.

-x
--hex-suffixes[=from]

Like --numeric-suffixes, but use hexadecimal numbers (in lower case).

--additional-suffix=suffix

Append an additional suffix to output file names. suffix must not contain slash.

-e
--elide-empty-files

Suppress the generation of zero-length output files. This can happen with the --number option if a file is (truncated to be) shorter than the number requested, or if a line is so long as to completely span a chunk. The output file sequence numbers, always run consecutively even when this option is specified.

-t separator
--separator=separator

Use character separator as the record separator instead of the default newline character (ASCII LF). To specify ASCII NUL as the separator, use the two-character string ‘\0’, e.g., ‘split -t '\0'’.

-u
--unbuffered

Immediately copy input to output in --number r/… mode, which is a much slower mode of operation.

--verbose

Write a diagnostic just before each output file is opened.

An exit status of zero indicates success, and a nonzero value indicates failure.

Here are a few examples to illustrate how the --number (-n) option works:

Notice how, by default, one line may be split onto two or more:

$ seq -w 6 10 > k; split -n3 k; head xa?
==> xaa <==
06
07
==> xab <==

08
0
==> xac <==
9
10

Use the "l/" modifier to suppress that:

$ seq -w 6 10 > k; split -nl/3 k; head xa?
==> xaa <==
06
07

==> xab <==
08
09

==> xac <==
10

Use the "r/" modifier to distribute lines in a round-robin fashion:

$ seq -w 6 10 > k; split -nr/3 k; head xa?
==> xaa <==
06
09

==> xab <==
07
10

==> xac <==
08

You can also extract just the Kth chunk. This extracts and prints just the 7th "chunk" of 33:

$ seq 100 > k; split -nl/7/33 k
20
21
22

5.4 csplit: Split a file into context-determined pieces

csplit creates zero or more output files containing sections of input (standard input if input is ‘-’). Synopsis:

csplit [option]… input pattern

The contents of the output files are determined by the pattern arguments, as detailed below. An error occurs if a pattern argument refers to a nonexistent line of the input file (e.g., if no remaining line matches a given regular expression). After every pattern has been matched, any remaining input is copied into one last output file.

By default, csplit prints the number of bytes written to each output file after it has been created.

The types of pattern arguments are:

n

Create an output file containing the input up to but not including line n (a positive integer). If followed by a repeat count, also create an output file containing the next n lines of the input file once for each repeat.

/regexp/[offset]

Create an output file containing the current line up to (but not including) the next line of the input file that contains a match for regexp. The optional offset is an integer, that can be preceded by ‘+’ or ‘-’. If it is given, the input up to (but not including) the matching line plus or minus offset is put into the output file, and the line after that begins the next section of input. Note lines within a negative offset of a regexp pattern are not matched in subsequent regexp patterns.

%regexp%[offset]

Like the previous type, except that it does not create an output file, so that section of the input file is effectively ignored.

{repeat-count}

Repeat the previous pattern repeat-count additional times. The repeat-count can either be a positive integer or an asterisk, meaning repeat as many times as necessary until the input is exhausted.

The output files’ names consist of a prefix (‘xx’ by default) followed by a suffix. By default, the suffix is an ascending sequence of two-digit decimal numbers from ‘00’ to ‘99’. In any case, concatenating the output files in sorted order by file name produces the original input file, excluding portions skipped with a %regexp% pattern or the --suppress-matched option.

By default, if csplit encounters an error or receives a hangup, interrupt, quit, or terminate signal, it removes any output files that it has created so far before it exits.

The program accepts the following options. Also see Common options.

-f prefix
--prefix=prefix

Use prefix as the output file name prefix.

-b format
--suffix-format=format

Use format as the output file name suffix. When this option is specified, the suffix string must include exactly one printf(3)-style conversion specification, possibly including format specification flags, a field width, a precision specification, or all of these kinds of modifiers. The format letter must convert a binary unsigned integer argument to readable form. The format letters ‘d’ and ‘i’ are aliases for ‘u’, and the ‘u’, ‘o’, ‘x’, and ‘X’ conversions are allowed. The entire format is given (with the current output file number) to sprintf(3) to form the file name suffixes for each of the individual output files in turn. If this option is used, the --digits option is ignored.

-n digits
--digits=digits

Use output file names containing numbers that are digits digits long instead of the default 2.

-k
--keep-files

Do not remove output files when errors are encountered.

--suppress-matched

Do not output lines matching the specified pattern. I.e., suppress the boundary line from the start of the second and subsequent splits.

-z
--elide-empty-files

Suppress the generation of zero-length output files. (In cases where the section delimiters of the input file are supposed to mark the first lines of each of the sections, the first output file will generally be a zero-length file unless you use this option.) The output file sequence numbers always run consecutively starting from 0, even when this option is specified.

-s
-q
--silent
--quiet

Do not print counts of output file sizes.

An exit status of zero indicates success, and a nonzero value indicates failure.

Here is an example of its usage. First, create an empty directory for the exercise, and cd into it:

$ mkdir d && cd d

Now, split the sequence of 1..14 on lines that end with 0 or 5:

$ seq 14 | csplit - '/[05]$/' '{*}'
8
10
15

Each number printed above is the size of an output file that csplit has just created. List the names of those output files:

$ ls
xx00  xx01  xx02

Use head to show their contents:

$ head xx*
==> xx00 <==
1
2
3
4

==> xx01 <==
5
6
7
8
9

==> xx02 <==
10
11
12
13
14

Example of splitting input by empty lines:

$ csplit --suppress-matched input.txt '/^$/' '{*}'

6 Summarizing files

These commands generate just a few numbers representing entire contents of files.


6.1 wc: Print newline, word, and byte counts

wc counts the number of bytes, characters, words, and newlines in each given file, or standard input if none are given or for a file of ‘-’. A word is a nonzero length sequence of printable characters delimited by white space. Synopsis:

wc [option]… [file]…

wc prints one line of counts for each file, and if the file was given as an argument, it prints the file name following the counts. If more than one file is given, wc prints a final line containing the cumulative counts, with the file name total. The counts are printed in this order: newlines, words, characters, bytes, maximum line length. Each count is printed right-justified in a field with at least one space between fields so that the numbers and file names normally line up nicely in columns. The width of the count fields varies depending on the inputs, so you should not depend on a particular field width. However, as a GNU extension, if only one count is printed, it is guaranteed to be printed without leading spaces.

By default, wc prints three counts: the newline, words, and byte counts. Options can specify that only certain counts be printed. Options do not undo others previously given, so

wc --bytes --words

prints both the byte counts and the word counts.

With the --max-line-length option, wc prints the length of the longest line per file, and if there is more than one file it prints the maximum (not the sum) of those lengths. The line lengths here are measured in screen columns, according to the current locale and assuming tab positions in every 8th column.

The program accepts the following options. Also see Common options.

-c
--bytes

Print only the byte counts.

-m
--chars

Print only the character counts, as per the current locale. Invalid characters are not counted.

-w
--words

Print only the word counts. A word is a nonzero length sequence of printable characters separated by white space.

-l
--lines

Print only the newline character counts. Note a file without a trailing newline character, will not have that last portion included in the line count.

-L
--max-line-length

Print only the maximum display widths. Tabs are set at every 8th column. Display widths of wide characters are considered. Non-printable characters are given 0 width.

--files0-from=file

Disallow processing files named on the command line, and instead process those named in file file; each name being terminated by a zero byte (ASCII NUL). This is useful when the list of file names is so long that it may exceed a command line length limitation. In such cases, running wc via xargs is undesirable because it splits the list into pieces and makes wc print a total for each sublist rather than for the entire list. One way to produce a list of ASCII NUL terminated file names is with GNU find, using its -print0 predicate. If file is ‘-’ then the ASCII NUL terminated file names are read from standard input.

For example, to find the length of the longest line in any .c or .h file in the current hierarchy, do this:

find . -name '*.[ch]' -print0 |
  wc -L --files0-from=- | tail -n1

An exit status of zero indicates success, and a nonzero value indicates failure.


6.2 sum: Print checksum and block counts

sum computes a 16-bit checksum for each given file, or standard input if none are given or for a file of ‘-’. Synopsis:

sum [option]… [file]…

sum prints the checksum for each file followed by the number of blocks in the file (rounded up). If at least one file is given, file names are also printed.

By default, GNU sum computes checksums using an algorithm compatible with BSD sum and prints file sizes in units of 1024-byte blocks.

The program accepts the following options. Also see Common options.

-r

Use the default (BSD compatible) algorithm. This option is included for compatibility with the System V sum. Unless -s was also given, it has no effect.

-s
--sysv

Compute checksums using an algorithm compatible with System V sum’s default, and print file sizes in units of 512-byte blocks.

sum is provided for compatibility; the cksum program (see next section) is preferable in new applications.

An exit status of zero indicates success, and a nonzero value indicates failure.


6.3 cksum: Print and verify file checksums

cksum by default computes a cyclic redundancy check (CRC) checksum for each given file, or standard input if none are given or for a file of ‘-’.

cksum also supports the -a,--algorithm option to select the digest algorithm to use. cksum is the preferred interface to these digests, subsuming the other standalone checksumming utilities, which can be emulated using cksum -a md5 --untagged "$@" etc. Synopsis:

cksum [option]… [file]…

cksum is typically used to ensure that files have not been corrupted, by comparing the cksum output for the received files with the cksum output for the original files (typically given in the distribution).

cksum by default prints the POSIX standard CRC checksum for each file along with the number of bytes in the file, and the file name unless no arguments were given.

The same usage and options as the b2sum command are supported. See b2sum: Print or check BLAKE2 digests. In addition cksum supports the following options.

-a
--algorithm

Compute checksums using the specified digest algorithm.

Supported legacy checksums (which are not supported by --check):

sysv’      equivalent to sum -sbsd’       equivalent to sum -rcrc’       equivalent to cksum (the default)

Supported more modern digest algorithms are:

md5’       equivalent to md5sumsha1’      equivalent to sha1sumsha224’    equivalent to sha224sumsha256’    equivalent to sha256sumsha384’    equivalent to sha384sumsha512’    equivalent to sha512sumblake2b’   equivalent to b2sumsm3’       only available through cksum
--debug

Output extra information to stderr, like the checksum implementation being used.

--untagged

Output using the original Coreutils format used by the other standalone checksum utilities like md5sum for example. This format has the checksum at the start of the line, and may be more amenable to further processing by other utilities, especially in combination with the --zero option. Note this does not identify the digest algorithm used for the checksum. See md5sum: Print or check MD5 digests for details of this format.


6.4 b2sum: Print or check BLAKE2 digests

b2sum computes a 512-bit checksum for each specified file. The same usage and options as the md5sum command are supported. See md5sum: Print or check MD5 digests. In addition b2sum supports the following options.

-l
--length

Change (shorten) the default digest length. This is specified in bits and thus must be a multiple of 8. This option is ignored when --check is specified, as the length is automatically determined when checking.


6.5 md5sum: Print or check MD5 digests

md5sum computes a 128-bit checksum (or fingerprint or message-digest) for each specified file.

Note: The MD5 digest is more reliable than a simple CRC (provided by the cksum command) for detecting accidental file corruption, as the chances of accidentally having two files with identical MD5 are vanishingly small. However, it should not be considered secure against malicious tampering: although finding a file with a given MD5 fingerprint is considered infeasible at the moment, it is known how to modify certain files, including digital certificates, so that they appear valid when signed with an MD5 digest. For more secure hashes, consider using SHA-2, or the newer b2sum command. See sha2 utilities: Print or check SHA-2 digests. See b2sum: Print or check BLAKE2 digests.

If a file is specified as ‘-’ or if no files are given md5sum computes the checksum for the standard input. md5sum can also determine whether a file and checksum are consistent. Synopsis:

md5sum [option]… [file]…

For each file, ‘md5sum’ outputs by default, the MD5 checksum, a space, a flag indicating binary or text input mode, and the file name. Binary mode is indicated with ‘*’, text mode with ‘ ’ (space). Binary mode is the default on systems where it’s significant, otherwise text mode is the default. The cksum command always uses binary mode and a ‘ ’ (space) flag.

Without --zero, if file contains a backslash, newline, or carriage return, the line is started with a backslash, and each problematic character in the file name is escaped with a backslash, making the output unambiguous even in the presence of arbitrary file names.

If file is omitted or specified as ‘-’, standard input is read.

The program accepts the following options. Also see Common options.

-b
--binary

Note this option is not supported by the cksum command, as it operates in binary mode exclusively. Treat each input file as binary, by reading it in binary mode and outputting a ‘*’ flag. This is the inverse of --text. On systems like GNU that do not distinguish between binary and text files, this option merely flags each input mode as binary: the MD5 checksum is unaffected. This option is the default on systems like MS-DOS that distinguish between binary and text files, except for reading standard input when standard input is a terminal.

-c
--check

Read file names and checksum information (not data) from each file (or from standard input if no file was specified) and report whether the checksums match the contents of the named files. The input to this mode of md5sum is usually the output of a prior, checksum-generating run of ‘md5sum’.

Three input formats are supported. Either the default output format described above, the --tag output format, or the BSD reversed mode format which is similar to the default mode, but doesn’t use a character to distinguish binary and text modes.

For the cksum command, the --check option supports auto-detecting the digest algorithm to use, when presented with checksum information in the --tag output format.

Output with --zero enabled is not supported by --check.


For each such line, md5sum reads the named file and computes its MD5 checksum. Then, if the computed message digest does not match the one on the line with the file name, the file is noted as having failed the test. Otherwise, the file passes the test. By default, for each valid line, one line is written to standard output indicating whether the named file passed the test. After all checks have been performed, if there were any failures, a warning is issued to standard error. Use the --status option to inhibit that output. If any listed file cannot be opened or read, if any valid line has an MD5 checksum inconsistent with the associated file, or if no valid line is found, md5sum exits with nonzero status. Otherwise, it exits successfully. Note the cksum command doesn’t support --check with the older ‘sysv’, ‘bsd’, or ‘crc’ algorithms.

--ignore-missing

This option is useful only when verifying checksums. When verifying checksums, don’t fail or report any status for missing files. This is useful when verifying a subset of downloaded files given a larger list of checksums.

--quiet

This option is useful only when verifying checksums. When verifying checksums, don’t generate an ’OK’ message per successfully checked file. Files that fail the verification are reported in the default one-line-per-file format. If there is any checksum mismatch, print a warning summarizing the failures to standard error.

--status

This option is useful only when verifying checksums. When verifying checksums, don’t generate the default one-line-per-file diagnostic and don’t output the warning summarizing any failures. Failures to open or read a file still evoke individual diagnostics to standard error. If all listed files are readable and are consistent with the associated MD5 checksums, exit successfully. Otherwise exit with a status code indicating there was a failure.

--tag

Output BSD style checksums, which indicate the checksum algorithm used. As a GNU extension, if --zero is not used, file names with problematic characters are escaped as described above, with the same escaping indicator of ‘\’ at the start of the line, being used. The --tag option implies binary mode, and is disallowed with --text mode as supporting that would unnecessarily complicate the output format, while providing little benefit. The cksum command, uses --tag as its default output format.

-t
--text

Note this option is not supported by the cksum command. Treat each input file as text, by reading it in text mode and outputting a ‘ ’ flag. This is the inverse of --binary. This option is the default on systems like GNU that do not distinguish between binary and text files. On other systems, it is the default for reading standard input when standard input is a terminal. This mode is never defaulted to if --tag is used.

-w
--warn

When verifying checksums, warn about improperly formatted MD5 checksum lines. This option is useful only if all but a few lines in the checked input are valid.

--strict

When verifying checksums, if one or more input line is invalid, exit nonzero after all warnings have been issued.

-z
--zero

Output a zero byte (ASCII NUL) at the end of each line, rather than a newline. This option enables other programs to parse the output even when that output would contain data with embedded newlines. Also file name escaping is not used.

An exit status of zero indicates success, and a nonzero value indicates failure.


6.6 sha1sum: Print or check SHA-1 digests

sha1sum computes a 160-bit checksum for each specified file. The usage and options of this command are precisely the same as for md5sum. See md5sum: Print or check MD5 digests.

Note: The SHA-1 digest is more reliable than a simple CRC (provided by the cksum command) for detecting accidental file corruption, as the chances of accidentally having two files with identical SHA-1 are vanishingly small. However, it should not be considered secure against malicious tampering: although finding a file with a given SHA-1 fingerprint is considered infeasible at the moment, it is known how to modify certain files, including digital certificates, so that they appear valid when signed with an SHA-1 digest. For more secure hashes, consider using SHA-2, or the newer b2sum command. See sha2 utilities: Print or check SHA-2 digests. See b2sum: Print or check BLAKE2 digests.


6.7 sha2 utilities: Print or check SHA-2 digests

The commands sha224sum, sha256sum, sha384sum and sha512sum compute checksums of various lengths (respectively 224, 256, 384 and 512 bits), collectively known as the SHA-2 hashes. The usage and options of these commands are precisely the same as for md5sum and sha1sum. See md5sum: Print or check MD5 digests.


7 Operating on sorted files

These commands work with (or produce) sorted files.


7.1 sort: Sort text files

sort sorts, merges, or compares all the lines from the given files, or standard input if none are given or for a file of ‘-’. By default, sort writes the results to standard output. Synopsis:

sort [option]… [file]…

Many options affect how sort compares lines; if the results are unexpected, try the --debug option to see what happened. A pair of lines is compared as follows: sort compares each pair of fields (see --key), in the order specified on the command line, according to the associated ordering options, until a difference is found or no fields are left. If no key fields are specified, sort uses a default key of the entire line. Finally, as a last resort when all keys compare equal, sort compares entire lines as if no ordering options other than --reverse (-r) were specified. The --stable (-s) option disables this last-resort comparison so that lines in which all fields compare equal are left in their original relative order. The --unique (-u) option also disables the last-resort comparison.

Unless otherwise specified, all comparisons use the character collating sequence specified by the LC_COLLATE locale.2 A line’s trailing newline is not part of the line for comparison purposes. If the final byte of an input file is not a newline, GNU sort silently supplies one. GNU sort (as specified for all GNU utilities) has no limit on input line length or restrictions on bytes allowed within lines.

sort has three modes of operation: sort (the default), merge, and check for sortedness. The following options change the operation mode:

-c
--check
--check=diagnose-first

Check whether the given file is already sorted: if it is not all sorted, print a diagnostic containing the first out-of-order line and exit with a status of 1. Otherwise, exit successfully. At most one input file can be given.

-C
--check=quiet
--check=silent

Exit successfully if the given file is already sorted, and exit with status 1 otherwise. At most one input file can be given. This is like -c, except it does not print a diagnostic.

-m
--merge

Merge the given files by sorting them as a group. Each input file must always be individually sorted. It always works to sort instead of merge; merging is provided because it is faster, in the case where it works.

Exit status:

0 if no error occurred
1 if invoked with -c or -C and the input is not sorted
2 if an error occurred

If the environment variable TMPDIR is set, sort uses its value as the directory for temporary files instead of /tmp. The --temporary-directory (-T) option in turn overrides the environment variable.

The following options affect the ordering of output lines. They may be specified globally or as part of a specific key field. If no key fields are specified, global options apply to comparison of entire lines; otherwise the global options are inherited by key fields that do not specify any special options of their own. In pre-POSIX versions of sort, global options affect only later key fields, so portable shell scripts should specify global options first.

-b
--ignore-leading-blanks

Ignore leading blanks when finding sort keys in each line. By default a blank is a space or a tab, but the LC_CTYPE locale can change this. Note blanks may be ignored by your locale’s collating rules, but without this option they will be significant for character positions specified in keys with the -k option.

-d
--dictionary-order

Sort in phone directory order: ignore all characters except letters, digits and blanks when sorting. By default letters and digits are those of ASCII and a blank is a space or a tab, but the LC_CTYPE locale can change this.

-f
--ignore-case

Fold lowercase characters into the equivalent uppercase characters when comparing so that, for example, ‘b’ and ‘B’ sort as equal. The LC_CTYPE locale determines character types. When used with --unique those lower case equivalent lines are thrown away. (There is currently no way to throw away the upper case equivalent instead. (Any --reverse given would only affect the final result, after the throwing away.))

-g
--general-numeric-sort
--sort=general-numeric

Sort numerically, converting a prefix of each line to a long double-precision floating point number. See Floating point numbers. Do not report overflow, underflow, or conversion errors. Use the following collating sequence:

  • Lines that do not start with numbers (all considered to be equal).
  • NaNs (“Not a Number” values, in IEEE floating point arithmetic) in a consistent but machine-dependent order.
  • Minus infinity.
  • Finite numbers in ascending numeric order (with -0 and +0 equal).
  • Plus infinity.

Use this option only if there is no alternative; it is much slower than --numeric-sort (-n) and it can lose information when converting to floating point.

You can use this option to sort hexadecimal numbers prefixed with ‘0x’ or ‘0X’, where those numbers are not fixed width, or of varying case. However for hex numbers of consistent case, and left padded with ‘0’ to a consistent width, a standard lexicographic sort will be faster.

-h
--human-numeric-sort
--sort=human-numeric

Sort numerically, first by numeric sign (negative, zero, or positive); then by SI suffix (either empty, or ‘k’ or ‘K’, or one of ‘MGTPEZY’, in that order; see Block size); and finally by numeric value. For example, ‘1023M’ sorts before ‘1G’ because ‘M’ (mega) precedes ‘G’ (giga) as an SI suffix. This option sorts values that are consistently scaled to the nearest suffix, regardless of whether suffixes denote powers of 1000 or 1024, and it therefore sorts the output of any single invocation of the df, du, or ls commands that are invoked with their --human-readable or --si options. The syntax for numbers is the same as for the --numeric-sort option; the SI suffix must immediately follow the number. Note also the numfmt command, which can be used to reformat numbers to human format after the sort, thus often allowing sort to operate on more accurate numbers.

-i
--ignore-nonprinting

Ignore nonprinting characters. The LC_CTYPE locale determines character types. This option has no effect if the stronger --dictionary-order (-d) option is also given.

-M
--month-sort
--sort=month

An initial string, consisting of any amount of blanks, followed by a month name abbreviation, is folded to UPPER case and compared in the order ‘JAN’ < ‘FEB’ < … < ‘DEC’. Invalid names compare low to valid names. The LC_TIME locale category determines the month spellings. By default a blank is a space or a tab, but the LC_CTYPE locale can change this.

-n
--numeric-sort
--sort=numeric

Sort numerically. The number begins each line and consists of optional blanks, an optional ‘-’ sign, and zero or more digits possibly separated by thousands separators, optionally followed by a decimal-point character and zero or more digits. An empty number is treated as ‘0’. The LC_NUMERIC locale specifies the decimal-point character and thousands separator. By default a blank is a space or a tab, but the LC_CTYPE locale can change this.

Comparison is exact; there is no rounding error.

Neither a leading ‘+’ nor exponential notation is recognized. To compare such strings numerically, use the --general-numeric-sort (-g) option.

-V
--version-sort

Sort by version name and number. It behaves like a standard sort, except that each sequence of decimal digits is treated numerically as an index/version number. (See Version sort ordering.)

-r
--reverse

Reverse the result of comparison, so that lines with greater key values appear earlier in the output instead of later.

-R
--random-sort
--sort=random

Sort by hashing the input keys and then sorting the hash values. Choose the hash function at random, ensuring that it is free of collisions so that differing keys have differing hash values. This is like a random permutation of the inputs (see shuf: Shuffling text), except that keys with the same value sort together.

If multiple random sort fields are specified, the same random hash function is used for all fields. To use different random hash functions for different fields, you can invoke sort more than once.

The choice of hash function is affected by the --random-source option.

Other options are:

--compress-program=prog

Compress any temporary files with the program prog.

With no arguments, prog must compress standard input to standard output, and when given the -d option it must decompress standard input to standard output.

Terminate with an error if prog exits with nonzero status.

White space and the backslash character should not appear in prog; they are reserved for future use.

--files0-from=file

Disallow processing files named on the command line, and instead process those named in file file; each name being terminated by a zero byte (ASCII NUL). This is useful when the list of file names is so long that it may exceed a command line length limitation. In such cases, running sort via xargs is undesirable because it splits the list into pieces and makes sort print sorted output for each sublist rather than for the entire list. One way to produce a list of ASCII NUL terminated file names is with GNU find, using its -print0 predicate. If file is ‘-’ then the ASCII NUL terminated file names are read from standard input.

-k pos1[,pos2]
--key=pos1[,pos2]

Specify a sort field that consists of the part of the line between pos1 and pos2 (or the end of the line, if pos2 is omitted), inclusive.

In its simplest form pos specifies a field number (starting with 1), with fields being separated by runs of blank characters, and by default those blanks being included in the comparison at the start of each field. To adjust the handling of blank characters see the -b and -t options.

More generally, each pos has the form ‘f[.c][opts]’, where f is the number of the field to use, and c is the number of the first character from the beginning of the field. Fields and character positions are numbered starting with 1; a character position of zero in pos2 indicates the field’s last character. If ‘.c’ is omitted from pos1, it defaults to 1 (the beginning of the field); if omitted from pos2, it defaults to 0 (the end of the field). opts are ordering options, allowing individual keys to be sorted according to different rules; see below for details. Keys can span multiple fields.

Example: To sort on the second field, use --key=2,2 (-k 2,2). See below for more notes on keys and more examples. See also the --debug option to help determine the part of the line being used in the sort.

--debug

Highlight the portion of each line used for sorting. Also issue warnings about questionable usage to standard error.

--batch-size=nmerge

Merge at most nmerge inputs at once.

When sort has to merge more than nmerge inputs, it merges them in groups of nmerge, saving the result in a temporary file, which is then used as an input in a subsequent merge.

A large value of nmerge may improve merge performance and decrease temporary storage utilization at the expense of increased memory usage and I/O. Conversely a small value of nmerge may reduce memory requirements and I/O at the expense of temporary storage consumption and merge performance.

The value of nmerge must be at least 2. The default value is currently 16, but this is implementation-dependent and may change in the future.

The value of nmerge may be bounded by a resource limit for open file descriptors. The commands ‘ulimit -n’ or ‘getconf OPEN_MAX’ may display limits for your systems; these limits may be modified further if your program already has some files open, or if the operating system has other limits on the number of open files. If the value of nmerge exceeds the resource limit, sort silently uses a smaller value.

-o output-file
--output=output-file

Write output to output-file instead of standard output. Normally, sort reads all input before opening output-file, so you can sort a file in place by using commands like sort -o F F and cat F | sort -o F. However, it is often safer to output to an otherwise-unused file, as data may be lost if the system crashes or sort encounters an I/O or other serious error while a file is being sorted in place. Also, sort with --merge (-m) can open the output file before reading all input, so a command like cat F | sort -m -o F - G is not safe as sort might start writing F before cat is done reading it.

On newer systems, -o cannot appear after an input file if POSIXLY_CORRECT is set, e.g., ‘sort F -o F’. Portable scripts should specify -o output-file before any input files.

--random-source=file

Use file as a source of random data used to determine which random hash function to use with the -R option. See Sources of random data.

-s
--stable

Make sort stable by disabling its last-resort comparison. This option has no effect if no fields or global ordering options other than --reverse (-r) are specified.

-S size
--buffer-size=size

Use a main-memory sort buffer of the given size. By default, size is in units of 1024 bytes. Appending ‘%’ causes size to be interpreted as a percentage of physical memory. Appending ‘K’ multiplies size by 1024 (the default), ‘M’ by 1,048,576, ‘G’ by 1,073,741,824, and so on for ‘T’, ‘P’, ‘E’, ‘Z’, and ‘Y’. Appending ‘b’ causes size to be interpreted as a byte count, with no multiplication.

This option can improve the performance of sort by causing it to start with a larger or smaller sort buffer than the default. However, this option affects only the initial buffer size. The buffer grows beyond size if sort encounters input lines larger than size.

-t separator
--field-separator=separator

Use character separator as the field separator when finding the sort keys in each line. By default, fields are separated by the empty string between a non-blank character and a blank character. By default a blank is a space or a tab, but the LC_CTYPE locale can change this.

That is, given the input line ‘ foo bar, sort breaks it into fields ‘ foo and ‘ bar. The field separator is not considered to be part of either the field preceding or the field following, so with ‘sort -t " "’ the same input line has three fields: an empty field, ‘foo’, and ‘bar’. However, fields that extend to the end of the line, as -k 2, or fields consisting of a range, as -k 2,3, retain the field separators present between the endpoints of the range.

To specify ASCII NUL as the field separator, use the two-character string ‘\0’, e.g., ‘sort -t '\0'’.

-T tempdir
--temporary-directory=tempdir

Use directory tempdir to store temporary files, overriding the TMPDIR environment variable. If this option is given more than once, temporary files are stored in all the directories given. If you have a large sort or merge that is I/O-bound, you can often improve performance by using this option to specify directories on different file systems.

--parallel=n

Set the number of sorts run in parallel to n. By default, n is set to the number of available processors, but limited to 8, as there are diminishing performance gains after that. Note also that using n threads increases the memory usage by a factor of log n. Also see nproc: Print the number of available processors.

-u
--unique

Normally, output only the first of a sequence of lines that compare equal. For the --check (-c or -C) option, check that no pair of consecutive lines compares equal.

This option also disables the default last-resort comparison.

The commands sort -u and sort | uniq are equivalent, but this equivalence does not extend to arbitrary sort options. For example, sort -n -u inspects only the value of the initial numeric string when checking for uniqueness, whereas sort -n | uniq inspects the entire line. See uniq: Uniquify files.

-z
--zero-terminated

Delimit items with a zero byte rather than a newline (ASCII LF). I.e., treat input as items separated by ASCII NUL and terminate output items with ASCII NUL. This option can be useful in conjunction with ‘perl -0’ or ‘find -print0’ and ‘xargs -0’ which do the same in order to reliably handle arbitrary file names (even those containing blanks or other special characters).

Historical (BSD and System V) implementations of sort have differed in their interpretation of some options, particularly -b, -f, and -n. GNU sort follows the POSIX behavior, which is usually (but not always!) like the System V behavior. According to POSIX, -n no longer implies -b. For consistency, -M has been changed in the same way. This may affect the meaning of character positions in field specifications in obscure cases. The only fix is to add an explicit -b.

A position in a sort field specified with -k may have any of the option letters ‘MbdfghinRrV’ appended to it, in which case no global ordering options are inherited by that particular field. The -b option may be independently attached to either or both of the start and end positions of a field specification, and if it is inherited from the global options it will be attached to both. If input lines can contain leading or adjacent blanks and -t is not used, then -k is typically combined with -b or an option that implicitly ignores leading blanks (‘Mghn’) as otherwise the varying numbers of leading blanks in fields can cause confusing results.

If the start position in a sort field specifier falls after the end of the line or after the end field, the field is empty. If the -b option was specified, the ‘.c’ part of a field specification is counted from the first nonblank character of the field.

On systems not conforming to POSIX 1003.1-2001, sort supports a traditional origin-zero syntax ‘+pos1 [-pos2]’ for specifying sort keys. The traditional command ‘sort +a.x -b.y’ is equivalent to ‘sort -k a+1.x+1,b’ if y is ‘0’ or absent, otherwise it is equivalent to ‘sort -k a+1.x+1,b+1.y’.

This traditional behavior can be controlled with the _POSIX2_VERSION environment variable (see Standards conformance); it can also be enabled when POSIXLY_CORRECT is not set by using the traditional syntax with ‘-pos2’ present.

Scripts intended for use on standard hosts should avoid traditional syntax and should use -k instead. For example, avoid ‘sort +2’, since it might be interpreted as either ‘sort ./+2’ or ‘sort -k 3’. If your script must also run on hosts that support only the traditional syntax, it can use a test like ‘if sort -k 1 </dev/null >/dev/null 2>&1; then …’ to decide which syntax to use.

Here are some examples to illustrate various combinations of options.

  • Sort in descending (reverse) numeric order.
    sort -n -r
    
  • Run no more than 4 sorts concurrently, using a buffer size of 10M.
    sort --parallel=4 -S 10M
    
  • Sort alphabetically, omitting the first and second fields and the blanks at the start of the third field. This uses a single key composed of the characters beginning at the start of the first nonblank character in field three and extending to the end of each line.
    sort -k 3b
    
  • Sort numerically on the second field and resolve ties by sorting alphabetically on the third and fourth characters of field five. Use ‘:’ as the field delimiter.
    sort -t : -k 2,2n -k 5.3,5.4
    

    Note that if you had written -k 2n instead of -k 2,2n sort would have used all characters beginning in the second field and extending to the end of the line as the primary numeric key. For the large majority of applications, treating keys spanning more than one field as numeric will not do what you expect.

    Also note that the ‘n’ modifier was applied to the field-end specifier for the first key. It would have been equivalent to specify -k 2n,2 or -k 2n,2n. All modifiers except ‘b’ apply to the associated field, regardless of whether the modifier character is attached to the field-start and/or the field-end part of the key specifier.

  • Sort the password file on the fifth field and ignore any leading blanks. Sort lines with equal values in field five on the numeric user ID in field three. Fields are separated by ‘:’.
    sort -t : -k 5b,5 -k 3,3n /etc/passwd
    sort -t : -n -k 5b,5 -k 3,3 /etc/passwd
    sort -t : -b -k 5,5 -k 3,3n /etc/passwd
    

    These three commands have equivalent effect. The first specifies that the first key’s start position ignores leading blanks and the second key is sorted numerically. The other two commands rely on global options being inherited by sort keys that lack modifiers. The inheritance works in this case because -k 5b,5b and -k 5b,5 are equivalent, as the location of a field-end lacking a ‘.c’ character position is not affected by whether initial blanks are skipped.

  • Sort a set of log files, primarily by IPv4 address and secondarily by timestamp. If two lines’ primary and secondary keys are identical, output the lines in the same order that they were input. The log files contain lines that look like this:
    4.150.156.3 - - [01/Apr/2020:06:31:51 +0000] message 1
    211.24.3.231 - - [24/Apr/2020:20:17:39 +0000] message 2
    

    Fields are separated by exactly one space. Sort IPv4 addresses lexicographically, e.g., 212.61.52.2 sorts before 212.129.233.201 because 61 is less than 129.

    sort -s -t ' ' -k 4.9n -k 4.5M -k 4.2n -k 4.14,4.21 file*.log |
    sort -s -t '.' -k 1,1n -k 2,2n -k 3,3n -k 4,4n
    

    This example cannot be done with a single sort invocation, since IPv4 address components are separated by ‘.’ while dates come just after a space. So it is broken down into two invocations of sort: the first sorts by timestamp and the second by IPv4 address. The timestamp is sorted by year, then month, then day, and finally by hour-minute-second field, using -k to isolate each field. Except for hour-minute-second there’s no need to specify the end of each key field, since the ‘n’ and ‘M’ modifiers sort based on leading prefixes that cannot cross field boundaries. The IPv4 addresses are sorted lexicographically. The second sort uses ‘-s’ so that ties in the primary key are broken by the secondary key; the first sort uses ‘-s’ so that the combination of the two sorts is stable.

  • Generate a tags file in case-insensitive sorted order.
    find src -type f -print0 | sort -z -f | xargs -0 etags --append
    

    The use of -print0, -z, and -0 in this case means that file names that contain blanks or other special characters are not broken up by the sort operation.

  • Use the common DSU, Decorate Sort Undecorate idiom to sort lines according to their length.
    awk '{print length, $0}' /etc/passwd | sort -n | cut -f2- -d' '
    

    In general this technique can be used to sort data that the sort command does not support, or is inefficient at, sorting directly.

  • Shuffle a list of directories, but preserve the order of files within each directory. For instance, one could use this to generate a music playlist in which albums are shuffled but the songs of each album are played in order.
    ls */* | sort -t / -k 1,1R -k 2,2
    

7.2 shuf: Shuffling text

shuf shuffles its input by outputting a random permutation of its input lines. Each output permutation is equally likely. Synopses:

shuf [option]… [file]
shuf -e [option]… [arg]…
shuf -i lo-hi [option]…

shuf has three modes of operation that affect where it obtains its input lines. By default, it reads lines from standard input. The following options change the operation mode:

-e
--echo

Treat each command-line operand as an input line.

-i lo-hi
--input-range=lo-hi

Act as if input came from a file containing the range of unsigned decimal integers lohi, one per line.

shuf’s other options can affect its behavior in all operation modes:

-n count
--head-count=count

Output at most count lines. By default, all input lines are output.

-o output-file
--output=output-file

Write output to output-file instead of standard output. shuf reads all input before opening output-file, so you can safely shuffle a file in place by using commands like shuf -o F <F and cat F | shuf -o F.

--random-source=file

Use file as a source of random data used to determine which permutation to generate. See Sources of random data.

-r
--repeat

Repeat output values, that is, select with replacement. With this option the output is not a permutation of the input; instead, each output line is randomly chosen from all the inputs. This option is typically combined with --head-count; if --head-count is not given, shuf repeats indefinitely.

-z
--zero-terminated

Delimit items with a zero byte rather than a newline (ASCII LF). I.e., treat input as items separated by ASCII NUL and terminate output items with ASCII NUL. This option can be useful in conjunction with ‘perl -0’ or ‘find -print0’ and ‘xargs -0’ which do the same in order to reliably handle arbitrary file names (even those containing blanks or other special characters).

For example:

shuf <<EOF
A man,
a plan,
a canal:
Panama!
EOF

might produce the output

Panama!
A man,
a canal:
a plan,

Similarly, the command:

shuf -e clubs hearts diamonds spades

might output:

clubs
diamonds
spades
hearts

and the command ‘shuf -i 1-4’ might output:

4
2
1
3

The above examples all have four input lines, so shuf might produce any of the twenty-four possible permutations of the input. In general, if there are n input lines, there are n! (i.e., n factorial, or n * (n - 1) * … * 1) possible output permutations.

To output 50 random numbers each in the range 0 through 9, use:

shuf -r -n 50 -i 0-9

To simulate 100 coin flips, use:

shuf -r -n 100 -e Head Tail

An exit status of zero indicates success, and a nonzero value indicates failure.


7.3 uniq: Uniquify files

uniq writes the unique lines in the given input, or standard input if nothing is given or for an input name of ‘-’. Synopsis:

uniq [option]… [input [output]]

By default, uniq prints its input lines, except that it discards all but the first of adjacent repeated lines, so that no output lines are repeated. Optionally, it can instead discard lines that are not repeated, or all repeated lines.

The input need not be sorted, but repeated input lines are detected only if they are adjacent. If you want to discard non-adjacent duplicate lines, perhaps you want to use sort -u. See sort: Sort text files.

Comparisons honor the rules specified by the LC_COLLATE locale category.

If no output file is specified, uniq writes to standard output.

The program accepts the following options. Also see Common options.

-f n
--skip-fields=n

Skip n fields on each line before checking for uniqueness. Use a null string for comparison if a line has fewer than n fields. Fields are sequences of non-space non-tab characters that are separated from each other by at least one space or tab.

For compatibility uniq supports a traditional option syntax -n. New scripts should use -f n instead.

-s n
--skip-chars=n

Skip n characters before checking for uniqueness. Use a null string for comparison if a line has fewer than n characters. If you use both the field and character skipping options, fields are skipped over first.

On systems not conforming to POSIX 1003.1-2001, uniq supports a traditional option syntax +n. Although this traditional behavior can be controlled with the _POSIX2_VERSION environment variable (see Standards conformance), portable scripts should avoid commands whose behavior depends on this variable. For example, use ‘uniq ./+10’ or ‘uniq -s 10’ rather than the ambiguous ‘uniq +10’.

-c
--count

Print the number of times each line occurred along with the line.

-i
--ignore-case

Ignore differences in case when comparing lines.

-d
--repeated

Discard lines that are not repeated. When used by itself, this option causes uniq to print the first copy of each repeated line, and nothing else.

-D
--all-repeated[=delimit-method]

Do not discard the second and subsequent repeated input lines, but discard lines that are not repeated. This option is useful mainly in conjunction with other options e.g., to ignore case or to compare only selected fields. The optional delimit-method, supported with the long form option, specifies how to delimit groups of repeated lines, and must be one of the following:

none

Do not delimit groups of repeated lines. This is equivalent to --all-repeated (-D).

prepend

Output a newline before each group of repeated lines. With --zero-terminated (-z), use a zero byte (ASCII NUL) instead of a newline as the delimiter.

separate

Separate groups of repeated lines with a single newline. This is the same as using ‘prepend’, except that no delimiter is inserted before the first group, and hence may be better suited for output direct to users. With --zero-terminated (-z), use a zero byte (ASCII NUL) instead of a newline as the delimiter.

Note that when groups are delimited and the input stream contains blank lines, then the output is ambiguous. To avoid that, filter the input through ‘tr -s '\n'’ to remove blank lines.

This is a GNU extension.

--group[=delimit-method]

Output all lines, and delimit each unique group. With --zero-terminated (-z), use a zero byte (ASCII NUL) instead of a newline as the delimiter. The optional delimit-method specifies how to delimit groups, and must be one of the following:

separate

Separate unique groups with a single delimiter. This is the default delimiting method if none is specified, and better suited for output direct to users.

prepend

Output a delimiter before each group of unique items.

append

Output a delimiter after each group of unique items.

both

Output a delimiter around each group of unique items.

Note that when groups are delimited and the input stream contains blank lines, then the output is ambiguous. To avoid that, filter the input through ‘tr -s '\n'’ to remove blank lines.

This is a GNU extension.

-u
--unique

Discard the last line that would be output for a repeated input group. When used by itself, this option causes uniq to print unique lines, and nothing else.

-w n
--check-chars=n

Compare at most n characters on each line (after skipping any specified fields and characters). By default the entire rest of the lines are compared.

-z
--zero-terminated

Delimit items with a zero byte rather than a newline (ASCII LF). I.e., treat input as items separated by ASCII NUL and terminate output items with ASCII NUL. This option can be useful in conjunction with ‘perl -0’ or ‘find -print0’ and ‘xargs -0’ which do the same in order to reliably handle arbitrary file names (even those containing blanks or other special characters). Note with -z the newline character is treated as a field separator.

An exit status of zero indicates success, and a nonzero value indicates failure.


7.4 comm: Compare two sorted files line by line

comm writes to standard output lines that are common, and lines that are unique, to two input files; a file name of ‘-’ means standard input. Synopsis:

comm [option]… file1 file2

Before comm can be used, the input files must be sorted using the collating sequence specified by the LC_COLLATE locale. If an input file ends in a non-newline character, a newline is silently appended. The sort command with no options always outputs a file that is suitable input to comm.

With no options, comm produces three-column output. Column one contains lines unique to file1, column two contains lines unique to file2, and column three contains lines common to both files. Columns are separated by a single TAB character.

The options -1, -2, and -3 suppress printing of the corresponding columns (and separators). Also see Common options.

Unlike some other comparison utilities, comm has an exit status that does not depend on the result of the comparison. Upon normal completion comm produces an exit code of zero. If there is an error it exits with nonzero status.

If the --check-order option is given, unsorted inputs will cause a fatal error message. If the option --nocheck-order is given, unsorted inputs will never cause an error message. If neither of these options is given, wrongly sorted inputs are diagnosed only if an input file is found to contain unpairable lines. If an input file is diagnosed as being unsorted, the comm command will exit with a nonzero status (and the output should not be used).

Forcing comm to process wrongly sorted input files containing unpairable lines by specifying --nocheck-order is not guaranteed to produce any particular output. The output will probably not correspond with whatever you hoped it would be.

--check-order

Fail with an error message if either input file is wrongly ordered.

--nocheck-order

Do not check that both input files are in sorted order.

Other options are:

--output-delimiter=str

Print str between adjacent output columns, rather than the default of a single TAB character.

The delimiter str may not be empty.

--total

Output a summary at the end.

Similar to the regular output, column one contains the total number of lines unique to file1, column two contains the total number of lines unique to file2, and column three contains the total number of lines common to both files, followed by the word ‘total’ in the additional column four.

In the following example, comm omits the regular output (-123), thus just printing the summary:

$ printf '%s\n' a b c d e     > file1
$ printf '%s\n'   b c d e f g > file2
$ comm --total -123 file1 file2
1       2       4       total

This option is a GNU extension. Portable scripts should use wc to get the totals, e.g. for the above example files:

$ comm -23 file1 file2 | wc -l    # number of lines only in file1
1
$ comm -13 file1 file2 | wc -l    # number of lines only in file2
2
$ comm -12 file1 file2 | wc -l    # number of lines common to both files
4
-z
--zero-terminated

Delimit items with a zero byte rather than a newline (ASCII LF). I.e., treat input as items separated by ASCII NUL and terminate output items with ASCII NUL. This option can be useful in conjunction with ‘perl -0’ or ‘find -print0’ and ‘xargs -0’ which do the same in order to reliably handle arbitrary file names (even those containing blanks or other special characters).


7.5 ptx: Produce permuted indexes

ptx reads a text file and essentially produces a permuted index, with each keyword in its context. The calling sketch is either one of:

ptx [option …] [file …]
ptx -G [option …] [input [output]]

The -G (or its equivalent: --traditional) option disables all GNU extensions and reverts to traditional mode, thus introducing some limitations and changing several of the program’s default option values. When -G is not specified, GNU extensions are always enabled. GNU extensions to ptx are documented wherever appropriate in this document. See The GNU extensions to ptx, for the full list.

Individual options are explained in the following sections.

When GNU extensions are enabled, there may be zero, one or several files after the options. If there is no file, the program reads the standard input. If there is one or several files, they give the name of input files which are all read in turn, as if all the input files were concatenated. However, there is a full contextual break between each file and, when automatic referencing is requested, file names and line numbers refer to individual text input files. In all cases, the program outputs the permuted index to the standard output.

When GNU extensions are not enabled, that is, when the program operates in traditional mode, there may be zero, one or two parameters besides the options. If there are no parameters, the program reads the standard input and outputs the permuted index to the standard output. If there is only one parameter, it names the text input to be read instead of the standard input. If two parameters are given, they give respectively the name of the input file to read and the name of the output file to produce. Be very careful to note that, in this case, the contents of file given by the second parameter is destroyed. This behavior is dictated by System V ptx compatibility; GNU Standards normally discourage output parameters not introduced by an option.

Note that for any file named as the value of an option or as an input text file, a single dash ‘-’ may be used, in which case standard input is assumed. However, it would not make sense to use this convention more than once per program invocation.


7.5.1 General options

-G
--traditional

As already explained, this option disables all GNU extensions to ptx and switches to traditional mode.

--help

Print a short help on standard output, then exit without further processing.

--version

Print the program version on standard output, then exit without further processing.

An exit status of zero indicates success, and a nonzero value indicates failure.


7.5.2 Charset selection

As it is set up now, ptx assumes that the input file is coded using 8-bit characters, and it may not work well in multibyte locales. In a single-byte locale, the default regular expression for a keyword allows foreign or diacriticized letters. Keyword sorting, however, is still crude; it obeys the underlying character set ordering quite blindly.

The output of ptx assumes the locale’s character encoding. For example, with ptx’s -T option, if the locale uses the Latin-1 encoding you may need a LaTeX directive like ‘\usepackage[latin1]{inputenc}’ to render non-ASCII characters correctly.

-f
--ignore-case

Fold lower case letters to upper case for sorting.


7.5.3 Word selection and input processing

-b file
--break-file=file

This option provides an alternative (to -W) method of describing which characters make up words. It introduces the name of a file which contains a list of characters which cannot be part of one word; this file is called the Break file. Any character which is not part of the Break file is a word constituent. If both options -b and -W are specified, then -W has precedence and -b is ignored.

When GNU extensions are enabled, the only way to avoid newline as a break character is to write all the break characters in the file with no newline at all, not even at the end of the file. When GNU extensions are disabled, spaces, tabs and newlines are always considered as break characters even if not included in the Break file.

-i file
--ignore-file=file

The file associated with this option contains a list of words which will never be taken as keywords in concordance output. It is called the Ignore file. The file contains exactly one word in each line; the end of line separation of words is not subject to the value of the -S option.

-o file
--only-file=file

The file associated with this option contains a list of words which will be retained in concordance output; any word not mentioned in this file is ignored. The file is called the Only file. The file contains exactly one word in each line; the end of line separation of words is not subject to the value of the -S option.

There is no default for the Only file. When both an Only file and an Ignore file are specified, a word is considered a keyword only if it is listed in the Only file and not in the Ignore file.

-r
--references

On each input line, the leading sequence of non-white space characters will be taken to be a reference that has the purpose of identifying this input line in the resulting permuted index. See Output formatting, for more information about reference production. Using this option changes the default value for option -S.

Using this option, the program does not try very hard to remove references from contexts in output, but it succeeds in doing so when the context ends exactly at the newline. If option -r is used with -S default value, or when GNU extensions are disabled, this condition is always met and references are completely excluded from the output contexts.

-S regexp
--sentence-regexp=regexp

This option selects which regular expression will describe the end of a line or the end of a sentence. In fact, this regular expression is not the only distinction between end of lines or end of sentences, and input line boundaries have no special significance outside this option. By default, when GNU extensions are enabled and if -r option is not used, end of sentences are used. In this case, this regex is imported from GNU Emacs:

[.?!][]\"')}]*\\($\\|\t\\|  \\)[ \t\n]*

Whenever GNU extensions are disabled or if -r option is used, end of lines are used; in this case, the default regexp is just:

\n

Using an empty regexp is equivalent to completely disabling end of line or end of sentence recognition. In this case, the whole file is considered to be a single big line or sentence. The user might want to disallow all truncation flag generation as well, through option -F "". See Syntax of Regular Expressions in The GNU Emacs Manual.

When the keywords happen to be near the beginning of the input line or sentence, this often creates an unused area at the beginning of the output context line; when the keywords happen to be near the end of the input line or sentence, this often creates an unused area at the end of the output context line. The program tries to fill those unused areas by wrapping around context in them; the tail of the input line or sentence is used to fill the unused area on the left of the output line; the head of the input line or sentence is used to fill the unused area on the right of the output line.

As a matter of convenience to the user, many usual backslashed escape sequences from the C language are recognized and converted to the corresponding characters by ptx itself.

-W regexp
--word-regexp=regexp

This option selects which regular expression will describe each keyword. By default, if GNU extensions are enabled, a word is a sequence of letters; the regexp used is ‘\w+’. When GNU extensions are disabled, a word is by default anything which ends with a space, a tab or a newline; the regexp used is ‘[^ \t\n]+’.

An empty regexp is equivalent to not using this option. See Syntax of Regular Expressions in The GNU Emacs Manual.

As a matter of convenience to the user, many usual backslashed escape sequences, as found in the C language, are recognized and converted to the corresponding characters by ptx itself.


7.5.4 Output formatting

Output format is mainly controlled by the -O and -T options described in the table below. When neither -O nor -T are selected, and if GNU extensions are enabled, the program chooses an output format suitable for a dumb terminal. Each keyword occurrence is output to the center of one line, surrounded by its left and right contexts. Each field is properly justified, so the concordance output can be readily observed. As a special feature, if automatic references are selected by option -A and are output before the left context, that is, if option -R is not selected, then a colon is added after the reference; this nicely interfaces with GNU Emacs next-error processing. In this default output format, each white space character, like newline and tab, is merely changed to exactly one space, with no special attempt to compress consecutive spaces. This might change in the future. Except for those white space characters, every other character of the underlying set of 256 characters is transmitted verbatim.

Output format is further controlled by the following options.

-g number
--gap-size=number

Select the size of the minimum white space gap between the fields on the output line.

-w number
--width=number

Select the maximum output width of each final line. If references are used, they are included or excluded from the maximum output width depending on the value of option -R. If this option is not selected, that is, when references are output before the left context, the maximum output width takes into account the maximum length of all references. If this option is selected, that is, when references are output after the right context, the maximum output width does not take into account the space taken by references, nor the gap that precedes them.

-A
--auto-reference

Select automatic references. Each input line will have an automatic reference made up of the file name and the line ordinal, with a single colon between them. However, the file name will be empty when standard input is being read. If both -A and -r are selected, then the input reference is still read and skipped, but the automatic reference is used at output time, overriding the input reference.

-R
--right-side-refs

In the default output format, when option -R is not used, any references produced by the effect of options -r or -A are placed to the far right of output lines, after the right context. With default output format, when the -R option is specified, references are rather placed at the beginning of each output line, before the left context. For any other output format, option -R is ignored, with one exception: with -R the width of references is not taken into account in total output width given by -w.

This option is automatically selected whenever GNU extensions are disabled.

-F string
--flag-truncation=string

This option will request that any truncation in the output be reported using the string string. Most output fields theoretically extend towards the beginning or the end of the current line, or current sentence, as selected with option -S. But there is a maximum allowed output line width, changeable through option -w, which is further divided into space for various output fields. When a field has to be truncated because it cannot extend beyond the beginning or the end of the current line to fit in, then a truncation occurs. By default, the string used is a single slash, as in -F /.

string may have more than one character, as in -F …. Also, in the particular case when string is empty (-F ""), truncation flagging is disabled, and no truncation marks are appended in this case.

As a matter of convenience to the user, many usual backslashed escape sequences, as found in the C language, are recognized and converted to the corresponding characters by ptx itself.

-M string
--macro-name=string

Select another string to be used instead of ‘xx’, while generating output suitable for nroff, troff or TeX.

-O
--format=roff

Choose an output format suitable for nroff or troff processing. Each output line will look like:

.xx "tail" "before" "keyword_and_after" "head" "ref"

so it will be possible to write a ‘.xx’ roff macro to take care of the output typesetting. This is the default output format when GNU extensions are disabled. Option -M can be used to change ‘xx’ to another macro name.

In this output format, each non-graphical character, like newline and tab, is merely changed to exactly one space, with no special attempt to compress consecutive spaces. Each quote character ‘"’ is doubled so it will be correctly processed by nroff or troff.

-T
--format=tex

Choose an output format suitable for TeX processing. Each output line will look like:

\xx {tail}{before}{keyword}{after}{head}{ref}

so it will be possible to write a \xx definition to take care of the output typesetting. Note that when references are not being produced, that is, neither option -A nor option -r is selected, the last parameter of each \xx call is inhibited. Option -M can be used to change ‘xx’ to another macro name.

In this output format, some special characters, like ‘$’, ‘%’, ‘&’, ‘#’ and ‘_’ are automatically protected with a backslash. Curly brackets ‘{’, ‘}’ are protected with a backslash and a pair of dollar signs (to force mathematical mode). The backslash itself produces the sequence \backslash{}. Circumflex and tilde diacritical marks produce the sequence ^\{ } and ~\{ } respectively. Other diacriticized characters of the underlying character set produce an appropriate TeX sequence as far as possible. The other non-graphical characters, like newline and tab, and all other characters which are not part of ASCII, are merely changed to exactly one space, with no special attempt to compress consecutive spaces. Let me know how to improve this special character processing for TeX.


7.5.5 The GNU extensions to ptx

This version of ptx contains a few features which do not exist in System V ptx. These extra features are suppressed by using the -G command line option, unless overridden by other command line options. Some GNU extensions cannot be recovered by overriding, so the simple rule is to avoid -G if you care about GNU extensions. Here are the differences between this program and System V ptx.

  • This program can read many input files at once, it always writes the resulting concordance on standard output. On the other hand, System V ptx reads only one file and sends the result to standard output or, if a second file parameter is given on the command, to that file.

    Having output parameters not introduced by options is a dangerous practice which GNU avoids as far as possible. So, for using ptx portably between GNU and System V, you should always use it with a single input file, and always expect the result on standard output. You might also want to automatically configure in a -G option to ptx calls in products using ptx, if the configurator finds that the installed ptx accepts -G.

  • The only options available in System V ptx are options -b, -f, -g, -i, -o, -r, -t and -w. All other options are GNU extensions and are not repeated in this enumeration. Moreover, some options have a slightly different meaning when GNU extensions are enabled, as explained below.
  • By default, concordance output is not formatted for troff or nroff. It is rather formatted for a dumb terminal. troff or nroff output may still be selected through option -O.
  • Unless -R option is used, the maximum reference width is subtracted from the total output line width. With GNU extensions disabled, width of references is not taken into account in the output line width computations.
  • All 256 bytes, even ASCII NUL bytes, are always read and processed from input file with no adverse effect, even if GNU extensions are disabled. However, System V ptx does not accept 8-bit characters, a few control characters are rejected, and the tilde ‘~’ is also rejected.
  • Input line length is only limited by available memory, even if GNU extensions are disabled. However, System V ptx processes only the first 200 characters in each line.
  • The break (non-word) characters default to be every character except all letters of the underlying character set, diacriticized or not. When GNU extensions are disabled, the break characters default to space, tab and newline only.
  • The program makes better use of output line width. If GNU extensions are disabled, the program rather tries to imitate System V ptx, but still, there are some slight disposition glitches this program does not completely reproduce.
  • The user can specify both an Ignore file and an Only file. This is not allowed with System V ptx.

7.6 tsort: Topological sort

tsort performs a topological sort on the given file, or standard input if no input file is given or for a file of ‘-’. For more details and some history, see tsort: Background. Synopsis:

tsort [option] [file]

tsort reads its input as pairs of strings, separated by blanks, indicating a partial ordering. The output is a total ordering that corresponds to the given partial ordering.

For example

tsort <<EOF
a b c
d
e f
b c d e
EOF

will produce the output

a
b
c
d
e
f

Consider a more realistic example. You have a large set of functions all in one file, and they may all be declared static except one. Currently that one (say main) is the first function defined in the file, and the ones it calls directly follow it, followed by those they call, etc. Let’s say that you are determined to take advantage of prototypes, so you have to choose between declaring all of those functions (which means duplicating a lot of information from the definitions) and rearranging the functions so that as many as possible are defined before they are used. One way to automate the latter process is to get a list for each function of the functions it calls directly. Many programs can generate such lists. They describe a call graph. Consider the following list, in which a given line indicates that the function on the left calls the one on the right directly.

main parse_options
main tail_file
main tail_forever
tail_file pretty_name
tail_file write_header
tail_file tail
tail_forever recheck
tail_forever pretty_name
tail_forever write_header
tail_forever dump_remainder
tail tail_lines
tail tail_bytes
tail_lines start_lines
tail_lines dump_remainder
tail_lines file_lines
tail_lines pipe_lines
tail_bytes xlseek
tail_bytes start_bytes
tail_bytes dump_remainder
tail_bytes pipe_bytes
file_lines dump_remainder
recheck pretty_name

then you can use tsort to produce an ordering of those functions that satisfies your requirement.

example$ tsort call-graph | tac
dump_remainder
start_lines
file_lines
pipe_lines
xlseek
start_bytes
pipe_bytes
tail_lines
tail_bytes
pretty_name
write_header
tail
recheck
parse_options
tail_file
tail_forever
main

tsort detects any cycles in the input and writes the first cycle encountered to standard error.

Note that for a given partial ordering, generally there is no unique total ordering. In the context of the call graph above, the function parse_options may be placed anywhere in the list as long as it precedes main.

The only options are --help and --version. See Common options.

An exit status of zero indicates success, and a nonzero value indicates failure.


7.6.1 tsort: Background

tsort exists because very early versions of the Unix linker processed an archive file exactly once, and in order. As ld read each object in the archive, it decided whether it was needed in the program based on whether it defined any symbols which were undefined at that point in the link.

This meant that dependencies within the archive had to be handled specially. For example, scanf probably calls read. That means that in a single pass through an archive, it was important for scanf.o to appear before read.o, because otherwise a program which calls scanf but not read might end up with an unexpected unresolved reference to read.

The way to address this problem was to first generate a set of dependencies of one object file on another. This was done by a shell script called lorder. The GNU tools don’t provide a version of lorder, as far as I know, but you can still find it in BSD distributions.

Then you ran tsort over the lorder output, and you used the resulting sort to define the order in which you added objects to the archive.

This whole procedure has been obsolete since about 1980, because Unix archives now contain a symbol table (traditionally built by ranlib, now generally built by ar itself), and the Unix linker uses the symbol table to effectively make multiple passes over an archive file.

Anyhow, that’s where tsort came from. To solve an old problem with the way the linker handled archive files, which has since been solved in different ways.


8 Operating on fields


8.1 cut: Print selected parts of lines

cut writes to standard output selected parts of each line of each input file, or standard input if no files are given or for a file name of ‘-’. Synopsis:

cut option… [file]…

In the table which follows, the byte-list, character-list, and field-list are one or more numbers or ranges (two numbers separated by a dash) separated by commas. Bytes, characters, and fields are numbered starting at 1. Incomplete ranges may be given: -m means ‘1-m’; ‘n-’ means ‘n’ through end of line or last field. The list elements can be repeated, can overlap, and can be specified in any order; but the selected input is written in the same order that it is read, and is written exactly once.

The program accepts the following options. Also see Common options.

-b byte-list
--bytes=byte-list

Select for printing only the bytes in positions listed in byte-list. Tabs and backspaces are treated like any other character; they take up 1 byte. If an output delimiter is specified, (see the description of --output-delimiter), then output that string between ranges of selected bytes.

-c character-list
--characters=character-list

Select for printing only the characters in positions listed in character-list. The same as -b for now, but internationalization will change that. Tabs and backspaces are treated like any other character; they take up 1 character. If an output delimiter is specified, (see the description of --output-delimiter), then output that string between ranges of selected bytes.

-f field-list
--fields=field-list

Select for printing only the fields listed in field-list. Fields are separated by a TAB character by default. Also print any line that contains no delimiter character, unless the --only-delimited (-s) option is specified.

Note awk supports more sophisticated field processing, like reordering fields, and handling fields aligned with blank characters. By default awk uses (and discards) runs of blank characters to separate fields, and ignores leading and trailing blanks.

awk '{print $2}'      # print the second field
awk '{print $(NF-1)}' # print the penultimate field
awk '{print $2,$1}'   # reorder the first two fields

Note while cut accepts field specifications in arbitrary order, output is always in the order encountered in the file.

In the unlikely event that awk is unavailable, one can use the join command, to process blank characters as awk does above.

join -a1 -o 1.2     - /dev/null # print the second field
join -a1 -o 1.2,1.1 - /dev/null # reorder the first two fields
-d input_delim_byte
--delimiter=input_delim_byte

With -f, use the first byte of input_delim_byte as the input fields separator (default is TAB).

-n

Do not split multi-byte characters (no-op for now).

-s
--only-delimited

For -f, do not print lines that do not contain the field separator character. Normally, any line without a field separator is printed verbatim.

--output-delimiter=output_delim_string

With -f, output fields are separated by output_delim_string. The default with -f is to use the input delimiter. When using -b or -c to select ranges of byte or character offsets (as opposed to ranges of fields), output output_delim_string between non-overlapping ranges of selected bytes.

--complement

This option is a GNU extension. Select for printing the complement of the bytes, characters or fields selected with the -b, -c or -f options. In other words, do not print the bytes, characters or fields specified via those options. This option is useful when you have many fields and want to print all but a few of them.

-z
--zero-terminated

Delimit items with a zero byte rather than a newline (ASCII LF). I.e., treat input as items separated by ASCII NUL and terminate output items with ASCII NUL. This option can be useful in conjunction with ‘perl -0’ or ‘find -print0’ and ‘xargs -0’ which do the same in order to reliably handle arbitrary file names (even those containing blanks or other special characters).

An exit status of zero indicates success, and a nonzero value indicates failure.


8.2 paste: Merge lines of files

paste writes to standard output lines consisting of sequentially corresponding lines of each given file, separated by a TAB character. Standard input is used for a file name of ‘-’ or if no input files are given.

Synopsis:

paste [option]… [file]…

For example, with:

$ cat num2
1
2
$ cat let3
a
b
c

Take lines sequentially from each file:

$ paste num2 let3
1       a
2       b
        c

Duplicate lines from a file:

$ paste num2 let3 num2
1       a      1
2       b      2
        c

Intermix lines from standard input:

$ paste - let3 - < num2
1       a      2
        b
        c

Join consecutive lines with a space:

$ seq 4 | paste -d ' ' - -
1 2
3 4

The program accepts the following options. Also see Common options.

-s
--serial

Paste the lines of one file at a time rather than one line from each file. Using the above example data:

$ paste -s num2 let3
1       2
a       b       c
-d delim-list
--delimiters=delim-list

Consecutively use the characters in delim-list instead of TAB to separate merged lines. When delim-list is exhausted, start again at its beginning. Using the above example data:

$ paste -d '%_' num2 let3 num2
1%a_1
2%b_2
%c_
-z
--zero-terminated

Delimit items with a zero byte rather than a newline (ASCII LF). I.e., treat input as items separated by ASCII NUL and terminate output items with ASCII NUL. This option can be useful in conjunction with ‘perl -0’ or ‘find -print0’ and ‘xargs -0’ which do the same in order to reliably handle arbitrary file names (even those containing blanks or other special characters).

An exit status of zero indicates success, and a nonzero value indicates failure.


8.3 join: Join lines on a common field

join writes to standard output a line for each pair of input lines that have identical join fields. Synopsis:

join [option]… file1 file2

Either file1 or file2 (but not both) can be ‘-’, meaning standard input. file1 and file2 should be sorted on the join fields.

$ cat file1
a 1
b 2
e 5

$ cat file2
a X
e Y
f Z

$ join file1 file2
a 1 X
e 5 Y

join’s default behavior (when no options are given):

  • the join field is the first field in each line;
  • fields in the input are separated by one or more blanks, with leading blanks on the line ignored;
  • fields in the output are separated by a space;
  • each output line consists of the join field, the remaining fields from file1, then the remaining fields from file2.

8.3.1 General options

The program accepts the following options. Also see Common options.

-a file-number

Print a line for each unpairable line in file file-number (either ‘1’ or ‘2’), in addition to the normal output.

--check-order

Fail with an error message if either input file is wrongly ordered.

--nocheck-order

Do not check that both input files are in sorted order. This is the default.

-e string

Replace those output fields that are missing in the input with string. I.e., missing fields specified with the -12jo options.

--header

Treat the first line of each input file as a header line. The header lines will be joined and printed as the first output line. If -o is used to specify output format, the header line will be printed according to the specified format. The header lines will not be checked for ordering even if --check-order is specified. Also if the header lines from each file do not match, the heading fields from the first file will be used.

-i
--ignore-case

Ignore differences in case when comparing keys. With this option, the lines of the input files must be ordered in the same way. Use ‘sort -f’ to produce this ordering.

-1 field

Join on field field (a positive integer) of file 1.

-2 field

Join on field field (a positive integer) of file 2.

-j field

Equivalent to -1 field -2 field.

-o field-list
-o auto

If the keyword ‘auto’ is specified, infer the output format from the first line in each file. This is the same as the default output format but also ensures the same number of fields are output for each line. Missing fields are replaced with the -e option and extra fields are discarded.

Otherwise, construct each output line according to the format in field-list. Each element in field-list is either the single character ‘0’ or has the form m.n where the file number, m, is ‘1’ or ‘2’ and n is a positive field number.

A field specification of ‘0’ denotes the join field. In most cases, the functionality of the ‘0’ field spec may be reproduced using the explicit m.n that corresponds to the join field. However, when printing unpairable lines (using either of the -a or -v options), there is no way to specify the join field using m.n in field-list if there are unpairable lines in both files. To give join that functionality, POSIX invented the ‘0’ field specification notation.

The elements in field-list are separated by commas or blanks. Blank separators typically need to be quoted for the shell. For example, the commands ‘join -o 1.2,2.2’ and ‘join -o '1.2 2.2'’ are equivalent.

All output lines—including those printed because of any -a or -v option—are subject to the specified field-list.

-t char

Use character char as the input and output field separator. Treat as significant each occurrence of char in the input file. Use ‘sort -t char’, without the -b option of ‘sort’, to produce this ordering. If ‘join -t ''’ is specified, the whole line is considered, matching the default operation of sort. If ‘-t '\0'’ is specified then the ASCII NUL character is used to delimit the fields.

-v file-number

Print a line for each unpairable line in file file-number (either ‘1’ or ‘2’), instead of the normal output.

-z
--zero-terminated

Delimit items with a zero byte rather than a newline (ASCII LF). I.e., treat input as items separated by ASCII NUL and terminate output items with ASCII NUL. This option can be useful in conjunction with ‘perl -0’ or ‘find -print0’ and ‘xargs -0’ which do the same in order to reliably handle arbitrary file names (even those containing blanks or other special characters). Note with -z the newline character is treated as a field separator.

An exit status of zero indicates success, and a nonzero value indicates failure.

If the --check-order option is given, unsorted inputs will cause a fatal error message. If the option --nocheck-order is given, unsorted inputs will never cause an error message. If neither of these options is given, wrongly sorted inputs are diagnosed only if an input file is found to contain unpairable lines, and when both input files are non empty. If an input file is diagnosed as being unsorted, the join command will exit with a nonzero status (and the output should not be used).

Forcing join to process wrongly sorted input files containing unpairable lines by specifying --nocheck-order is not guaranteed to produce any particular output. The output will probably not correspond with whatever you hoped it would be.


8.3.2 Pre-sorting

join requires sorted input files. Each input file should be sorted according to the key (=field/column number) used in join. The recommended sorting option is ‘sort -k 1b,1’ (assuming the desired key is in the first column).

Typical usage:

$ sort -k 1b,1 file1 > file1.sorted
$ sort -k 1b,1 file2 > file2.sorted
$ join file1.sorted file2.sorted > file3

Normally, the sort order is that of the collating sequence specified by the LC_COLLATE locale. Unless the -t option is given, the sort comparison ignores blanks at the start of the join field, as in sort -b. If the --ignore-case option is given, the sort comparison ignores the case of characters in the join field, as in sort -f:

$ sort -k 1bf,1 file1 > file1.sorted
$ sort -k 1bf,1 file2 > file2.sorted
$ join --ignore-case file1.sorted file2.sorted > file3

The sort and join commands should use consistent locales and options if the output of sort is fed to join. You can use a command like ‘sort -k 1b,1’ to sort a file on its default join field, but if you select a non-default locale, join field, separator, or comparison options, then you should do so consistently between join and sort.

To avoid any locale-related issues, it is recommended to use the ‘C’ locale for both commands:

$ LC_ALL=C sort -k 1b,1 file1 > file1.sorted
$ LC_ALL=C sort -k 1b,1 file2 > file2.sorted
$ LC_ALL=C join file1.sorted file2.sorted > file3

8.3.3 Working with fields

Use -1,-2 to set the key fields for each of the input files. Ensure the preceding sort commands operated on the same fields.

The following example joins two files, using the values from seventh field of the first file and the third field of the second file:

$ sort -k 7b,7 file1 > file1.sorted
$ sort -k 3b,3 file2 > file2.sorted
$ join -1 7 -2 3 file1.sorted file2.sorted > file3

If the field number is the same for both files, use -j:

$ sort -k4b,4 file1 > file1.sorted
$ sort -k4b,4 file2 > file2.sorted
$ join -j4    file1.sorted file2.sorted > file3

Both sort and join operate of whitespace-delimited fields. To specify a different delimiter, use -t in both:

$ sort -t, -k3b,3 file1 > file1.sorted
$ sort -t, -k3b,3 file2 > file2.sorted
$ join -t, -j3    file1.sorted file2.sorted > file3

To specify a tab (ASCII 0x09) character instead of whitespace, use 3:

$ sort -t$'\t' -k3b,3 file1 > file1.sorted
$ sort -t$'\t' -k3b,3 file2 > file2.sorted
$ join -t$'\t' -j3    file1.sorted file2.sorted > file3

If ‘join -t ''’ is specified then the whole line is considered which matches the default operation of sort:

$ sort file1 > file1.sorted
$ sort file2 > file2.sorted
$ join -t '' file1.sorted file2.sorted > file3

8.3.4 Controlling join’s field matching

In this section the sort commands are omitted for brevity. Sorting the files before joining is still required.

join’s default behavior is to print only lines common to both input files. Use -a and -v to print unpairable lines from one or both files.

All examples below use the following two (pre-sorted) input files:

$ cat file1
a 1
b 2
$ cat file2
a A
c C
CommandOutcome
$ join file1 file2
a 1 A
common lines (intersection)
$ join -a 1 file1 file2
a 1 A
b 2
common lines and unpaired lines from the first file
$ join -a 2 file1 file2
a 1 A
c C
common lines and unpaired lines from the second file
$ join -a 1 -a 2 file1 file2
a 1 A
b 2
c C
all lines (paired and unpaired) from both files (union).
see note below regarding -o auto.
$ join -v 1 file1 file2
b 2
unpaired lines from the first file (difference)
$ join -v 2 file1 file2
c C
unpaired lines from the second file (difference)
$ join -v 1 -v 2 file1 file2
b 2
c C
unpaired lines from both files, omitting common lines (symmetric difference).

The -o auto -e X options are useful when dealing with unpaired lines. The following example prints all lines (common and unpaired) from both files. Without -o auto it is not easy to discern which fields originate from which file:

$ join -a 1 -a 2 file1 file2
a 1 A
b 2
c C

$ join -o auto -e X -a 1 -a 2 file1 file2
a 1 A
b 2 X
c X C

If the input has no unpairable lines, a GNU extension is available; the sort order can be any order that considers two fields to be equal if and only if the sort comparison described above considers them to be equal. For example:

$ cat file1
a a1
c c1
b b1

$ cat file2
a a2
c c2
b b2

$ join file1 file2
a a1 a2
c c1 c2
b b1 b2

8.3.5 Header lines

The --header option can be used when the files to join have a header line which is not sorted:

$ cat file1
Name     Age
Alice    25
Charlie  34

$ cat file2
Name   Country
Alice  France
Bob    Spain

$ join --header -o auto -e NA -a1 -a2 file1 file2
Name     Age   Country
Alice    25    France
Bob      NA    Spain
Charlie  34    NA

To sort a file with a header line, use GNU sed -u. The following example sort the files but keeps the first line of each file in place:

$ ( sed -u 1q ; sort -k2b,2 ) < file1 > file1.sorted
$ ( sed -u 1q ; sort -k2b,2 ) < file2 > file2.sorted
$ join --header -o auto -e NA -a1 -a2 file1.sorted file2.sorted > file3

8.3.6 Union, Intersection and Difference of files

Combine sort, uniq and join to perform the equivalent of set operations on files:

Commandoutcome
sort -u file1 file2Union of unsorted files
sort file1 file2 | uniq -dIntersection of unsorted files
sort file1 file1 file2 | uniq -uDifference of unsorted files
sort file1 file2 | uniq -uSymmetric Difference of unsorted files
join -t '' -a1 -a2 file1 file2Union of sorted files
join -t '' file1 file2Intersection of sorted files
join -t '' -v2 file1 file2Difference of sorted files
join -t '' -v1 -v2 file1 file2Symmetric Difference of sorted files

All examples above operate on entire lines and not on specific fields: sort without -k and join -t '' both consider entire lines as the key.


9 Operating on characters

These commands operate on individual characters.


9.1 tr: Translate, squeeze, and/or delete characters

Synopsis:

tr [option]… string1 [string2]

tr copies standard input to standard output, performing one of the following operations:

  • translate, and optionally squeeze repeated characters in the result,
  • squeeze repeated characters,
  • delete characters,
  • delete characters, then squeeze repeated characters from the result.

The string1 and string2 operands define arrays of characters array1 and array2. By default array1 lists input characters that tr operates on, and array2 lists corresponding translations. In some cases the second operand is omitted.

The program accepts the following options. Also see Common options. Options must precede operands.

-c
-C
--complement

Instead of array1, use its complement (all characters not specified by string1), in ascending order. Use this option with caution in multibyte locales where its meaning is not always clear or portable; see Specifying arrays of characters.

-d
--delete

Delete characters in array1; do not translate.

-s
--squeeze-repeats

Replace each sequence of a repeated character that is listed in the last specified array, with a single occurrence of that character.

-t
--truncate-set1

Truncate array1 to the length of array2.

An exit status of zero indicates success, and a nonzero value indicates failure.


9.1.1 Specifying arrays of characters

The string1 and string2 operands are not regular expressions, even though they may look similar. Instead, they merely represent arrays of characters. As a GNU extension to POSIX, an empty string operand represents an empty array of characters.

The interpretation of string1 and string2 depends on locale. GNU tr fully supports only safe single-byte locales, where each possible input byte represents a single character. Unfortunately, this means GNU tr will not handle commands like ‘tr $'\u7530' $'\u68EE'’ the way you might expect, since (assuming a UTF-8 encoding) this is equivalent to ‘tr '\347\224\260' '\346\243\256'’ and GNU tr will simply transliterate all ‘\347’ bytes to ‘\346’ bytes, etc. POSIX does not clearly specify the behavior of tr in locales where characters are represented by byte sequences instead of by individual bytes, or where data might contain invalid bytes that are encoding errors. To avoid problems in this area, you can run tr in a safe single-byte locale by using a shell command like ‘LC_ALL=C tr’ instead of plain tr.

Although most characters simply represent themselves in string1 and string2, the strings can contain shorthands listed below, for convenience. Some shorthands can be used only in string1 or string2, as noted below.

Backslash escapes

The following backslash escape sequences are recognized:

\a

Bell (BEL, Control-G).

\b

Backspace (BS, Control-H).

\f

Form feed (FF, Control-L).

\n

Newline (LF, Control-J).

\r

Carriage return (CR, Control-M).

\t

Tab (HT, Control-I).

\v

Vertical tab (VT, Control-K).

\ooo

The eight-bit byte with the value given by ooo, which is the longest sequence of one to three octal digits following the backslash. For portability, ooo should represent a value that fits in eight bits. As a GNU extension to POSIX, if the value would not fit, then only the first two digits of ooo are used, e.g., ‘\400’ is equivalent to ‘\0400’ and represents a two-byte sequence.

\\

A backslash.

It is an error if no character follows an unescaped backslash. As a GNU extension, a backslash followed by a character not listed above is interpreted as that character, removing any special significance; this can be used to escape the characters ‘[’ and ‘-’ when they would otherwise be special.

Ranges

The notation ‘m-n’ expands to the characters from m through n, in ascending order. m should not collate after n; if it does, an error results. As an example, ‘0-9’ is the same as ‘0123456789’.

GNU tr does not support the System V syntax that uses square brackets to enclose ranges. Translations specified in that format sometimes work as expected, since the brackets are often transliterated to themselves. However, they should be avoided because they sometimes behave unexpectedly. For example, ‘tr -d '[0-9]'’ deletes brackets as well as digits.

Many historically common and even accepted uses of ranges are not fully portable. For example, on EBCDIC hosts using the ‘A-Z’ range will not do what most would expect because ‘A’ through ‘Z’ are not contiguous as they are in ASCII. One way to work around this is to use character classes (see below). Otherwise, it is most portable (and most ugly) to enumerate the members of the ranges.

Repeated characters

The notation ‘[c*n]’ in string2 expands to n copies of character c. Thus, ‘[y*6]’ is the same as ‘yyyyyy’. The notation ‘[c*]’ in string2 expands to as many copies of c as are needed to make array2 as long as array1. If n begins with ‘0’, it is interpreted in octal, otherwise in decimal. A zero-valued n is treated as if it were absent.

Character classes

The notation ‘[:class:]’ expands to all characters in the (predefined) class class. When the --delete (-d) and --squeeze-repeats (-s) options are both given, any character class can be used in string2. Otherwise, only the character classes lower and upper are accepted in string2, and then only if the corresponding character class (upper and lower, respectively) is specified in the same relative position in string1. Doing this specifies case conversion. Except for case conversion, a class’s characters appear in no particular order. The class names are given below; an error results when an invalid class name is given.

alnum

Letters and digits.

alpha

Letters.

blank

Horizontal whitespace.

cntrl

Control characters.

digit

Digits.

graph

Printable characters, not including space.

lower

Lowercase letters.

print

Printable characters, including space.

punct

Punctuation characters.

space

Horizontal or vertical whitespace.

upper

Uppercase letters.

xdigit

Hexadecimal digits.

Equivalence classes

The syntax ‘[=c=]’ expands to all characters equivalent to c, in no particular order. These equivalence classes are allowed in string2 only when --delete (-d) and --squeeze-repeats -s are both given.

Although equivalence classes are intended to support non-English alphabets, there seems to be no standard way to define them or determine their contents. Therefore, they are not fully implemented in GNU tr; each character’s equivalence class consists only of that character, which is of no particular use.


9.1.2 Translating

tr performs translation when string1 and string2 are both given and the --delete (-d) option is not given. tr translates each character of its input that is in array1 to the corresponding character in array2. Characters not in array1 are passed through unchanged.

As a GNU extension to POSIX, when a character appears more than once in array1, only the final instance is used. For example, these two commands are equivalent:

tr aaa xyz
tr a z

A common use of tr is to convert lowercase characters to uppercase. This can be done in many ways. Here are three of them:

tr abcdefghijklmnopqrstuvwxyz ABCDEFGHIJKLMNOPQRSTUVWXYZ
tr a-z A-Z
tr '[:lower:]' '[:upper:]'

However, ranges like a-z are not portable outside the C locale.

When tr is performing translation, array1 and array2 typically have the same length. If array1 is shorter than array2, the extra characters at the end of array2 are ignored.

On the other hand, making array1 longer than array2 is not portable; POSIX says that the result is undefined. In this situation, BSD tr pads array2 to the length of array1 by repeating the last character of array2 as many times as necessary. System V tr truncates array1 to the length of array2.

By default, GNU tr handles this case like BSD tr. When the --truncate-set1 (-t) option is given, GNU tr handles this case like the System V tr instead. This option is ignored for operations other than translation.

Acting like System V tr in this case breaks the relatively common BSD idiom:

tr -cs A-Za-z0-9 '\012'

because it converts only zero bytes (the first element in the complement of array1), rather than all non-alphanumerics, to newlines.

By the way, the above idiom is not portable because it uses ranges, and it assumes that the octal code for newline is 012. Here is a better way to write it:

tr -cs '[:alnum:]' '[\n*]'

9.1.3 Squeezing repeats and deleting

When given just the --delete (-d) option, tr removes any input characters that are in array1.

When given just the --squeeze-repeats (-s) option and not translating, tr replaces each input sequence of a repeated character that is in array1 with a single occurrence of that character.

When given both --delete and --squeeze-repeats, tr first performs any deletions using array1, then squeezes repeats from any remaining characters using array2.

The --squeeze-repeats option may also be used when translating, in which case tr first performs translation, then squeezes repeats from any remaining characters using array2.

Here are some examples to illustrate various combinations of options:

  • Remove all zero bytes:
    tr -d '\0'
    
  • Put all words on lines by themselves. This converts all non-alphanumeric characters to newlines, then squeezes each string of repeated newlines into a single newline:
    tr -cs '[:alnum:]' '[\n*]'
    
  • Convert each sequence of repeated newlines to a single newline. I.e., delete empty lines:
    tr -s '\n'
    
  • Find doubled occurrences of words in a document. For example, people often write “the the” with the repeated words separated by a newline. The Bourne shell script below works first by converting each sequence of punctuation and blank characters to a single newline. That puts each “word” on a line by itself. Next it maps all uppercase characters to lower case, and finally it runs uniq with the -d option to print out only the words that were repeated.
    #!/bin/sh
    cat -- "$@" \
      | tr -s '[:punct:][:blank:]' '[\n*]' \
      | tr '[:upper:]' '[:lower:]' \
      | uniq -d
    
  • Deleting a small set of characters is usually straightforward. For example, to remove all ‘a’s, ‘x’s, and ‘M’s you would do this:
    tr -d axM
    

    However, when ‘-’ is one of those characters, it can be tricky because ‘-’ has special meanings. Performing the same task as above but also removing all ‘-’ characters, we might try tr -d -axM, but that would fail because tr would try to interpret -a as a command-line option. Alternatively, we could try putting the hyphen inside the string, tr -d a-xM, but that wouldn’t work either because it would make tr interpret a-x as the range of characters ‘a’…‘x’ rather than the three. One way to solve the problem is to put the hyphen at the end of the list of characters:

    tr -d axM-
    

    Or you can use ‘--’ to terminate option processing:

    tr -d -- -axM
    

9.2 expand: Convert tabs to spaces

expand writes the contents of each given file, or standard input if none are given or for a file of ‘-’, to standard output, with tab characters converted to the appropriate number of spaces. Synopsis:

expand [option]… [file]…

By default, expand converts all tabs to spaces. It preserves backspace characters in the output; they decrement the column count for tab calculations. The default action is equivalent to -t 8 (set tabs every 8 columns).

The program accepts the following options. Also see Common options.

-t tab1[,tab2]…
--tabs=tab1[,tab2]…

If only one tab stop is given, set the tabs tab1 spaces apart (default is 8). Otherwise, set the tabs at columns tab1, tab2, … (numbered from 0), and replace any tabs beyond the last tab stop given with single spaces. Tab stops can be separated by blanks as well as by commas.

As a GNU extension the last tab specified can be prefixed with a ‘/’ to indicate a tab size to use for remaining positions. For example, --tabs=2,4,/8 will set tab stops at position 2 and 4, and every multiple of 8 after that.

Also the last tab specified can be prefixed with a ‘+’ to indicate a tab size to use for remaining positions, offset from the final explicitly specified tab stop. For example, to ignore the 1 character gutter present in diff output, one can specify a 1 character offset using --tabs=1,+8, which will set tab stops at positions 1,9,17,…

For compatibility, GNU expand also accepts the obsolete option syntax, -t1[,t2]…. New scripts should use -t t1[,t2]… instead.

-i
--initial

Only convert initial tabs (those that precede all non-space or non-tab characters) on each line to spaces.

An exit status of zero indicates success, and a nonzero value indicates failure.


9.3 unexpand: Convert spaces to tabs

unexpand writes the contents of each given file, or standard input if none are given or for a file of ‘-’, to standard output, converting blanks at the beginning of each line into as many tab characters as needed. In the default POSIX locale, a blank is a space or a tab; other locales may specify additional blank characters. Synopsis:

unexpand [option]… [file]…

By default, unexpand converts only initial blanks (those that precede all non-blank characters) on each line. It preserves backspace characters in the output; they decrement the column count for tab calculations. By default, tabs are set at every 8th column.

The program accepts the following options. Also see Common options.

-t tab1[,tab2]…
--tabs=tab1[,tab2]…

If only one tab stop is given, set the tabs tab1 columns apart instead of the default 8. Otherwise, set the tabs at columns tab1, tab2, … (numbered from 0), and leave blanks beyond the tab stops given unchanged. Tab stops can be separated by blanks as well as by commas.

As a GNU extension the last tab specified can be prefixed with a ‘/’ to indicate a tab size to use for remaining positions. For example, --tabs=2,4,/8 will set tab stops at position 2 and 4, and every multiple of 8 after that.

Also the last tab specified can be prefixed with a ‘+’ to indicate a tab size to use for remaining positions, offset from the final explicitly specified tab stop. For example, to ignore the 1 character gutter present in diff output, one can specify a 1 character offset using --tabs=1,+8, which will set tab stops at positions 1,9,17,…

This option implies the -a option.

For compatibility, GNU unexpand supports the obsolete option syntax, -tab1[,tab2]…, where tab stops must be separated by commas. (Unlike -t, this obsolete option does not imply -a.) New scripts should use --first-only -t tab1[,tab2]… instead.

-a
--all

Also convert all sequences of two or more blanks just before a tab stop, even if they occur after non-blank characters in a line.

An exit status of zero indicates success, and a nonzero value indicates failure.


10 Directory listing

This chapter describes the ls command and its variants dir and vdir, which list information about files.


10.1 ls: List directory contents

The ls program lists information about files (of any type, including directories). Options and file arguments can be intermixed arbitrarily, as usual. Later options override earlier options that are incompatible.

For non-option command-line arguments that are directories, by default ls lists the contents of directories, not recursively, and omitting files with names beginning with ‘.’. For other non-option arguments, by default ls lists just the file name. If no non-option argument is specified, ls operates on the current directory, acting as if it had been invoked with a single argument of ‘.’.

By default, the output is sorted alphabetically, according to the locale settings in effect.4 If standard output is a terminal, the output is in columns (sorted vertically) and control characters are output as question marks; otherwise, the output is listed one per line and control characters are output as-is.

Because ls is such a fundamental program, it has accumulated many options over the years. They are described in the subsections below; within each section, options are listed alphabetically (ignoring case). The division of options into the subsections is not absolute, since some options affect more than one aspect of ls’s operation.

Exit status:

0 success
1 minor problems  (e.g., failure to access a file or directory not
  specified as a command line argument.  This happens when listing a
  directory in which entries are actively being removed or renamed.)
2 serious trouble (e.g., memory exhausted, invalid option, failure
  to access a file or directory specified as a command line argument
  or a directory loop)

Also see Common options.


10.1.1 Which files are listed

These options determine which files ls lists information for. By default, ls lists files and the contents of any directories on the command line, except that in directories it ignores files whose names start with ‘.’.

-a
--all

In directories, do not ignore file names that start with ‘.’.

-A
--almost-all

In directories, do not ignore all file names that start with ‘.’; ignore only . and ... The --all (-a) option overrides this option.

-B
--ignore-backups

In directories, ignore files that end with ‘~’. This option is equivalent to ‘--ignore='*~' --ignore='.*~'’.

-d
--directory

List just the names of directories, as with other types of files, rather than listing their contents. Do not follow symbolic links listed on the command line unless the --dereference-command-line (-H), --dereference (-L), or --dereference-command-line-symlink-to-dir options are specified.

-H
--dereference-command-line

If a command line argument specifies a symbolic link, show information for the file the link references rather than for the link itself.

Do not dereference symbolic links, with one exception: if a command line argument specifies a symbolic link that refers to a directory, show information for that directory rather than for the link itself. This is the default behavior unless long format is being used or any of the following options is in effect: --classify (-F), --directory (-d), --dereference (-L), or --dereference-command-line (-H)).

--group-directories-first

Group all the directories before the files and then sort the directories and the files separately using the selected sort key (see –sort option). That is, this option specifies a primary sort key, and the –sort option specifies a secondary key. However, any use of --sort=none (-U) disables this option altogether.

--hide=PATTERN

In directories, ignore files whose names match the shell pattern pattern, unless the --all (-a) or --almost-all (-A) is also given. This option acts like --ignore=pattern except that it has no effect if --all (-a) or --almost-all (-A) is also given.

This option can be useful in shell aliases. For example, if lx is an alias for ‘ls --hide='*~'’ and ly is an alias for ‘ls --ignore='*~'’, then the command ‘lx -A’ lists the file README~ even though ‘ly -A’ would not.

-I pattern
--ignore=pattern

In directories, ignore files whose names match the shell pattern (not regular expression) pattern. As in the shell, an initial ‘.’ in a file name does not match a wildcard at the start of pattern. Sometimes it is useful to give this option several times. For example,

$ ls --ignore='.??*' --ignore='.[^.]' --ignore='#*'

The first option ignores names of length 3 or more that start with ‘.’, the second ignores all two-character names that start with ‘.’ except ‘..’, and the third ignores names that start with ‘#’.

-L
--dereference

When showing file information for a symbolic link, show information for the file the link references rather than the link itself. However, even with this option, ls still prints the name of the link itself, not the name of the file that the link points to.

-R
--recursive

List the contents of all directories recursively.


10.1.2 What information is listed

These options affect the information that ls displays. By default, only file names are shown.

--author

In long format, list each file’s author. In GNU/Hurd, file authors can differ from their owners, but in other operating systems the two are the same.

-D
--dired

Print an additional line after the main output:

//DIRED// beg1 end1 beg2 end2

The begn and endn are unsigned integers that record the byte position of the beginning and end of each file name in the output. This makes it easy for Emacs to find the names, even when they contain unusual characters such as space or newline, without fancy searching.

If directories are being listed recursively via --recursive (-R), output a similar line with offsets for each subdirectory name:

//SUBDIRED// beg1 end1

Finally, output a line of the form:

//DIRED-OPTIONS// --quoting-style=word

where word is the quoting style (see Formatting the file names).

Here is an actual example:

$ mkdir -p a/sub/deeper a/sub2
$ touch a/f1 a/f2
$ touch a/sub/deeper/file
$ ls -gloRF --dired a
  a:
  total 8
  -rw-r--r-- 1    0 Jun 10 12:27 f1
  -rw-r--r-- 1    0 Jun 10 12:27 f2
  drwxr-xr-x 3 4096 Jun 10 12:27 sub/
  drwxr-xr-x 2 4096 Jun 10 12:27 sub2/

  a/sub:
  total 4
  drwxr-xr-x 2 4096 Jun 10 12:27 deeper/

  a/sub/deeper:
  total 0
  -rw-r--r-- 1 0 Jun 10 12:27 file

  a/sub2:
  total 0
//DIRED// 48 50 84 86 120 123 158 162 217 223 282 286
//SUBDIRED// 2 3 167 172 228 240 290 296
//DIRED-OPTIONS// --quoting-style=literal

The pairs of offsets on the ‘//DIRED//’ line above delimit these names: f1, f2, sub, sub2, deeper, file. The offsets on the ‘//SUBDIRED//’ line delimit the following directory names: a, a/sub, a/sub/deeper, a/sub2.

Here is an example of how to extract the fifth entry name, ‘deeper’, corresponding to the pair of offsets, 222 and 228:

$ ls -gloRF --dired a > out
$ dd bs=1 skip=222 count=6 < out 2>/dev/null; echo
deeper

Although the listing above includes a trailing slash for the ‘deeper’ entry, the offsets select the name without the trailing slash. However, if you invoke ls with --dired (-D) along with an option like --escape (-b) and operate on a file whose name contains special characters, the backslash is included:

$ touch 'a b'
$ ls -blog --dired 'a b'
  -rw-r--r-- 1 0 Jun 10 12:28 a\ b
//DIRED// 30 34
//DIRED-OPTIONS// --quoting-style=escape

If you use a quoting style like --quoting-style=c (-Q) that adds quote marks, then the offsets include the quote marks. So beware that the user may select the quoting style via the environment variable QUOTING_STYLE. Hence, applications using --dired should either specify an explicit --quoting-style=literal (-N) option on the command line, or else be prepared to parse the escaped names.

The --dired (-D) option has well-defined behavior only when long format is in effect and hyperlinks are disabled (e.g., --hyperlink=none).

--full-time

Produce long format, and list times in full. It is equivalent to using --format=long (-l) with --time-style=full-iso (see Formatting file timestamps).

-g

Produce long format, but omit owner information.

-G
--no-group

Inhibit display of group information in long format. (This is the default in some non-GNU versions of ls, so we provide this option for compatibility.)

-h
--human-readable

Append a size letter to each size, such as ‘M’ for mebibytes. Powers of 1024 are used, not 1000; ‘M’ stands for 1,048,576 bytes. This option is equivalent to --block-size=human-readable. Use the --si option if you prefer powers of 1000.

-i
--inode

Print the inode number (also called the file serial number and index number) of each file to the left of the file name. (This number uniquely identifies each file within a particular file system.)

-l
--format=long
--format=verbose

Produce long format. In addition to the name of each file, print the file type, file mode bits, number of hard links, owner name, group name, size, and timestamp (see Formatting file timestamps), normally the modification timestamp (the mtime, see File timestamps). If the owner or group name cannot be determined, print the owner or group ID instead, right-justified as a cue that it is a number rather than a textual name. Print question marks for other information that cannot be determined.

Normally the size is printed as a byte count without punctuation, but this can be overridden (see Block size). For example, --human-readable (-h) prints an abbreviated, human-readable count, and ‘--block-size="'1"’ prints a byte count with the thousands separator of the current locale.

For each directory that is listed, preface the files with a line ‘total blocks’, where blocks is the file system allocation for all files in that directory. The block size currently defaults to 1024 bytes, but this can be overridden (see Block size). The blocks computed counts each hard link separately; this is arguably a deficiency.

The file type is one of the following characters:

-

regular file

b

block special file

c

character special file

C

high performance (“contiguous data”) file

d

directory

D

door (Solaris)

l

symbolic link

M

off-line (“migrated”) file (Cray DMF)

n

network special file (HP-UX)

p

FIFO (named pipe)

P

port (Solaris)

s

socket

?

some other file type

The file mode bits listed are similar to symbolic mode specifications (see Symbolic Modes). But ls combines multiple bits into the third character of each set of permissions as follows:

s

If the set-user-ID or set-group-ID bit and the corresponding executable bit are both set.

S

If the set-user-ID or set-group-ID bit is set but the corresponding executable bit is not set.

t

If the restricted deletion flag or sticky bit, and the other-executable bit, are both set. The restricted deletion flag is another name for the sticky bit. See Structure of File Mode Bits.

T

If the restricted deletion flag or sticky bit is set but the other-executable bit is not set.

x

If the executable bit is set and none of the above apply.

-

Otherwise.

Following the file mode bits is a single character that specifies whether an alternate access method such as an access control list applies to the file. When the character following the file mode bits is a space, there is no alternate access method. When it is a printing character, then there is such a method.

GNU ls uses a ‘.’ character to indicate a file with a security context, but no other alternate access method.

A file with any other combination of alternate access methods is marked with a ‘+’ character.

-n
--numeric-uid-gid

Produce long format, but display right-justified numeric user and group IDs instead of left-justified owner and group names.

-o

Produce long format, but omit group information. It is equivalent to using --format=long (-l) with --no-group (-G).

-s
--size

Print the file system allocation of each file to the left of the file name. This is the amount of file system space used by the file, which is usually a bit more than the file’s size, but it can be less if the file has holes.

Normally the allocation is printed in units of 1024 bytes, but this can be overridden (see Block size).

For files that are NFS-mounted from an HP-UX system to a BSD system, this option reports sizes that are half the correct values. On HP-UX systems, it reports sizes that are twice the correct values for files that are NFS-mounted from BSD systems. This is due to a flaw in HP-UX; it also affects the HP-UX ls program.

--si

Append an SI-style abbreviation to each size, such as ‘M’ for megabytes. Powers of 1000 are used, not 1024; ‘M’ stands for 1,000,000 bytes. This option is equivalent to --block-size=si. Use the -h or --human-readable option if you prefer powers of 1024.

-Z
--context

Display the SELinux security context or ‘?’ if none is found. In long format, print the security context to the left of the size column.


10.1.3 Sorting the output

These options change the order in which ls sorts the information it outputs. By default, sorting is done by character code (e.g., ASCII order).

-c
--time=ctime
--time=status

In long format, print the status change timestamp (the ctime) instead of the mtime. When sorting by time or when not using long format, sort according to the ctime. See File timestamps.

-f

Produce an unsorted directory listing. This is equivalent to the combination of --all (-a), --sort=none (-U), -1, --color=none, and --hyperlink=none, while also disabling any previous use of --size (-s).

-r
--reverse

Reverse whatever the sorting method is—e.g., list files in reverse alphabetical order, youngest first, smallest first, or whatever. This option has no effect when --sort=none (-U) is in effect.

-S
--sort=size

Sort by file size, largest first.

-t
--sort=time

Sort by modification timestamp (mtime) by default, newest first. The timestamp to order by can be changed with the --time option. See File timestamps.

-u
--time=atime
--time=access
--time=use

In long format, print the last access timestamp (the atime). When sorting by time or when not using long format, sort according to the atime. See File timestamps.

--time=birth
--time=creation

In long format, print the file creation timestamp if available. When sorting by time or when not using long format, sort according to the birth time. See File timestamps.

-U
--sort=none

Do not sort; list the files in whatever order they are stored in the directory. (Do not do any of the other unrelated things that -f does.) This can be useful when listing large directories, where sorting can take some time.

-v
--sort=version

Sort by version name and number, lowest first. It behaves like a default sort, except that each sequence of decimal digits is treated numerically as an index/version number. See Version sort ordering.

--sort=width

Sort by printed width of file names. This can be useful with the --format=vertical (-C) output format, to most densely display the listed files.

-X
--sort=extension

Sort directory contents alphabetically by file extension (characters after the last ‘.’); files with no extension are sorted first.


10.1.4 General output formatting

These options affect the appearance of the overall output.

--format=single-column

List one file name per line, with no other information. This is the default for ls when standard output is not a terminal. See also the --escape (-b), --hide-control-chars (-q), and --zero options to disambiguate output of file names containing newline characters.

-1

List one file per line. This is like --format=single-column except that it has no effect if long format is also in effect.

-C
--format=vertical

List files in columns, sorted vertically, with no other information. This is the default for ls if standard output is a terminal. It is always the default for the dir program. GNU ls uses variable width columns to display as many files as possible in the fewest lines.

--color [=when]

Specify whether to use color for distinguishing file types; when may be omitted, or one of:

  • none - Do not use color at all. This is the default.
  • auto - Only use color if standard output is a terminal.
  • always - Always use color.

Specifying --color and no when is equivalent to --color=always. If piping a colored listing through a pager like less, use the pager’s -R option to pass the color codes to the terminal.

Using the --color option may incur a noticeable performance penalty when run in a large directory, because the default settings require that ls stat every single file it lists. However, if you would like most of the file-type coloring but can live without the other coloring options (e.g., executable, orphan, sticky, other-writable, capability), use dircolors to set the LS_COLORS environment variable like this,

eval $(dircolors -p | perl -pe \
  's/^((CAP|S[ET]|O[TR]|M|E)\w+).*/$1 00/' | dircolors -)

and on a dirent.d_type-capable file system, ls will perform only one stat call per command line argument.

-F
--classify [=when]
--indicator-style=classify

Append a character to each file name indicating the file type. Also, for regular files that are executable, append ‘*’. The file type indicators are ‘/’ for directories, ‘@’ for symbolic links, ‘|’ for FIFOs, ‘=’ for sockets, ‘>’ for doors, and nothing for regular files. when may be omitted, or one of:

  • none - Do not classify. This is the default.
  • auto - Only classify if standard output is a terminal.
  • always - Always classify.

Specifying --classify and no when is equivalent to --classify=always. Do not follow symbolic links listed on the command line unless the --dereference-command-line (-H), --dereference (-L), or --dereference-command-line-symlink-to-dir options are specified.

--file-type
--indicator-style=file-type

Append a character to each file name indicating the file type. This is like --classify (-F, except that executables are not marked.

Output codes recognized by some terminals to link to files using the ‘file://’ URI format. when may be omitted, or one of:

  • none - Do not use hyperlinks at all. This is the default.
  • auto - Only use hyperlinks if standard output is a terminal.
  • always - Always use hyperlinks.

Specifying --hyperlink and no when is equivalent to --hyperlink=always.

--indicator-style=word

Append a character indicator with style word to entry names, as follows:

none

Do not append any character indicator; this is the default.

slash

Append ‘/’ for directories. This is the same as the -p option.

file-type

Append ‘/’ for directories, ‘@’ for symbolic links, ‘|’ for FIFOs, ‘=’ for sockets, and nothing for regular files. This is the same as the --file-type option.

classify

Append ‘*’ for executable regular files, otherwise behave as for ‘file-type’. This is the same as the --classify (-F) option.

-k
--kibibytes

Set the default block size to its normal value of 1024 bytes, overriding any contrary specification in environment variables (see Block size). If --block-size, --human-readable (-h), or --si options are used, they take precedence even if --kibibytes (-k) is placed after

The --kibibytes (-k) option affects the per-directory block count written in long format, and the file system allocation written by the --size (-s) option. It does not affect the file size in bytes that is written in long format.

-m
--format=commas

List files horizontally, with as many as will fit on each line, separated by ‘, ’ (a comma and a space), and with no other information.

-p
--indicator-style=slash

Append a ‘/’ to directory names.

-x
--format=across
--format=horizontal

List the files in columns, sorted horizontally.

-T cols
--tabsize=cols

Assume that each tab stop is cols columns wide. The default is 8. ls uses tabs where possible in the output, for efficiency. If cols is zero, do not use tabs at all.

Some terminal emulators might not properly align columns to the right of a TAB following a non-ASCII byte. You can avoid that issue by using the -T0 option or put TABSIZE=0 in your environment, to tell ls to align using spaces, not tabs.

-w cols
--width=cols

Assume the screen is cols columns wide. The default is taken from the terminal settings if possible; otherwise the environment variable COLUMNS is used if it is set; otherwise the default is 80. With a cols value of ‘0’, there is no limit on the length of the output line, and that single output line will be delimited with spaces, not tabs.

--zero

Output a zero byte (ASCII NUL) at the end of each line, rather than a newline. This option enables other programs to parse the output even when that output would contain data with embedded newlines. This option is incompatible with the --dired (-D) option. This option also implies the options --show-control-chars, -1, --color=none, and --quoting-style=literal (-N).


10.1.5 Formatting file timestamps

By default, file timestamps are listed in abbreviated form, using a date like ‘Mar 30  2020’ for non-recent timestamps, and a date-without-year and time like ‘Mar 30 23:45’ for recent timestamps. This format can change depending on the current locale as detailed below.

A timestamp is considered to be recent if it is less than six months old, and is not dated in the future. If a timestamp dated today is not listed in recent form, the timestamp is in the future, which means you probably have clock skew problems which may break programs like make that rely on file timestamps. See File timestamps.

Timestamps are listed according to the time zone rules specified by the TZ environment variable, or by the system default rules if TZ is not set. See Specifying the Time Zone with TZ in The GNU C Library Reference Manual.

The following option changes how file timestamps are printed.

--time-style=style

List timestamps in style style. The style should be one of the following:

+format

List timestamps using format, where format is interpreted like the format argument of date (see date: Print or set system date and time). For example, --time-style="+%Y-%m-%d %H:%M:%S" causes ls to list timestamps like ‘2020-03-30 23:45:56’. As with date, format’s interpretation is affected by the LC_TIME locale category.

If format contains two format strings separated by a newline, the former is used for non-recent files and the latter for recent files; if you want output columns to line up, you may need to insert spaces in one of the two formats.

full-iso

List timestamps in full using ISO 8601-like date, time, and time zone components with nanosecond precision, e.g., ‘2020-07-21 23:45:56.477817180 -0400’. This style is equivalent to ‘+%Y-%m-%d %H:%M:%S.%N %z’.

This is useful because the time output includes all the information that is available from the operating system. For example, this can help explain make’s behavior, since GNU make uses the full timestamp to determine whether a file is out of date.

long-iso

List ISO 8601 date and time components with minute precision, e.g., ‘2020-03-30 23:45’. These timestamps are shorter than ‘full-iso’ timestamps, and are usually good enough for everyday work. This style is equivalent to ‘+%Y-%m-%d %H:%M’.

iso

List ISO 8601 dates for non-recent timestamps (e.g., ‘2020-03-30 ’), and ISO 8601-like month, day, hour, and minute for recent timestamps (e.g., ‘03-30 23:45’). These timestamps are uglier than ‘long-iso’ timestamps, but they carry nearly the same information in a smaller space and their brevity helps ls output fit within traditional 80-column output lines. The following two ls invocations are equivalent:

newline='
'
ls -l --time-style="+%Y-%m-%d $newline%m-%d %H:%M"
ls -l --time-style="iso"
locale

List timestamps in a locale-dependent form. For example, a French locale might list non-recent timestamps like ‘30 mars   2020’ and recent timestamps like ‘30 mars  23:45’. Locale-dependent timestamps typically consume more space than ‘iso’ timestamps and are harder for programs to parse because locale conventions vary so widely, but they are easier for many people to read.

The LC_TIME locale category specifies the timestamp format. The default POSIX locale uses timestamps like ‘Mar 30  2020’ and ‘Mar 30 23:45’; in this locale, the following two ls invocations are equivalent:

newline='
'
ls -l --time-style="+%b %e  %Y$newline%b %e %H:%M"
ls -l --time-style="locale"

Other locales behave differently. For example, in a German locale, --time-style="locale" might be equivalent to --time-style="+%e. %b %Y $newline%e. %b %H:%M" and might generate timestamps like ‘30. Mär 2020 ’ and ‘30. Mär 23:45’.

posix-style

List POSIX-locale timestamps if the LC_TIME locale category is POSIX, style timestamps otherwise. For example, the ‘posix-long-iso’ style lists timestamps like ‘Mar 30  2020’ and ‘Mar 30 23:45’ when in the POSIX locale, and like ‘2020-03-30 23:45’ otherwise.

You can specify the default value of the --time-style option with the environment variable TIME_STYLE; if TIME_STYLE is not set the default style is ‘locale’. GNU Emacs 21.3 and later use the --dired option and therefore can parse any date format, but if you are using Emacs 21.1 or 21.2 and specify a non-POSIX locale you may need to set ‘TIME_STYLE="posix-long-iso"’.

To avoid certain denial-of-service attacks, timestamps that would be longer than 1000 bytes may be treated as errors.


10.1.6 Formatting the file names

These options change how file names themselves are printed.

-b
--escape
--quoting-style=escape

Quote nongraphic characters in file names using alphabetic and octal backslash sequences like those used in C.

-N
--literal
--quoting-style=literal

Do not quote file names. However, with ls nongraphic characters are still printed as question marks if the output is a terminal and you do not specify the --show-control-chars option.

-q
--hide-control-chars

Print question marks instead of nongraphic characters in file names. This is the default if the output is a terminal and the program is ls.

-Q
--quote-name
--quoting-style=c

Enclose file names in double quotes and quote nongraphic characters as in C.

--quoting-style=word

Use style word to quote file names and other strings that may contain arbitrary characters. The word should be one of the following:

literal

Output strings as-is; this is the same as the --literal (-N) option.

shell

Quote strings for the shell if they contain shell metacharacters or would cause ambiguous output. The quoting is suitable for POSIX-compatible shells like bash, but it does not always work for incompatible shells like csh.

shell-always

Quote strings for the shell, even if they would normally not require quoting.

shell-escape

Like ‘shell’, but also quoting non-printable characters using the POSIX proposed ‘$''’ syntax suitable for most shells.

shell-escape-always

Like ‘shell-escape’, but quote strings even if they would normally not require quoting.

c

Quote strings as for C character string literals, including the surrounding double-quote characters; this is the same as the --quote-name (-Q) option.

escape

Quote strings as for C character string literals, except omit the surrounding double-quote characters; this is the same as the --escape (-b) option.

clocale

Quote strings as for C character string literals, except use surrounding quotation marks appropriate for the locale.

locale

Quote strings as for C character string literals, except use surrounding quotation marks appropriate for the locale, and quote 'like this' instead of "like this" in the default C locale. This looks nicer on many displays.

You can specify the default value of the --quoting-style option with the environment variable QUOTING_STYLE. If that environment variable is not set, the default value is ‘shell-escape’ when the output is a terminal, and ‘literal’ otherwise.

--show-control-chars

Print nongraphic characters as-is in file names. This is the default unless the output is a terminal and the program is ls.


10.2 dir: Briefly list directory contents

dir is equivalent to ls -C -b; that is, by default files are listed in columns, sorted vertically, and special characters are represented by backslash escape sequences.

See ls.


10.3 vdir: Verbosely list directory contents

vdir is equivalent to ls -l -b; that is, by default files are listed in long format and special characters are represented by backslash escape sequences.

See ls.


10.4 dircolors: Color setup for ls

dircolors outputs a sequence of shell commands to set up the terminal for color output from ls (and dir, etc.). Typical usage:

eval "$(dircolors [option]… [file])"

If file is specified, dircolors reads it to determine which colors to use for which file types and extensions. Otherwise, a precompiled database is used. For details on the format of these files, run ‘dircolors --print-database’.

To make dircolors read a ~/.dircolors file if it exists, you can put the following lines in your ~/.bashrc (or adapt them to your favorite shell):

d=.dircolors
test -r $d && eval "$(dircolors $d)"

The output is a shell command to set the LS_COLORS environment variable. You can specify the shell syntax to use on the command line, or dircolors will guess it from the value of the SHELL environment variable.

The program accepts the following options. Also see Common options.

-b
--sh
--bourne-shell

Output Bourne shell commands. This is the default if the SHELL environment variable is set and does not end with ‘csh’ or ‘tcsh’.

-c
--csh
--c-shell

Output C shell commands. This is the default if SHELL ends with csh or tcsh.

-p
--print-database

Print the (compiled-in) default color configuration database. This output is itself a valid configuration file, and is fairly descriptive of the possibilities.

--print-ls-colors

Print the LS_COLORS entries on separate lines, each colored as per the color they represent.

An exit status of zero indicates success, and a nonzero value indicates failure.


11 Basic operations

This chapter describes the commands for basic file manipulation: copying, moving (renaming), and deleting (removing).


11.1 cp: Copy files and directories

cp copies files (or, optionally, directories). The copy is completely independent of the original. You can either copy one file to another, or copy arbitrarily many files to a destination directory. Synopses:

cp [option]… [-T] source dest
cp [option]… sourcedirectory
cp [option]… -t directory source
  • If two file names are given, cp copies the first file to the second.
  • If the --target-directory (-t) option is given, or failing that if the last file is a directory and the --no-target-directory (-T) option is not given, cp copies each source file to the specified directory, using the sources’ names.

Generally, files are written just as they are read. For exceptions, see the --sparse option below.

By default, cp does not copy directories. However, the -R, -a, and -r options cause cp to copy recursively by descending into source directories and copying files to corresponding destination directories.

When copying from a symbolic link, cp normally follows the link only when not copying recursively or when --link (-l) is used. This default can be overridden with the --archive (-a), -d, --dereference (-L), --no-dereference (-P), and -H options. If more than one of these options is specified, the last one silently overrides the others.

When copying to a symbolic link, cp follows the link only when it refers to an existing regular file. However, when copying to a dangling symbolic link, cp refuses by default, and fails with a diagnostic, since the operation is inherently dangerous. This behavior is contrary to historical practice and to POSIX. Set POSIXLY_CORRECT to make cp attempt to create the target of a dangling destination symlink, in spite of the possible risk. Also, when an option like --backup or --link acts to rename or remove the destination before copying, cp renames or removes the symbolic link rather than the file it points to.

By default, cp copies the contents of special files only when not copying recursively. This default can be overridden with the --copy-contents option.

cp generally refuses to copy a file onto itself, with the following exception: if --force --backup is specified with source and dest identical, and referring to a regular file, cp will make a backup file, either regular or numbered, as specified in the usual ways (see Backup options). This is useful when you simply want to make a backup of an existing file before changing it.

The program accepts the following options. Also see Common options.

-a
--archive

Preserve as much as possible of the structure and attributes of the original files in the copy (but do not attempt to preserve internal directory structure; i.e., ‘ls -U’ may list the entries in a copied directory in a different order). Try to preserve SELinux security context and extended attributes (xattr), but ignore any failure to do that and print no corresponding diagnostic. Equivalent to -dR --preserve=all with the reduced diagnostics.

--attributes-only

Copy only the specified attributes of the source file to the destination. If the destination already exists, do not alter its contents. See the --preserve option for controlling which attributes to copy.

-b
--backup[=method]

See Backup options. Make a backup of each file that would otherwise be overwritten or removed. As a special case, cp makes a backup of source when the force and backup options are given and source and dest are the same name for an existing, regular file. One useful application of this combination of options is this tiny Bourne shell script:

#!/bin/sh
# Usage: backup FILE...
# Create a GNU-style backup of each listed FILE.
fail=0
for i; do
  cp --backup --force --preserve=all -- "$i" "$i" || fail=1
done
exit $fail
--copy-contents

If copying recursively, copy the contents of any special files (e.g., FIFOs and device files) as if they were regular files. This means trying to read the data in each source file and writing it to the destination. It is usually a mistake to use this option, as it normally has undesirable effects on special files like FIFOs and the ones typically found in the /dev directory. In most cases, cp -R --copy-contents will hang indefinitely trying to read from FIFOs and special files like /dev/console, and it will fill up your destination file system if you use it to copy /dev/zero. This option has no effect unless copying recursively, and it does not affect the copying of symbolic links.

-d

Copy symbolic links as symbolic links rather than copying the files that they point to, and preserve hard links between source files in the copies. Equivalent to --no-dereference --preserve=links.

-f
--force

When copying without this option and an existing destination file cannot be opened for writing, the copy fails. However, with --force, when a destination file cannot be opened, cp then tries to recreate the file by first removing it. Note --force alone will not remove dangling symlinks. When this option is combined with --link (-l) or --symbolic-link (-s), the destination link is replaced, and unless --backup (-b) is also given there is no brief moment when the destination does not exist. Also see the description of --remove-destination.

This option is independent of the --interactive or -i option: neither cancels the effect of the other.

This option is ignored when the --no-clobber or -n option is also used.

-H

If a command line argument specifies a symbolic link, then copy the file it points to rather than the symbolic link itself. However, copy (preserving its nature) any symbolic link that is encountered via recursive traversal.

-i
--interactive

When copying a file other than a directory, prompt whether to overwrite an existing destination file. The -i option overrides a previous -n option.

-l
--link

Make hard links instead of copies of non-directories.

-L
--dereference

Follow symbolic links when copying from them. With this option, cp cannot create a symbolic link. For example, a symlink (to regular file) in the source tree will be copied to a regular file in the destination tree.

-n
--no-clobber

Do not overwrite an existing file; silently do nothing instead. This option overrides a previous -i option. This option is mutually exclusive with -b or --backup option.

-P
--no-dereference

Copy symbolic links as symbolic links rather than copying the files that they point to. This option affects only symbolic links in the source; symbolic links in the destination are always followed if possible.

-p
--preserve[=attribute_list]

Preserve the specified attributes of the original files. If specified, the attribute_list must be a comma-separated list of one or more of the following strings:

mode

Preserve the file mode bits and access control lists.

ownership

Preserve the owner and group. On most modern systems, only users with appropriate privileges may change the owner of a file, and ordinary users may preserve the group ownership of a file only if they happen to be a member of the desired group.

timestamps

Preserve the times of last access and last modification, when possible. On older systems, it is not possible to preserve these attributes when the affected file is a symbolic link. However, many systems now provide the utimensat function, which makes it possible even for symbolic links.

links

Preserve in the destination files any links between corresponding source files. Note that with -L or -H, this option can convert symbolic links to hard links. For example,

$ mkdir c; : > a; ln -s a b; cp -aH a b c; ls -i1 c
74161745 a
74161745 b

Note the inputs: b is a symlink to regular file a, yet the files in destination directory, c/, are hard-linked. Since -a implies --no-dereference it would copy the symlink, but the later -H tells cp to dereference the command line arguments where it then sees two files with the same inode number. Then the --preserve=links option also implied by -a will preserve the perceived hard link.

Here is a similar example that exercises cp’s -L option:

$ mkdir b c; (cd b; : > a; ln -s a b); cp -aL b c; ls -i1 c/b
74163295 a
74163295 b
context

Preserve SELinux security context of the file, or fail with full diagnostics.

xattr

Preserve extended attributes of the file, or fail with full diagnostics. If cp is built without xattr support, ignore this option. If SELinux context, ACLs or Capabilities are implemented using xattrs, they are preserved implicitly by this option as well, i.e., even without specifying --preserve=mode or --preserve=context.

all

Preserve all file attributes. Equivalent to specifying all of the above, but with the difference that failure to preserve SELinux security context or extended attributes does not change cp’s exit status. In contrast to -a, all but ‘Operation not supported’ warnings are output.

Using --preserve with no attribute_list is equivalent to --preserve=mode,ownership,timestamps.

In the absence of this option, the permissions of existing destination files are unchanged. Each new file is created with the mode of the corresponding source file minus the set-user-ID, set-group-ID, and sticky bits as the create mode; the operating system then applies either the umask or a default ACL, possibly resulting in a more restrictive file mode. See File permissions.

--no-preserve=attribute_list

Do not preserve the specified attributes. The attribute_list has the same form as for --preserve.

--parents

Form the name of each destination file by appending to the target directory a slash and the specified name of the source file. The last argument given to cp must be the name of an existing directory. For example, the command:

cp --parents a/b/c existing_dir

copies the file a/b/c to existing_dir/a/b/c, creating any missing intermediate directories.

-R
-r
--recursive

Copy directories recursively. By default, do not follow symbolic links in the source unless used together with the --link (-l) option; see the --archive (-a), -d, --dereference (-L), --no-dereference (-P), and -H options. Special files are copied by creating a destination file of the same type as the source; see the --copy-contents option. It is not portable to use -r to copy symbolic links or special files. On some non-GNU systems, -r implies the equivalent of -L and --copy-contents for historical reasons. Also, it is not portable to use -R to copy symbolic links unless you also specify -P, as POSIX allows implementations that dereference symbolic links by default.

Perform a lightweight, copy-on-write (COW) copy, if supported by the file system. Once it has succeeded, beware that the source and destination files share the same data blocks as long as they remain unmodified. Thus, if an I/O error affects data blocks of one of the files, the other suffers the same fate.

The when value can be one of the following:

always

If the copy-on-write operation is not supported then report the failure for each file and exit with a failure status. Plain --reflink is equivalent to --reflink=when.

auto

If the copy-on-write operation is not supported then fall back to the standard copy behavior. This is the default if no --reflink option is given.

never

Disable copy-on-write operation and use the standard copy behavior.

This option is overridden by the --link, --symbolic-link and --attributes-only options, thus allowing it to be used to configure the default data copying behavior for cp.

--remove-destination

Remove each existing destination file before attempting to open it (contrast with -f above).

--sparse=when

A sparse file contains holes—a sequence of zero bytes that does not occupy any file system blocks; the ‘read’ system call reads these as zeros. This can both save considerable space and increase speed, since many binary files contain lots of consecutive zero bytes. By default, cp detects holes in input source files via a crude heuristic and makes the corresponding output file sparse as well. Only regular files may be sparse.

The when value can be one of the following:

auto

The default behavior: if the input file is sparse, attempt to make the output file sparse, too. However, if an output file exists but refers to a non-regular file, then do not attempt to make it sparse.

always

For each sufficiently long sequence of zero bytes in the input file, attempt to create a corresponding hole in the output file, even if the input file does not appear to be sparse. This is useful when the input file resides on a file system that does not support sparse files (for example, ‘efs’ file systems in SGI IRIX 5.3 and earlier), but the output file is on a type of file system that does support them. Holes may be created only in regular files, so if the destination file is of some other type, cp does not even try to make it sparse.

never

Never make the output file sparse. This is useful in creating a file for use with the mkswap command, since such a file must not have any holes.

For example, with the following alias, cp will use the minimum amount of space supported by the file system. (Older versions of cp can also benefit from --reflink=auto here.)

alias cp='cp --sparse=always'
--strip-trailing-slashes

Remove any trailing slashes from each source argument. See Trailing slashes.

-s
--symbolic-link

Make symbolic links instead of copies of non-directories. All source file names must be absolute (starting with ‘/’) unless the destination files are in the current directory. This option merely results in an error message on systems that do not support symbolic links.

-S suffix
--suffix=suffix

Append suffix to each backup file made with -b. See Backup options.

-t directory
--target-directory=directory

Specify the destination directory. See Target directory.

-T
--no-target-directory

Do not treat the last operand specially when it is a directory or a symbolic link to a directory. See Target directory.

-u
--update

Do not copy a non-directory that has an existing destination with the same or newer modification timestamp. If timestamps are being preserved, the comparison is to the source timestamp truncated to the resolutions of the destination file system and of the system calls used to update timestamps; this avoids duplicate work if several ‘cp -pu’ commands are executed with the same source and destination. This option is ignored if the -n or --no-clobber option is also specified. Also, if --preserve=links is also specified (like with ‘cp -au’ for example), that will take precedence; consequently, depending on the order that files are processed from the source, newer files in the destination may be replaced, to mirror hard links in the source.

-v
--verbose

Print the name of each file before copying it.

-x
--one-file-system

Skip subdirectories that are on different file systems from the one that the copy started on. However, mount point directories are copied.

-Z
--context[=context]

Without a specified context, adjust the SELinux security context according to the system default type for destination files, similarly to the restorecon command. The long form of this option with a specific context specified, will set the context for newly created files only. With a specified context, if both SELinux and SMACK are disabled, a warning is issued. This option is mutually exclusive with the --preserve=context option, and overrides the --preserve=all and -a options.

An exit status of zero indicates success, and a nonzero value indicates failure.


11.2 dd: Convert and copy a file

dd copies input to output with a changeable I/O block size, while optionally performing conversions on the data. Synopses:

dd [operand]…
dd option

The only options are --help and --version. See Common options.

By default, dd copies standard input to standard output. To copy, dd repeatedly does the following steps in order:

  1. Read an input block.
  2. If converting via ‘sync’, pad as needed to meet the input block size. Pad with spaces if converting via ‘block’ or ‘unblock’, NUL bytes otherwise.
  3. If ‘bs=’ is given and no conversion mentioned in steps (4) or (5) is given, output the data as a single block and skip all remaining steps.
  4. If the ‘swab’ conversion is given, swap each pair of input bytes. If the input data length is odd, preserve the last input byte (since there is nothing to swap it with).
  5. If any of the conversions ‘swab’, ‘block’, ‘unblock’, ‘lcase’, ‘ucase’, ‘ascii’, ‘ebcdic’ and ‘ibm’ are given, do these conversions. These conversions operate independently of input blocking, and might deal with records that span block boundaries.
  6. Aggregate the resulting data into output blocks of the specified size, and output each output block in turn. Do not pad the last output block; it can be shorter than usual.

dd accepts the following operands, whose syntax was inspired by the DD (data definition) statement of OS/360 JCL.

if=file

Read from file instead of standard input.

of=file

Write to file instead of standard output. Unless ‘conv=notrunc’ is given, truncate file before writing it.

ibs=bytes

Set the input block size to bytes. This makes dd read bytes per block. The default is 512 bytes.

obs=bytes

Set the output block size to bytes. This makes dd write bytes per block. The default is 512 bytes.

bs=bytes

Set both input and output block sizes to bytes. This makes dd read and write bytes per block, overriding any ‘ibs’ and ‘obs’ settings. In addition, if no data-transforming conv operand is specified, input is copied to the output as soon as it’s read, even if it is smaller than the block size.

cbs=bytes

Set the conversion block size to bytes. When converting variable-length records to fixed-length ones (conv=block) or the reverse (conv=unblock), use bytes as the fixed record length.

skip=n
iseek=n

Skip nibs’-byte blocks in the input file before copying. If n ends in the letter ‘B’, interpret n as a byte count rather than a block count. (‘B’ and the ‘iseek=’ spelling are GNU extensions to POSIX.)

seek=n
oseek=n

Skip nobs’-byte blocks in the output file before truncating or copying. If n ends in the letter ‘B’, interpret n as a byte count rather than a block count. (‘B’ and the ‘oseek=’ spelling are GNU extensions to POSIX.)

count=n

Copy nibs’-byte blocks from the input file, instead of everything until the end of the file. If n ends in the letter ‘B’, interpret n as a byte count rather than a block count; this is a GNU extension to POSIX. If short reads occur, as could be the case when reading from a pipe for example, ‘iflag=fullblock’ ensures that ‘count=’ counts complete input blocks rather than input read operations. As an extension to POSIX, ‘count=0’ copies zero blocks instead of copying all blocks.

status=level

Specify the amount of information printed. If this operand is given multiple times, the last one takes precedence. The level value can be one of the following:

none

Do not print any informational or warning messages to standard error. Error messages are output as normal.

noxfer

Do not print the final transfer rate and volume statistics that normally make up the last status line.

progress

Print the transfer rate and volume statistics on standard error, when processing each input block. Statistics are output on a single line at most once every second, but updates can be delayed when waiting on I/O.

Transfer information is normally output to standard error upon receipt of the ‘INFO’ signal or when dd exits, and defaults to the following form in the C locale:

7287+1 records in
116608+0 records out
59703296 bytes (60 MB, 57 MiB) copied, 0.0427974 s, 1.4 GB/s

The notation ‘w+p’ stands for w whole blocks and p partial blocks. A partial block occurs when a read or write operation succeeds but transfers less data than the block size. An additional line like ‘1 truncated record’ or ‘10 truncated records’ is output after the ‘records out’ line if ‘conv=block’ processing truncated one or more input records.

The ‘status=’ operand is a GNU extension to POSIX.

conv=conversion[,conversion]…

Convert the file as specified by the conversion argument(s). (No spaces around any comma(s).)

Conversions:

ascii

Convert EBCDIC to ASCII, using the conversion table specified by POSIX. This provides a 1:1 translation for all 256 bytes. This implies ‘conv=unblock’; input is converted to ASCII before trailing spaces are deleted.

ebcdic

Convert ASCII to EBCDIC. This is the inverse of the ‘ascii’ conversion. This implies ‘conv=block’; trailing spaces are added before being converted to EBCDIC.

ibm

This acts like ‘conv=ebcdic’, except it uses the alternate conversion table specified by POSIX. This is not a 1:1 translation, but reflects common historical practice for ‘~’, ‘[’, and ‘]’.

The ‘ascii’, ‘ebcdic’, and ‘ibm’ conversions are mutually exclusive. If you use any of these conversions, you should also use the ‘cbs=’ operand.

block

For each line in the input, output ‘cbs’ bytes, replacing the input newline with a space and truncating or padding input lines with spaces as necessary.

unblock

Remove any trailing spaces in each ‘cbs’-sized input block, and append a newline.

The ‘block’ and ‘unblock’ conversions are mutually exclusive. If you use either of these conversions, you should also use the ‘cbs=’ operand.

lcase

Change uppercase letters to lowercase.

ucase

Change lowercase letters to uppercase.

The ‘lcase’ and ‘ucase’ conversions are mutually exclusive.

sparse

Try to seek rather than write NUL output blocks. On a file system that supports sparse files, this will create sparse output when extending the output file. Be careful when using this conversion in conjunction with ‘conv=notrunc’ or ‘oflag=append’. With ‘conv=notrunc’, existing data in the output file corresponding to NUL blocks from the input, will be untouched. With ‘oflag=append’ the seeks performed will be ineffective. Similarly, when the output is a device rather than a file, NUL input blocks are not copied, and therefore this conversion is most useful with virtual or pre zeroed devices.

The ‘sparse’ conversion is a GNU extension to POSIX.

swab

Swap every pair of input bytes.

sync

Pad every input block to size of ‘ibs’ with trailing zero bytes. When used with ‘block’ or ‘unblock’, pad with spaces instead of zero bytes.

The following “conversions” are really file flags and don’t affect internal processing:

excl

Fail if the output file already exists; dd must create the output file itself.

nocreat

Do not create the output file; the output file must already exist.

The ‘excl’ and ‘nocreat’ conversions are mutually exclusive, and are GNU extensions to POSIX.

notrunc

Do not truncate the output file.

noerror

Continue after read errors.

fdatasync

Synchronize output data just before finishing, even if there were write errors. This forces a physical write of output data. This conversion is a GNU extension to POSIX.

fsync

Synchronize output data and metadata just before finishing, even if there were write errors. This forces a physical write of output data and metadata. This conversion is a GNU extension to POSIX.

iflag=flag[,flag]…

Access the input file using the flags specified by the flag argument(s). (No spaces around any comma(s).)

oflag=flag[,flag]…

Access the output file using the flags specified by the flag argument(s). (No spaces around any comma(s).)

Here are the flags.

append

Write in append mode, so that even if some other process is writing to this file, every dd write will append to the current contents of the file. This flag makes sense only for output. If you combine this flag with the ‘of=file’ operand, you should also specify ‘conv=notrunc’ unless you want the output file to be truncated before being appended to.

cio

Use concurrent I/O mode for data. This mode performs direct I/O and drops the POSIX requirement to serialize all I/O to the same file. A file cannot be opened in CIO mode and with a standard open at the same time.

direct

Use direct I/O for data, avoiding the buffer cache. Note that the kernel may impose restrictions on read or write buffer sizes. For example, with an ext4 destination file system and a Linux-based kernel, using ‘oflag=direct’ will cause writes to fail with EINVAL if the output buffer size is not a multiple of 512.

directory

Fail unless the file is a directory. Most operating systems do not allow I/O to a directory, so this flag has limited utility.

dsync

Use synchronized I/O for data. For the output file, this forces a physical write of output data on each write. For the input file, this flag can matter when reading from a remote file that has been written to synchronously by some other process. Metadata (e.g., last-access and last-modified time) is not necessarily synchronized.

sync

Use synchronized I/O for both data and metadata.

nocache

Request to discard the system data cache for a file. When count=0 all cached data for the file is specified, otherwise the cache is dropped for the processed portion of the file. Also when count=0, failure to discard the cache is diagnosed and reflected in the exit status.

Note data that is not already persisted to storage will not be discarded from cache, so note the use of the ‘sync’ conversions in the examples below, which are used to maximize the effectiveness of the ‘nocache’ flag.

Here are some usage examples:

# Advise to drop cache for whole file
dd if=ifile iflag=nocache count=0

# Ensure drop cache for the whole file
dd of=ofile oflag=nocache conv=notrunc,fdatasync count=0

# Advise to drop cache for part of file
# Note the kernel will only consider complete and
# already persisted pages.
dd if=ifile iflag=nocache skip=10 count=10 of=/dev/null

# Stream data using just the read-ahead cache.
# See also the ‘direct’ flag.
dd if=ifile of=ofile iflag=nocache oflag=nocache,sync
nonblock

Use non-blocking I/O.

noatime

Do not update the file’s access timestamp. See File timestamps. Some older file systems silently ignore this flag, so it is a good idea to test it on your files before relying on it.

noctty

Do not assign the file to be a controlling terminal for dd. This has no effect when the file is not a terminal. On many hosts (e.g., GNU/Linux hosts), this flag has no effect at all.

nofollow

Do not follow symbolic links.

Fail if the file has multiple hard links.

binary

Use binary I/O. This flag has an effect only on nonstandard platforms that distinguish binary from text I/O.

text

Use text I/O. Like ‘binary’, this flag has no effect on standard platforms.

fullblock

Accumulate full blocks from input. The read system call may return early if a full block is not available. When that happens, continue calling read to fill the remainder of the block. This flag can be used only with iflag. This flag is useful with pipes for example as they may return short reads. In that case, this flag is needed to ensure that a ‘count=’ argument is interpreted as a block count rather than a count of read operations.

These flags are all GNU extensions to POSIX. They are not supported on all systems, and ‘dd’ rejects attempts to use them when they are not supported. When reading from standard input or writing to standard output, the ‘nofollow’ and ‘noctty’ flags should not be specified, and the other flags (e.g., ‘nonblock’) can affect how other processes behave with the affected file descriptors, even after dd exits.

The behavior of dd is unspecified if operands other than ‘conv=’, ‘iflag=’, ‘oflag=’, and ‘status=’ are specified more than once.

The numeric-valued strings above (n and bytes) are unsigned decimal integers that can be followed by a multiplier: ‘b’=512, ‘c’=1, ‘w’=2, ‘xm’=m, or any of the standard block size suffixes like ‘k’=1024 (see Block size). These multipliers are GNU extensions to POSIX, except that POSIX allows bytes to be followed by ‘k’, ‘b’, and ‘xm’. Block sizes (i.e., specified by bytes strings) must be nonzero.

Any block size you specify via ‘bs=’, ‘ibs=’, ‘obs=’, ‘cbs=’ should not be too large—values larger than a few megabytes are generally wasteful or (as in the gigabyte..exabyte case) downright counterproductive or error-inducing.

To process data with offset or size that is not a multiple of the I/O block size, you can use a numeric string n that ends in the letter ‘B’. For example, the following shell commands copy data in 1 MiB blocks between a flash drive and a tape, but do not save or restore a 512-byte area at the start of the flash drive:

flash=/dev/sda
tape=/dev/st0

# Copy all but the initial 512 bytes from flash to tape.
dd if=$flash iseek=512B bs=1MiB of=$tape

# Copy from tape back to flash, leaving initial 512 bytes alone.
dd if=$tape bs=1MiB of=$flash oseek=512B

For failing storage devices, other tools come with a great variety of extra functionality to ease the saving of as much data as possible before the device finally dies, e.g. GNU ddrescue. However, in some cases such a tool is not available or the administrator feels more comfortable with the handling of dd. As a simple rescue method, call dd as shown in the following example: the operand ‘conv=noerror,sync’ is used to continue after read errors and to pad out bad reads with NULs, while ‘iflag=fullblock’ caters for short reads (which traditionally never occur on flash or similar devices):

# Rescue data from an (unmounted!) partition of a failing device.
dd conv=noerror,sync iflag=fullblock </dev/sda1 > /mnt/rescue.img

Sending an ‘INFO’ signal (or ‘USR1’ signal where that is unavailable) to a running dd process makes it print I/O statistics to standard error and then resume copying. In the example below, dd is run in the background to copy 5GB of data. The kill command makes it output intermediate I/O statistics, and when dd completes normally or is killed by the SIGINT signal, it outputs the final statistics.

# Ignore the signal so we never inadvertently terminate the dd child.
# Note this is not needed when SIGINFO is available.
trap '' USR1

# Run dd with the fullblock iflag to avoid short reads
# which can be triggered by reception of signals.
dd iflag=fullblock if=/dev/zero of=/dev/null count=5000000 bs=1000 & pid=$!

# Output stats every second.
while kill -s USR1 $pid 2>/dev/null; do sleep 1; done

The above script will output in the following format:

3441325+0 records in
3441325+0 records out
3441325000 bytes (3.4 GB, 3.2 GiB) copied, 1.00036 s, 3.4 GB/s
5000000+0 records in
5000000+0 records out
5000000000 bytes (5.0 GB, 4.7 GiB) copied, 1.44433 s, 3.5 GB/s

The ‘status=progress’ operand periodically updates the last line of the transfer statistics above.

On systems lacking the ‘INFO’ signal dd responds to the ‘USR1’ signal instead, unless the POSIXLY_CORRECT environment variable is set.

An exit status of zero indicates success, and a nonzero value indicates failure.


11.3 install: Copy files and set attributes

install copies files while setting their file mode bits and, if possible, their owner and group. Synopses:

install [option]… [-T] source dest
install [option]… sourcedirectory
install [option]… -t directory source…
install [option]… -d directory
  • If two file names are given, install copies the first file to the second.
  • If the --target-directory (-t) option is given, or failing that if the last file is a directory and the --no-target-directory (-T) option is not given, install copies each source file to the specified directory, using the sources’ names.
  • If the --directory (-d) option is given, install creates each directory and any missing parent directories. Parent directories are created with mode ‘u=rwx,go=rx’ (755), regardless of the -m option or the current umask. See Directories and the Set-User-ID and Set-Group-ID Bits, for how the set-user-ID and set-group-ID bits of parent directories are inherited.

install is similar to cp, but allows you to control the attributes of destination files. It is typically used in Makefiles to copy programs into their destination directories. It refuses to copy files onto themselves.

install never preserves extended attributes (xattr).

The program accepts the following options. Also see Common options.

-b
--backup[=method]

See Backup options. Make a backup of each file that would otherwise be overwritten or removed.

-C
--compare

Compare content of source and destination files, and if there would be no change to the destination content, owner, group, permissions, and possibly SELinux context, then do not modify the destination at all. Note this option is best used in conjunction with --user, --group and --mode options, lest install incorrectly determines the default attributes that installed files would have (as it doesn’t consider setgid directories and POSIX default ACLs for example). This could result in redundant copies or attributes that are not reset to the correct defaults.

-c

Ignored; for compatibility with old Unix versions of install.

-D

Create any missing parent directories of dest, then copy source to dest. Explicitly specifying the --target-directory=dir will similarly ensure the presence of that hierarchy before copying source arguments.

-d
--directory

Create any missing parent directories, giving them the default attributes. Then create each given directory, setting their owner, group and mode as given on the command line or to the defaults.

-g group
--group=group

Set the group ownership of installed files or directories to group. The default is the process’s current group. group may be either a group name or a numeric group ID.

-m mode
--mode=mode

Set the file mode bits for the installed file or directory to mode, which can be either an octal number, or a symbolic mode as in chmod, with ‘a=’ (no access allowed to anyone) as the point of departure (see File permissions). The default mode is ‘u=rwx,go=rx,a-s’—read, write, and execute for the owner, read and execute for group and other, and with set-user-ID and set-group-ID disabled. This default is not quite the same as ‘755’, since it disables instead of preserving set-user-ID and set-group-ID on directories. See Directories and the Set-User-ID and Set-Group-ID Bits.

-o owner
--owner=owner

If install has appropriate privileges (is run as root), set the ownership of installed files or directories to owner. The default is root. owner may be either a user name or a numeric user ID.

--preserve-context

Preserve the SELinux security context of files and directories. Failure to preserve the context in all of the files or directories will result in an exit status of 1. If SELinux is disabled then print a warning and ignore the option.

-p
--preserve-timestamps

Set the time of last access and the time of last modification of each installed file to match those of each corresponding original file. When a file is installed without this option, its last access and last modification timestamps are both set to the time of installation. This option is useful if you want to use the last modification timestamps of installed files to keep track of when they were last built as opposed to when they were last installed.

-s
--strip

Strip the symbol tables from installed binary executables.

--strip-program=program

Program used to strip binaries.

-S suffix
--suffix=suffix

Append suffix to each backup file made with -b. See Backup options.

-t directory
--target-directory=directory

Specify the destination directory. See Target directory. Also specifying the -D option will ensure the directory is present.

-T
--no-target-directory

Do not treat the last operand specially when it is a directory or a symbolic link to a directory. See Target directory.

-v
--verbose

Print the name of each file before copying it.

-Z
--context[=context]

Without a specified context, adjust the SELinux security context according to the system default type for destination files, similarly to the restorecon command. The long form of this option with a specific context specified, will set the context for newly created files only. With a specified context, if both SELinux and SMACK are disabled, a warning is issued. This option is mutually exclusive with the --preserve-context option.

An exit status of zero indicates success, and a nonzero value indicates failure.


11.4 mv: Move (rename) files

mv moves or renames files (or directories). Synopses:

mv [option]… [-T] source dest
mv [option]… sourcedirectory
mv [option]… -t directory source
  • If two file names are given, mv moves the first file to the second.
  • If the --target-directory (-t) option is given, or failing that if the last file is a directory and the --no-target-directory (-T) option is not given, mv moves each source file to the specified directory, using the sources’ names.

mv can move any type of file from one file system to another. Prior to version 4.0 of the fileutils, mv could move only regular files between file systems. For example, now mv can move an entire directory hierarchy including special device files from one partition to another. It first uses some of the same code that’s used by cp -a to copy the requested directories and files, then (assuming the copy succeeded) it removes the originals. If the copy fails, then the part that was copied to the destination partition is removed. If you were to copy three directories from one partition to another and the copy of the first directory succeeded, but the second didn’t, the first would be left on the destination partition and the second and third would be left on the original partition.

mv always tries to copy extended attributes (xattr), which may include SELinux context, ACLs or Capabilities. Upon failure all but ‘Operation not supported’ warnings are output.

If a destination file exists but is normally unwritable, standard input is a terminal, and the -f or --force option is not given, mv prompts the user for whether to replace the file. (You might own the file, or have write permission on its directory.) If the response is not affirmative, the file is skipped.

Warning: Avoid specifying a source name with a trailing slash, when it might be a symlink to a directory. Otherwise, mv may do something very surprising, since its behavior depends on the underlying rename system call. On a system with a modern Linux-based kernel, it fails with errno=ENOTDIR. However, on other systems (at least FreeBSD 6.1 and Solaris 10) it silently renames not the symlink but rather the directory referenced by the symlink. See Trailing slashes.

Note: mv will only replace empty directories in the destination. Conflicting populated directories are skipped with a diagnostic.

The program accepts the following options. Also see Common options.

-b
--backup[=method]

See Backup options. Make a backup of each file that would otherwise be overwritten or removed.

-f
--force

Do not prompt the user before removing a destination file. If you specify more than one of the -i, -f, -n options, only the final one takes effect.

-i
--interactive

Prompt whether to overwrite each existing destination file, regardless of its permissions. If the response is not affirmative, the file is skipped. If you specify more than one of the -i, -f, -n options, only the final one takes effect.

-n
--no-clobber

Do not overwrite an existing file; silently do nothing instead. If you specify more than one of the -i, -f, -n options, only the final one takes effect. This option is mutually exclusive with -b or --backup option.

-u
--update

Do not move a non-directory that has an existing destination with the same or newer modification timestamp. If the move is across file system boundaries, the comparison is to the source timestamp truncated to the resolutions of the destination file system and of the system calls used to update timestamps; this avoids duplicate work if several ‘mv -u’ commands are executed with the same source and destination. This option is ignored if the -n or --no-clobber option is also specified.

-v
--verbose

Print the name of each file before moving it.

--strip-trailing-slashes

Remove any trailing slashes from each source argument. See Trailing slashes.

-S suffix
--suffix=suffix

Append suffix to each backup file made with -b. See Backup options.

-t directory
--target-directory=directory

Specify the destination directory. See Target directory.

-T
--no-target-directory

Do not treat the last operand specially when it is a directory or a symbolic link to a directory. See Target directory.

-Z
--context

This option functions similarly to the restorecon command, by adjusting the SELinux security context according to the system default type for destination files and each created directory.

An exit status of zero indicates success, and a nonzero value indicates failure.


11.5 rm: Remove files or directories

rm removes each given file. By default, it does not remove directories. Synopsis:

rm [option]… [file]…

If the -I or --interactive=once option is given, and there are more than three files or the -r, -R, or --recursive are given, then rm prompts the user for whether to proceed with the entire operation. If the response is not affirmative, the entire command is aborted.

Otherwise, if a file is unwritable, standard input is a terminal, and the -f or --force option is not given, or the -i or --interactive=always option is given, rm prompts the user for whether to remove the file. If the response is not affirmative, the file is skipped.

Any attempt to remove a file whose last file name component is . or .. is rejected without any prompting, as mandated by POSIX.

Warning: If you use rm to remove a file, it is usually possible to recover the contents of that file. If you want more assurance that the contents are unrecoverable, consider using shred.

The program accepts the following options. Also see Common options.

-d
--dir

Remove the listed directories if they are empty.

-f
--force

Ignore nonexistent files and missing operands, and never prompt the user. Ignore any previous --interactive (-i) option.

-i

Prompt whether to remove each file. If the response is not affirmative, the file is skipped. Ignore any previous --force (-f) option. Equivalent to --interactive=always.

-I

Prompt once whether to proceed with the command, if more than three files are named or if a recursive removal is requested. Ignore any previous --force (-f) option. Equivalent to --interactive=once.

--interactive [=when]

Specify when to issue an interactive prompt. when may be omitted, or one of:

  • never - Do not prompt at all.
  • once - Prompt once if more than three files are named or if a recursive removal is requested. Equivalent to -I.
  • always - Prompt for every file being removed. Equivalent to -i.

--interactive with no when is equivalent to --interactive=always.

--one-file-system

When removing a hierarchy recursively, skip any directory that is on a file system different from that of the corresponding command line argument. This option is useful when removing a build “chroot” hierarchy, which normally contains no valuable data. However, it is not uncommon to bind-mount /home into such a hierarchy, to make it easier to use one’s start-up file. The catch is that it’s easy to forget to unmount /home. Then, when you use rm -rf to remove your normally throw-away chroot, that command will remove everything under /home, too. Use the --one-file-system option, and it will warn about and skip directories on other file systems. Of course, this will not save your /home if it and your chroot happen to be on the same file system. See also --preserve-root=all to protect command line arguments themselves.

--preserve-root [=all]

Fail upon any attempt to remove the root directory, /, when used with the --recursive option. This is the default behavior. See Treating / specially. When ‘all’ is specified, reject any command line argument that is not on the same file system as its parent.

--no-preserve-root

Do not treat / specially when removing recursively. This option is not recommended unless you really want to remove all the files on your computer. See Treating / specially.

-r
-R
--recursive

Remove the listed directories and their contents recursively.

-v
--verbose

Print the name of each file before removing it.

One common question is how to remove files whose names begin with a ‘-’. GNU rm, like every program that uses the getopt function to parse its arguments, lets you use the ‘--’ option to indicate that all following arguments are non-options. To remove a file called -f in the current directory, you could type either:

rm -- -f

or:

rm ./-f

The Unix rm program’s use of a single ‘-’ for this purpose predates the development of the getopt standard syntax.

An exit status of zero indicates success, and a nonzero value indicates failure.


11.6 shred: Remove files more securely

shred overwrites devices or files, to help prevent even extensive forensics from recovering the data.

Ordinarily when you remove a file (see rm: Remove files or directories), its data and metadata are not actually destroyed. Only the file’s directory entry is removed, and the file’s storage is reclaimed only when no process has the file open and no other directory entry links to the file. And even if file’s data and metadata’s storage space is freed for further reuse, there are undelete utilities that will attempt to reconstruct the file from the data in freed storage, and that can bring the file back if the storage was not rewritten.

On a busy system with a nearly-full device, space can get reused in a few seconds. But there is no way to know for sure. And although the undelete utilities and already-existing processes require insider or superuser access, you may be wary of the superuser, of processes running on your behalf, or of attackers that can physically access the storage device. So if you have sensitive data, you may want to be sure that recovery is not possible by plausible attacks like these.

The best way to remove something irretrievably is to destroy the media it’s on with acid, melt it down, or the like. For cheap removable media this is often the preferred method. However, some storage devices are expensive or are harder to destroy, so the shred utility tries to achieve a similar effect non-destructively, by overwriting the file with non-sensitive data.

Please note that shred relies on a crucial assumption: that the file system and hardware overwrite data in place. Although this is common and is the traditional way to do things, but many modern file system designs do not satisfy this assumption. Exceptions include:

  • Log-structured or journaled file systems, such as ext3/ext4 (in data=journal mode), Btrfs, NTFS, ReiserFS, XFS, ZFS, file systems supplied with AIX and Solaris, etc., when they are configured to journal data.
  • File systems that write redundant data and carry on even if some writes fail, such as RAID-based file systems.
  • File systems that make snapshots, such as Network Appliance’s NFS server.
  • File systems that cache in temporary locations, such as NFS version 3 clients.
  • Compressed file systems.

For ext3 and ext4 file systems, shred is less effective when the file system is in data=journal mode, which journals file data in addition to just metadata. In both the data=ordered (default) and data=writeback modes, shred works as usual. The ext3/ext4 journaling modes can be changed by adding the data=something option to the mount options for a particular file system in the /etc/fstab file, as documented in the mount man page (‘man mount’). Alternatively, if you know how large the journal is, you can shred the journal by shredding enough file data so that the journal cycles around and fills up with shredded data.

If you are not sure how your file system operates, then you should assume that it does not overwrite data in place, which means shred cannot reliably operate on regular files in your file system.

Generally speaking, it is more reliable to shred a device than a file, since this bypasses file system design issues mentioned above. However, devices are also problematic for shredding, for reasons such as the following:

  • Solid-state storage devices (SSDs) typically do wear leveling to prolong service life, and this means writes are distributed to other blocks by the hardware, so “overwritten” data blocks are still present in the underlying device.
  • Most storage devices map out bad blocks invisibly to the application; if the bad blocks contain sensitive data, shred won’t be able to destroy it.
  • With some obsolete storage technologies, it may be possible to take (say) a floppy disk back to a laboratory and use a lot of sensitive (and expensive) equipment to look for the faint “echoes” of the original data underneath the overwritten data. With these older technologies, if the file has been overwritten only once, it’s reputedly not even that hard. Luckily, this kind of data recovery has become difficult, and there is no public evidence that today’s higher-density storage devices can be analyzed in this way.

    The shred command can use many overwrite passes, with data patterns chosen to maximize the damage they do to the old data. By default the patterns are designed for best effect on hard drives using now-obsolete technology; for newer devices, a single pass should suffice. For more details, see the source code and Peter Gutmann’s paper Secure Deletion of Data from Magnetic and Solid-State Memory, from the proceedings of the Sixth USENIX Security Symposium (San Jose, California, July 22–25, 1996).

shred makes no attempt to detect or report these problems, just as it makes no attempt to do anything about backups. However, since it is more reliable to shred devices than files, shred by default does not deallocate or remove the output file. This default is more suitable for devices, which typically cannot be deallocated and should not be removed.

Finally, consider the risk of backups and mirrors. File system backups and remote mirrors may contain copies of the file that cannot be removed, and that will allow a shredded file to be recovered later. So if you keep any data you may later want to destroy using shred, be sure that it is not backed up or mirrored.

shred [option]… file[…]

The program accepts the following options. Also see Common options.

-f
--force

Override file permissions if necessary to allow overwriting.

-n number
--iterations=number

By default, shred uses 3 passes of overwrite. You can reduce this to save time, or increase it if you think it’s appropriate. After 25 passes all of the internal overwrite patterns will have been used at least once.

--random-source=file

Use file as a source of random data used to overwrite and to choose pass ordering. See Sources of random data.

-s bytes
--size=bytes

Shred the first bytes bytes of the file. The default is to shred the whole file. bytes can be followed by a size specification like ‘K’, ‘M’, or ‘G’ to specify a multiple. See Block size.

-u
--remove[=how]

After shredding a file, deallocate it (if possible) and then remove it. If a file has multiple links, only the named links will be removed. Often the file name is less sensitive than the file data, in which case the optional how parameter, supported with the long form option, gives control of how to more efficiently remove each directory entry. The ‘unlink’ parameter will just use a standard unlink call, ‘wipe’ will also first obfuscate bytes in the name, and ‘wipesync’ will also sync each obfuscated byte in the name to the file system. Note ‘wipesync’ is the default method, but can be expensive, requiring a sync for every character in every file. This can become significant with many files, or is redundant if your file system provides synchronous metadata updates.

-v
--verbose

Display to standard error all status updates as sterilization proceeds.

-x
--exact

By default, shred rounds the size of a regular file up to the next multiple of the file system block size to fully erase the slack space in the last block of the file. This space may contain portions of the current system memory on some systems for example. Use --exact to suppress that behavior. Thus, by default if you shred a 10-byte regular file on a system with 512-byte blocks, the resulting file will be 512 bytes long. With this option, shred does not increase the apparent size of the file.

-z
--zero

Normally, the last pass that shred writes is made up of random data. If this would be conspicuous on your storage device (for example, because it looks like encrypted data), or you just think it’s tidier, the --zero option adds an additional overwrite pass with all zero bits. This is in addition to the number of passes specified by the --iterations option.

You might use the following command to erase the file system you created on a USB flash drive. This command typically takes several minutes, depending on the drive’s size and write speed. On modern storage devices a single pass should be adequate, and will take one third the time of the default three-pass approach.

shred -v -n 1 /dev/sdd1

Similarly, to erase all data on a selected partition of your device, you could give a command like the following.

# 1 pass, write pseudo-random data; 3x faster than the default
shred -v -n1 /dev/sda5

To be on the safe side, use at least one pass that overwrites using pseudo-random data. I.e., don’t be tempted to use ‘-n0 --zero’, in case some device controller optimizes the process of writing blocks of all zeros, and thereby does not clear all bytes in a block. Some SSDs may do just that.

A file of ‘-’ denotes standard output. The intended use of this is to shred a removed temporary file. For example:

i=$(mktemp)
exec 3<>"$i"
rm -- "$i"
echo "Hello, world" >&3
shred - >&3
exec 3>-

However, the command ‘shred - >file’ does not shred the contents of file, since the shell truncates file before invoking shred. Use the command ‘shred file’ or (if using a Bourne-compatible shell) the command ‘shred - 1<>file’ instead.

An exit status of zero indicates success, and a nonzero value indicates failure.


12 Special file types

This chapter describes commands which create special types of files (and rmdir, which removes directories, one special file type).

Although Unix-like operating systems have markedly fewer special file types than others, not everything can be treated only as the undifferentiated byte stream of normal files. For example, when a file is created or removed, the system must record this information, which it does in a directory—a special type of file. Although you can read directories as normal files, if you’re curious, in order for the system to do its job it must impose a structure, a certain order, on the bytes of the file. Thus it is a “special” type of file.

Besides directories, other special file types include named pipes (FIFOs), symbolic links, sockets, and so-called special files.


ln makes links between files. By default, it makes hard links; with the -s option, it makes symbolic (or soft) links. Synopses:

ln [option]… [-T] target linkname
ln [option]… target
ln [option]… targetdirectory
ln [option]… -t directory target
  • If two file names are given, ln creates a link to the first file from the second.
  • If one target is given, ln creates a link to that file in the current directory.
  • If the --target-directory (-t) option is given, or failing that if the last file is a directory and the --no-target-directory (-T) option is not given, ln creates a link to each target file in the specified directory, using the targets’ names.

Normally ln does not replace existing files. Use the --force (-f) option to replace them unconditionally, the --interactive (-i) option to replace them conditionally, and the --backup (-b) option to rename them. Unless the --backup (-b) option is used there is no brief moment when the destination does not exist; this is an extension to POSIX.

A hard link is another name for an existing file; the link and the original are indistinguishable. Technically speaking, they share the same inode, and the inode contains all the information about a file—indeed, it is not incorrect to say that the inode is the file. Most systems prohibit making a hard link to a directory; on those where it is allowed, only the super-user can do so (and with caution, since creating a cycle will cause problems to many other utilities). Hard links cannot cross file system boundaries. (These restrictions are not mandated by POSIX, however.)

Symbolic links (symlinks for short), on the other hand, are a special file type (which not all kernels support: System V release 3 (and older) systems lack symlinks) in which the link file actually refers to a different file, by name. When most operations (opening, reading, writing, and so on) are passed the symbolic link file, the kernel automatically dereferences the link and operates on the target of the link. But some operations (e.g., removing) work on the link file itself, rather than on its target. The owner and group of a symlink are not significant to file access performed through the link, but do have implications on deleting a symbolic link from a directory with the restricted deletion bit set. On the GNU system, the mode of a symlink has no significance and cannot be changed, but on some BSD systems, the mode can be changed and will affect whether the symlink will be traversed in file name resolution. See Symbolic Links in The GNU C Library Reference Manual.

Symbolic links can contain arbitrary strings; a dangling symlink occurs when the string in the symlink does not resolve to a file. There are no restrictions against creating dangling symbolic links. There are trade-offs to using absolute or relative symlinks. An absolute symlink always points to the same file, even if the directory containing the link is moved. However, if the symlink is visible from more than one machine (such as on a networked file system), the file pointed to might not always be the same. A relative symbolic link is resolved in relation to the directory that contains the link, and is often useful in referring to files on the same device without regards to what name that device is mounted on when accessed via networked machines.

When creating a relative symlink in a different location than the current directory, the resolution of the symlink will be different than the resolution of the same string from the current directory. Therefore, many users prefer to first change directories to the location where the relative symlink will be created, so that tab-completion or other file resolution will find the same target as what will be placed in the symlink.

The program accepts the following options. Also see Common options.

-b
--backup[=method]

See Backup options. Make a backup of each file that would otherwise be overwritten or removed.

-d
-F
--directory

Allow users with appropriate privileges to attempt to make hard links to directories. However, note that this will probably fail due to system restrictions, even for the super-user.

-f
--force

Remove existing destination files.

-i
--interactive

Prompt whether to remove existing destination files.

-L
--logical

If -s is not in effect, and the source file is a symbolic link, create the hard link to the file referred to by the symbolic link, rather than the symbolic link itself.

-n
--no-dereference

Do not treat the last operand specially when it is a symbolic link to a directory. Instead, treat it as if it were a normal file.

When the destination is an actual directory (not a symlink to one), there is no ambiguity. The link is created in that directory. But when the specified destination is a symlink to a directory, there are two ways to treat the user’s request. ln can treat the destination just as it would a normal directory and create the link in it. On the other hand, the destination can be viewed as a non-directory—as the symlink itself. In that case, ln must delete or backup that symlink before creating the new link. The default is to treat a destination that is a symlink to a directory just like a directory.

This option is weaker than the --no-target-directory (-T) option, so it has no effect if both options are given.

-P
--physical

If -s is not in effect, and the source file is a symbolic link, create the hard link to the symbolic link itself. On platforms where this is not supported by the kernel, this option creates a symbolic link with identical contents; since symbolic link contents cannot be edited, any file name resolution performed through either link will be the same as if a hard link had been created.

-r
--relative

Make symbolic links relative to the link location. This option is only valid with the --symbolic option.

Example:

ln -srv /a/file /tmp
'/tmp/file' -> '../a/file'

Relative symbolic links are generated based on their canonicalized containing directory, and canonicalized targets. I.e., all symbolic links in these file names will be resolved. See realpath: Print the resolved file name., which gives greater control over relative file name generation, as demonstrated in the following example:

ln--relative() {
  test "$1" = --no-symlinks && { nosym=$1; shift; }
  target="$1";
  test -d "$2" && link="$2/." || link="$2"
  rtarget="$(realpath $nosym -m "$target" \
              --relative-to "$(dirname "$link")")"
  ln -s -v "$rtarget" "$link"
}
-s
--symbolic

Make symbolic links instead of hard links. This option merely produces an error message on systems that do not support symbolic links.

-S suffix
--suffix=suffix

Append suffix to each backup file made with -b. See Backup options.

-t directory
--target-directory=directory

Specify the destination directory. See Target directory.

-T
--no-target-directory

Do not treat the last operand specially when it is a directory or a symbolic link to a directory. See Target directory.

-v
--verbose

Print the name of each file after linking it successfully.

If -L and -P are both given, the last one takes precedence. If -s is also given, -L and -P are silently ignored. If neither option is given, then this implementation defaults to -P if the system link supports hard links to symbolic links (such as the GNU system), and -L if link follows symbolic links (such as on BSD).

An exit status of zero indicates success, and a nonzero value indicates failure.

Examples:

Bad Example:

# Create link ../a pointing to a in that directory.
# Not really useful because it points to itself.
ln -s a ..

Better Example:

# Change to the target before creating symlinks to avoid being confused.
cd ..
ln -s adir/a .

Bad Example:

# Hard coded file names don't move well.
ln -s $(pwd)/a /some/dir/

Better Example:

# Relative file names survive directory moves and also
# work across networked file systems.
ln -s afile anotherfile
ln -s ../adir/afile yetanotherfile

12.3 mkdir: Make directories

mkdir creates directories with the specified names. Synopsis:

mkdir [option]… name

mkdir creates each directory name in the order given. It reports an error if name already exists, unless the -p option is given and name is a directory.

The program accepts the following options. Also see Common options.

-m mode
--mode=mode

Set the file permission bits of created directories to mode, which uses the same syntax as in chmod and uses ‘a=rwx’ (read, write and execute allowed for everyone) for the point of the departure. See File permissions. This option affects only directories given on the command line; it does not affect any parents that may be created via the -p option.

Normally the directory has the desired file mode bits at the moment it is created. As a GNU extension, mode may also mention special mode bits, but in this case there may be a temporary window during which the directory exists but its special mode bits are incorrect. See Directories and the Set-User-ID and Set-Group-ID Bits, for how the set-user-ID and set-group-ID bits of directories are inherited unless overridden in this way.

-p
--parents

Make any missing parent directories for each argument, setting their file permission bits to ‘=rwx,u+wx’, that is, with the umask modified by ‘u+wx’. Ignore existing parent directories, and do not change their file permission bits.

If the -m option is also given, it does not affect file permission bits of any newly-created parent directories. To control these bits, set the umask before invoking mkdir. For example, if the shell command ‘(umask u=rwx,go=rx; mkdir -p P/Q)’ creates the parent P it sets the parent’s file permission bits to ‘u=rwx,go=rx’. (The umask must include ‘u=wx’ for this method to work.) To set a parent’s special mode bits as well, you can invoke chmod after mkdir. See Directories and the Set-User-ID and Set-Group-ID Bits, for how the set-user-ID and set-group-ID bits of newly-created parent directories are inherited.

-v
--verbose

Print a message for each created directory. This is most useful with --parents.

-Z
--context[=context]

Without a specified context, adjust the SELinux security context according to the system default type for destination files, similarly to the restorecon command. The long form of this option with a specific context specified, will set the context for newly created files only. With a specified context, if both SELinux and SMACK are disabled, a warning is issued.

An exit status of zero indicates success, and a nonzero value indicates failure.


12.4 mkfifo: Make FIFOs (named pipes)

mkfifo creates FIFOs (also called named pipes) with the specified names. Synopsis:

mkfifo [option] name

A FIFO is a special file type that permits independent processes to communicate. One process opens the FIFO file for writing, and another for reading, after which data can flow as with the usual anonymous pipe in shells or elsewhere.

The program accepts the following options. Also see Common options.

-m mode
--mode=mode

Set the mode of created FIFOs to mode, which is symbolic as in chmod and uses ‘a=rw’ (read and write allowed for everyone) for the point of departure. mode should specify only file permission bits. See File permissions.

-Z
--context[=context]

Without a specified context, adjust the SELinux security context according to the system default type for destination files, similarly to the restorecon command. The long form of this option with a specific context specified, will set the context for newly created files only. With a specified context, if both SELinux and SMACK are disabled, a warning is issued.

An exit status of zero indicates success, and a nonzero value indicates failure.


12.5 mknod: Make block or character special files

mknod creates a FIFO, character special file, or block special file with the specified name. Synopsis:

mknod [option]… name type [major minor]

Unlike the phrase “special file type” above, the term special file has a technical meaning on Unix: something that can generate or receive data. Usually this corresponds to a physical piece of hardware, e.g., a printer or a flash drive. (These files are typically created at system-configuration time.) The mknod command is what creates files of this type. Such devices can be read either a character at a time or a “block” (many characters) at a time, hence we say there are block special files and character special files.

Due to shell aliases and built-in mknod functions, using an unadorned mknod interactively or in a script may get you different functionality than that described here. Invoke it via env (i.e., env mknod …) to avoid interference from the shell.

The arguments after name specify the type of file to make:

p

for a FIFO

b

for a block special file

c

for a character special file

When making a block or character special file, the major and minor device numbers must be given after the file type. If a major or minor device number begins with ‘0x’ or ‘0X’, it is interpreted as hexadecimal; otherwise, if it begins with ‘0’, as octal; otherwise, as decimal.

The program accepts the following options. Also see Common options.

-m mode
--mode=mode

Set the mode of created files to mode, which is symbolic as in chmod and uses ‘a=rw’ as the point of departure. mode should specify only file permission bits. See File permissions.

-Z
--context[=context]

Without a specified context, adjust the SELinux security context according to the system default type for destination files, similarly to the restorecon command. The long form of this option with a specific context specified, will set the context for newly created files only. With a specified context, if both SELinux and SMACK are disabled, a warning is issued.

An exit status of zero indicates success, and a nonzero value indicates failure.


12.7 rmdir: Remove empty directories

rmdir removes empty directories. Synopsis:

rmdir [option]… directory

If any directory argument does not refer to an existing empty directory, it is an error.

The program accepts the following options. Also see Common options.

--ignore-fail-on-non-empty

Ignore each failure to remove a directory that is solely because the directory is non-empty.

-p
--parents

Remove directory, then try to remove each component of directory. So, for example, ‘rmdir -p a/b/c’ is similar to ‘rmdir a/b/c a/b a’. As such, it fails if any of those directories turns out not to be empty. Use the --ignore-fail-on-non-empty option to make it so such a failure does not evoke a diagnostic and does not cause rmdir to exit unsuccessfully.

-v
--verbose

Give a diagnostic for each successful removal. directory is removed.

See rm: Remove files or directories, for how to remove non-empty directories recursively.

To remove all empty directories under dirname, including directories that become empty because other directories are removed, you can use either of the following commands:

# This uses GNU extensions.
find dirname -type d -empty -delete

# This runs on any POSIX platform.
find dirname -depth -type d -exec rmdir {} +

An exit status of zero indicates success, and a nonzero value indicates failure.


13 Changing file attributes

A file is not merely its contents, a name, and a file type (see Special file types). A file also has an owner (a user ID), a group (a group ID), permissions (what the owner can do with the file, what people in the group can do, and what everyone else can do), various timestamps, and other information. Collectively, we call these a file’s attributes.

These commands change file attributes.


13.1 chown: Change file owner and group

chown changes the user and/or group ownership of each given file to new-owner or to the user and group of an existing reference file. Synopsis:

chown [option]… {new-owner | --reference=ref_file} file

If used, new-owner specifies the new owner and/or group as follows (with no embedded white space):

[owner] [ : [group] ]

Specifically:

owner

If only an owner (a user name or numeric user ID) is given, that user is made the owner of each given file, and the files’ group is not changed.

owner‘:’group

If the owner is followed by a colon and a group (a group name or numeric group ID), with no spaces between them, the group ownership of the files is changed as well (to group).

owner‘:

If a colon but no group name follows owner, that user is made the owner of the files and the group of the files is changed to owner’s login group.

:’group

If the colon and following group are given, but the owner is omitted, only the group of the files is changed; in this case, chown performs the same function as chgrp.

:

If only a colon is given, or if new-owner is empty, neither the owner nor the group is changed.

If owner or group is intended to represent a numeric user or group ID, then you may specify it with a leading ‘+’. See chown, chgrp, chroot, id: Disambiguating user names and IDs.

Some older scripts may still use ‘.’ in place of the ‘:’ separator. POSIX 1003.1-2001 (see Standards conformance) does not require support for that, but for backward compatibility GNU chown supports ‘.’ so long as no ambiguity results, although it issues a warning and support may be removed in future versions. New scripts should avoid the use of ‘.’ because it is not portable, and because it has undesirable results if the entire owner‘.’group happens to identify a user whose name contains ‘.’.

It is system dependent whether a user can change the group to an arbitrary one, or the more portable behavior of being restricted to setting a group of which the user is a member.

The chown command sometimes clears the set-user-ID or set-group-ID permission bits. This behavior depends on the policy and functionality of the underlying chown system call, which may make system-dependent file mode modifications outside the control of the chown command. For example, the chown command might not affect those bits when invoked by a user with appropriate privileges, or when the bits signify some function other than executable permission (e.g., mandatory locking). When in doubt, check the underlying system behavior.

The program accepts the following options. Also see Common options.

-c
--changes

Verbosely describe the action for each file whose ownership actually changes.

-f
--silent
--quiet

Do not print error messages about files whose ownership cannot be changed.

--from=old-owner

Change a file’s ownership only if it has current attributes specified by old-owner. old-owner has the same form as new-owner described above. This option is useful primarily from a security standpoint in that it narrows considerably the window of potential abuse. For example, to reflect a user ID numbering change for one user’s files without an option like this, root might run

find / -owner OLDUSER -print0 | xargs -0 chown -h NEWUSER

But that is dangerous because the interval between when the find tests the existing file’s owner and when the chown is actually run may be quite large. One way to narrow the gap would be to invoke chown for each file as it is found:

find / -owner OLDUSER -exec chown -h NEWUSER {} \;

But that is very slow if there are many affected files. With this option, it is safer (the gap is narrower still) though still not perfect:

chown -h -R --from=OLDUSER NEWUSER /
--dereference

Do not act on symbolic links themselves but rather on what they point to. This is the default when not operating recursively.

Combining this dereferencing option with the --recursive option may create a security risk: During the traversal of the directory tree, an attacker may be able to introduce a symlink to an arbitrary target; when the tool reaches that, the operation will be performed on the target of that symlink, possibly allowing the attacker to escalate privileges.

-h
--no-dereference

Act on symbolic links themselves instead of what they point to. This mode relies on the lchown system call. On systems that do not provide the lchown system call, chown fails when a file specified on the command line is a symbolic link. By default, no diagnostic is issued for symbolic links encountered during a recursive traversal, but see --verbose.

--preserve-root

Fail upon any attempt to recursively change the root directory, /. Without --recursive, this option has no effect. See Treating / specially.

--no-preserve-root

Cancel the effect of any preceding --preserve-root option. See Treating / specially.

--reference=ref_file

Change the user and group of each file to be the same as those of ref_file. If ref_file is a symbolic link, do not use the user and group of the symbolic link, but rather those of the file it refers to.

-v
--verbose

Output a diagnostic for every file processed. If a symbolic link is encountered during a recursive traversal on a system without the lchown system call, and --no-dereference is in effect, then issue a diagnostic saying neither the symbolic link nor its referent is being changed.

-R
--recursive

Recursively change ownership of directories and their contents.

-H

If --recursive (-R) is specified and a command line argument is a symbolic link to a directory, traverse it. See Traversing symlinks.

-L

In a recursive traversal, traverse every symbolic link to a directory that is encountered.

Combining this dereferencing option with the --recursive option may create a security risk: During the traversal of the directory tree, an attacker may be able to introduce a symlink to an arbitrary target; when the tool reaches that, the operation will be performed on the target of that symlink, possibly allowing the attacker to escalate privileges.

See Traversing symlinks.

-P

Do not traverse any symbolic links. This is the default if none of -H, -L, or -P is specified. See Traversing symlinks.

An exit status of zero indicates success, and a nonzero value indicates failure.

Examples:

# Change the owner of /u to "root".
chown root /u

# Likewise, but also change its group to "staff".
chown root:staff /u

# Change the owner of /u and subfiles to "root".
chown -hR root /u

13.2 chgrp: Change group ownership

chgrp changes the group ownership of each given file to group (which can be either a group name or a numeric group ID) or to the group of an existing reference file. See chown: Change file owner and group. Synopsis:

chgrp [option]… {group | --reference=ref_file} file

If group is intended to represent a numeric group ID, then you may specify it with a leading ‘+’. See chown, chgrp, chroot, id: Disambiguating user names and IDs.

It is system dependent whether a user can change the group to an arbitrary one, or the more portable behavior of being restricted to setting a group of which the user is a member.

The program accepts the following options. Also see Common options.

-c
--changes

Verbosely describe the action for each file whose group actually changes.

-f
--silent
--quiet

Do not print error messages about files whose group cannot be changed.

--dereference

Do not act on symbolic links themselves but rather on what they point to. This is the default when not operating recursively.

Combining this dereferencing option with the --recursive option may create a security risk: During the traversal of the directory tree, an attacker may be able to introduce a symlink to an arbitrary target; when the tool reaches that, the operation will be performed on the target of that symlink, possibly allowing the attacker to escalate privileges.

-h
--no-dereference

Act on symbolic links themselves instead of what they point to. This mode relies on the lchown system call. On systems that do not provide the lchown system call, chgrp fails when a file specified on the command line is a symbolic link. By default, no diagnostic is issued for symbolic links encountered during a recursive traversal, but see --verbose.

--preserve-root

Fail upon any attempt to recursively change the root directory, /. Without --recursive, this option has no effect. See Treating / specially.

--no-preserve-root

Cancel the effect of any preceding --preserve-root option. See Treating / specially.

--reference=ref_file

Change the group of each file to be the same as that of ref_file. If ref_file is a symbolic link, do not use the group of the symbolic link, but rather that of the file it refers to.

-v
--verbose

Output a diagnostic for every file processed. If a symbolic link is encountered during a recursive traversal on a system without the lchown system call, and --no-dereference is in effect, then issue a diagnostic saying neither the symbolic link nor its referent is being changed.

-R
--recursive

Recursively change the group ownership of directories and their contents.

-H

If --recursive (-R) is specified and a command line argument is a symbolic link to a directory, traverse it. See Traversing symlinks.

-L

In a recursive traversal, traverse every symbolic link to a directory that is encountered.

Combining this dereferencing option with the --recursive option may create a security risk: During the traversal of the directory tree, an attacker may be able to introduce a symlink to an arbitrary target; when the tool reaches that, the operation will be performed on the target of that symlink, possibly allowing the attacker to escalate privileges.

See Traversing symlinks.

-P

Do not traverse any symbolic links. This is the default if none of -H, -L, or -P is specified. See Traversing symlinks.

An exit status of zero indicates success, and a nonzero value indicates failure.

Examples:

# Change the group of /u to "staff".
chgrp staff /u

# Change the group of /u and subfiles to "staff".
chgrp -hR staff /u

13.3 chmod: Change access permissions

chmod changes the access permissions of the named files. Synopsis:

chmod [option]… {mode | --reference=ref_file} file

chmod never changes the permissions of symbolic links, since the chmod system call cannot change their permissions. This is not a problem since the permissions of symbolic links are never used. However, for each symbolic link listed on the command line, chmod changes the permissions of the pointed-to file. In contrast, chmod ignores symbolic links encountered during recursive directory traversals.

Only a process whose effective user ID matches the user ID of the file, or a process with appropriate privileges, is permitted to change the file mode bits of a file.

A successful use of chmod clears the set-group-ID bit of a regular file if the file’s group ID does not match the user’s effective group ID or one of the user’s supplementary group IDs, unless the user has appropriate privileges. Additional restrictions may cause the set-user-ID and set-group-ID bits of mode or ref_file to be ignored. This behavior depends on the policy and functionality of the underlying chmod system call. When in doubt, check the underlying system behavior.

If used, mode specifies the new file mode bits. For details, see the section on File permissions. If you really want mode to have a leading ‘-’, you should use -- first, e.g., ‘chmod -- -w file’. Typically, though, ‘chmod a-w file’ is preferable, and chmod -w file (without the --) complains if it behaves differently from what ‘chmod a-w file’ would do.

The program accepts the following options. Also see Common options.

-c
--changes

Verbosely describe the action for each file whose permissions actually change.

-f
--silent
--quiet

Do not print error messages about files whose permissions cannot be changed.

--preserve-root

Fail upon any attempt to recursively change the root directory, /. Without --recursive, this option has no effect. See Treating / specially.

--no-preserve-root

Cancel the effect of any preceding --preserve-root option. See Treating / specially.

-v
--verbose

Verbosely describe the action or non-action taken for every file.

--reference=ref_file

Change the mode of each file to be the same as that of ref_file. See File permissions. If ref_file is a symbolic link, do not use the mode of the symbolic link, but rather that of the file it refers to.

-R
--recursive

Recursively change permissions of directories and their contents.

An exit status of zero indicates success, and a nonzero value indicates failure.

Examples:

# Change file permissions of FOO to be world readable
# and user writable, with no other permissions.
chmod 644 foo
chmod a=r,u+w foo

# Add user and group execute permissions to FOO.
chmod +110 file
chmod ug+x file

# Set file permissions of DIR and subsidiary files to
# be the umask default, assuming execute permissions for
# directories and for files already executable.
chmod -R a=,+rwX dir

13.4 touch: Change file timestamps

touch changes the access and/or modification timestamps of the specified files. Synopsis:

touch [option]… file

Any file argument that does not exist is created empty, unless option --no-create (-c) or --no-dereference (-h) was in effect.

A file argument string of ‘-’ is handled specially and causes touch to change the times of the file associated with standard output.

By default, touch sets file timestamps to the current time. Because touch acts on its operands left to right, the resulting timestamps of earlier and later operands may disagree.

When setting file timestamps to the current time, touch can change the timestamps for files that the user does not own but has write permission for. Otherwise, the user must own the files. Some older systems have a further restriction: the user must own the files unless both the access and modification timestamps are being set to the current time.

The touch command cannot set a file’s status change timestamp to a user-specified value, and cannot change the file’s birth time (if supported) at all. Also, touch has issues similar to those affecting all programs that update file timestamps. For example, touch may set a file’s timestamp to a value that differs slightly from the requested time. See File timestamps.

Timestamps assume the time zone rules specified by the TZ environment variable, or by the system default rules if TZ is not set. See Specifying the Time Zone with TZ in The GNU C Library Reference Manual. You can avoid ambiguities during daylight saving transitions by using UTC timestamps.

The program accepts the following options. Also see Common options.

-a
--time=atime
--time=access
--time=use

Change the access timestamp only. See File timestamps.

-c
--no-create

Do not warn about or create files that do not exist.

-d time
--date=time

Use time instead of the current time. It can contain month names, time zones, ‘am’ and ‘pm’, ‘yesterday’, etc. For example, --date="2020-07-21 14:19:13.489392193 +0530" specifies the instant of time that is 489,392,193 nanoseconds after July 21, 2020 at 2:19:13 PM in a time zone that is 5 hours and 30 minutes east of UTC. See Date input formats. File systems that do not support high-resolution timestamps silently ignore any excess precision here.

-f

Ignored; for compatibility with BSD versions of touch.

-h
--no-dereference

Attempt to change the timestamps of a symbolic link, rather than what the link refers to. When using this option, empty files are not created, but option -c must also be used to avoid warning about files that do not exist. Not all systems support changing the timestamps of symlinks, since underlying system support for this action was not required until POSIX 2008. Also, on some systems, the mere act of examining a symbolic link changes the access timestamp, such that only changes to the modification timestamp will persist long enough to be observable. When coupled with option -r, a reference timestamp is taken from a symbolic link rather than the file it refers to.

-m
--time=mtime
--time=modify

Change the modification timestamp only.

-r file
--reference=file

Use the times of the reference file instead of the current time. If this option is combined with the --date=time (-d time) option, the reference file’s time is the origin for any relative times given, but is otherwise ignored. For example, ‘-r foo -d '-5 seconds'’ specifies a timestamp equal to five seconds before the corresponding timestamp for foo. If file is a symbolic link, the reference timestamp is taken from the target of the symlink, unless -h was also in effect.

-t [[cc]yy]mmddhhmm[.ss]

Use the argument (optional four-digit or two-digit years, months, days, hours, minutes, optional seconds) instead of the current time. If the year is specified with only two digits, then cc is 20 for years in the range 0 … 68, and 19 for years in 69 … 99. If no digits of the year are specified, the argument is interpreted as a date in the current year. On the atypical systems that support leap seconds, ss may be ‘60’.

On systems predating POSIX 1003.1-2001, touch supports an obsolete syntax, as follows. If no timestamp is given with any of the -d, -r, or -t options, and if there are two or more files and the first file is of the form ‘mmddhhmm[yy]’ and this would be a valid argument to the -t option (if the yy, if any, were moved to the front), and if the represented year is in the range 1969–1999, that argument is interpreted as the time for the other files instead of as a file name. Although this obsolete behavior can be controlled with the _POSIX2_VERSION environment variable (see Standards conformance), portable scripts should avoid commands whose behavior depends on this variable. For example, use ‘touch ./12312359 main.c’ or ‘touch -t 12312359 main.c’ rather than the ambiguous ‘touch 12312359 main.c’.

An exit status of zero indicates success, and a nonzero value indicates failure.


14 File space usage

No file system can hold an infinite amount of data. These commands report how much storage is in use or available, report other file and file status information, and write buffers to file systems.


14.1 df: Report file system space usage

df reports the amount of space used and available on file systems. Synopsis:

df [option]… [file]…

With no arguments, df reports the space used and available on all currently mounted file systems (of all types). Otherwise, df reports on the file system containing each argument file.

Normally the space is printed in units of 1024 bytes, but this can be overridden (see Block size). Non-integer quantities are rounded up to the next higher unit.

For bind mounts and without arguments, df only outputs the statistics for that device with the shortest mount point name in the list of file systems (mtab), i.e., it hides duplicate entries, unless the -a option is specified.

With the same logic, df elides a mount entry of a dummy pseudo device if there is another mount entry of a real block device for that mount point with the same device number, e.g. the early-boot pseudo file system ‘rootfs’ is not shown per default when already the real root device has been mounted.

If an argument file resolves to a special file containing a mounted file system, df shows the space available on that file system rather than on the file system containing the device node. GNU df does not attempt to determine the usage on unmounted file systems, because on most kinds of systems doing so requires extremely nonportable intimate knowledge of file system structures.

The program accepts the following options. Also see Common options.

-a
--all

Include in the listing dummy, duplicate, or inaccessible file systems, which are omitted by default. Dummy file systems are typically special purpose pseudo file systems such as ‘/proc’, with no associated storage. Duplicate file systems are local or remote file systems that are mounted at separate locations in the local file hierarchy, or bind mounted locations. Inaccessible file systems are those which are mounted but subsequently over-mounted by another file system at that point, or otherwise inaccessible due to permissions of the mount point etc.

-B size
--block-size=size

Scale sizes by size before printing them (see Block size). For example, -BG prints sizes in units of 1,073,741,824 bytes.

-h
--human-readable

Append a size letter to each size, such as ‘M’ for mebibytes. Powers of 1024 are used, not 1000; ‘M’ stands for 1,048,576 bytes. This option is equivalent to --block-size=human-readable. Use the --si option if you prefer powers of 1000.

-H

Equivalent to --si.

-i
--inodes

List inode usage information instead of block usage. An inode (short for index node) contains information about a file such as its owner, permissions, timestamps, and location on the file system.

-k

Print sizes in 1024-byte blocks, overriding the default block size (see Block size). This option is equivalent to --block-size=1K.

-l
--local

Limit the listing to local file systems. By default, remote file systems are also listed.

--no-sync

Do not invoke the sync system call before getting any usage data. This may make df run significantly faster on systems with many file systems, but on some systems (notably Solaris) the results may be slightly out of date. This is the default.

--output
--output[=field_list]

Use the output format defined by field_list, or print all fields if field_list is omitted. In the latter case, the order of the columns conforms to the order of the field descriptions below.

The use of the --output together with each of the options -i, -P, and -T is mutually exclusive.

FIELD_LIST is a comma-separated list of columns to be included in df’s output and therefore effectively controls the order of output columns. Each field can thus be used at the place of choice, but yet must only be used once.

Valid field names in the field_list are:

source

The source of the mount point, usually a device.

fstype

File system type.

itotal

Total number of inodes.

iused

Number of used inodes.

iavail

Number of available inodes.

ipcent

Percentage of iused divided by itotal.

size

Total number of blocks.

used

Number of used blocks.

avail

Number of available blocks.

pcent

Percentage of used divided by size.

file

The file name if specified on the command line.

target

The mount point.

The fields for block and inodes statistics are affected by the scaling options like -h as usual.

The definition of the field_list can even be split among several --output uses.

#!/bin/sh
# Print the TARGET (i.e., the mount point) along with their percentage
# statistic regarding the blocks and the inodes.
df --out=target --output=pcent,ipcent

# Print all available fields.
df --o
-P
--portability

Use the POSIX output format. This is like the default format except for the following:

  1. The information about each file system is always printed on exactly one line; a mount device is never put on a line by itself. This means that if the mount device name is more than 20 characters long (e.g., for some network mounts), the columns are misaligned.
  2. The labels in the header output line are changed to conform to POSIX.
  3. The default block size and output format are unaffected by the DF_BLOCK_SIZE, BLOCK_SIZE and BLOCKSIZE environment variables. However, the default block size is still affected by POSIXLY_CORRECT: it is 512 if POSIXLY_CORRECT is set, 1024 otherwise. See Block size.
--si

Append an SI-style abbreviation to each size, such as ‘M’ for megabytes. Powers of 1000 are used, not 1024; ‘M’ stands for 1,000,000 bytes. This option is equivalent to --block-size=si. Use the -h or --human-readable option if you prefer powers of 1024.

--sync

Invoke the sync system call before getting any usage data. On some systems (notably Solaris), doing this yields more up to date results, but in general this option makes df much slower, especially when there are many or very busy file systems.

--total

Print a grand total of all arguments after all arguments have been processed. This can be used to find out the total size, usage and available space of all listed devices. If no arguments are specified df will try harder to elide file systems insignificant to the total available space, by suppressing duplicate remote file systems.

For the grand total line, df prints ‘"total"’ into the source column, and ‘"-"’ into the target column. If there is no source column (see --output), then df prints ‘"total"’ into the target column, if present.

-t fstype
--type=fstype

Limit the listing to file systems of type fstype. Multiple file system types can be specified by giving multiple -t options. By default, nothing is omitted.

-T
--print-type

Print each file system’s type. The types printed here are the same ones you can include or exclude with -t and -x. The particular types printed are whatever is supported by the system. Here are some of the common names (this list is certainly not exhaustive):

nfs

An NFS file system, i.e., one mounted over a network from another machine. This is the one type name which seems to be used uniformly by all systems.

ext2, ext3, ext4, xfs, btrfs…

A file system on a locally-mounted device. (The system might even support more than one type here; GNU/Linux does.)

iso9660, cdfs

A file system on a CD or DVD drive. HP-UX uses ‘cdfs’, most other systems use ‘iso9660’.

ntfs,fat

File systems used by MS-Windows / MS-DOS.

-x fstype
--exclude-type=fstype

Limit the listing to file systems not of type fstype. Multiple file system types can be eliminated by giving multiple -x options. By default, no file system types are omitted.

-v

Ignored; for compatibility with System V versions of df.

df is installed only on systems that have usable mount tables, so portable scripts should not rely on its existence.

An exit status of zero indicates success, and a nonzero value indicates failure. Failure includes the case where no output is generated, so you can inspect the exit status of a command like ‘df -t ext3 -t reiserfs dir’ to test whether dir is on a file system of type ‘ext3’ or ‘reiserfs’.

Since the list of file systems (mtab) is needed to determine the file system type, failure includes the cases when that list cannot be read and one or more of the options -a, -l, -t or -x is used together with a file name argument.


14.2 du: Estimate file space usage

du reports the amount of file system space used by the set of specified files and for each subdirectory (of directory arguments). Synopsis:

du [option]… [file]…

With no arguments, du reports the file system space for the current directory. Normally the space is printed in units of 1024 bytes, but this can be overridden (see Block size). Non-integer quantities are rounded up to the next higher unit.

If two or more hard links point to the same file, only one of the hard links is counted. The file argument order affects which links are counted, and changing the argument order may change the numbers and entries that du outputs.

The program accepts the following options. Also see Common options.

-0
--null

Output a zero byte (ASCII NUL) at the end of each line, rather than a newline. This option enables other programs to parse the output even when that output would contain data with embedded newlines.

-a
--all

Show counts for all files, not just directories.

--apparent-size

Print apparent sizes, rather than file system usage. The apparent size of a file is the number of bytes reported by wc -c on regular files, or more generally, ls -l --block-size=1 or stat --format=%s. For example, a file containing the word ‘zoo’ with no newline would, of course, have an apparent size of 3. Such a small file may require anywhere from 0 to 16 KiB or more of file system space, depending on the type and configuration of the file system on which the file resides. However, a sparse file created with this command:

dd bs=1 seek=2GiB if=/dev/null of=big

has an apparent size of 2 GiB, yet on most modern file systems, it actually uses almost no space.

-B size
--block-size=size

Scale sizes by size before printing them (see Block size). For example, -BG prints sizes in units of 1,073,741,824 bytes.

-b
--bytes

Equivalent to --apparent-size --block-size=1.

-c
--total

Print a grand total of all arguments after all arguments have been processed. This can be used to find out the total file system usage of a given set of files or directories.

-D
--dereference-args

Dereference symbolic links that are command line arguments. Does not affect other symbolic links. This is helpful for finding out the file system usage of directories, such as /usr/tmp, which are often symbolic links.

-d depth
--max-depth=depth

Show the total for each directory (and file if –all) that is at most MAX_DEPTH levels down from the root of the hierarchy. The root is at level 0, so du --max-depth=0 is equivalent to du -s.

--files0-from=file

Disallow processing files named on the command line, and instead process those named in file file; each name being terminated by a zero byte (ASCII NUL). This is useful when the list of file names is so long that it may exceed a command line length limitation. In such cases, running du via xargs is undesirable because it splits the list into pieces and makes du print with the --total (-c) option for each sublist rather than for the entire list. One way to produce a list of ASCII NUL terminated file names is with GNU find, using its -print0 predicate. If file is ‘-’ then the ASCII NUL terminated file names are read from standard input.

-H

Equivalent to --dereference-args (-D).

-h
--human-readable

Append a size letter to each size, such as ‘M’ for mebibytes. Powers of 1024 are used, not 1000; ‘M’ stands for 1,048,576 bytes. This option is equivalent to --block-size=human-readable. Use the --si option if you prefer powers of 1000.

--inodes

List inode usage information instead of block usage. This option is useful for finding directories which contain many files, and therefore eat up most of the inodes space of a file system (see df, option --inodes). It can well be combined with the options -a, -c, -h, -l, -s, -S, -t and -x; however, passing other options regarding the block size, for example -b, -m and --apparent-size, is ignored.

-k

Print sizes in 1024-byte blocks, overriding the default block size (see Block size). This option is equivalent to --block-size=1K.

-L
--dereference

Dereference symbolic links (show the file system space used by the file or directory that the link points to instead of the space used by the link).

-l
--count-links

Count the size of all files, even if they have appeared already (as a hard link).

-m

Print sizes in 1,048,576-byte blocks, overriding the default block size (see Block size). This option is equivalent to --block-size=1M.

-P
--no-dereference

For each symbolic link encountered by du, consider the file system space used by the symbolic link itself.

-S
--separate-dirs

Normally, in the output of du (when not using --summarize), the size listed next to a directory name, d, represents the sum of sizes of all entries beneath d as well as the size of d itself. With --separate-dirs, the size reported for a directory name, d, will exclude the size of any subdirectories.

--si

Append an SI-style abbreviation to each size, such as ‘M’ for megabytes. Powers of 1000 are used, not 1024; ‘M’ stands for 1,000,000 bytes. This option is equivalent to --block-size=si. Use the -h or --human-readable option if you prefer powers of 1024.

-s
--summarize

Display only a total for each argument.

-t size
--threshold=size

Exclude entries based on a given size. The size refers to used blocks in normal mode (see Block size), or inodes count in conjunction with the --inodes option.

If size is positive, then du will only print entries with a size greater than or equal to that.

If size is negative, then du will only print entries with a size smaller than or equal to that.

Although GNU find can be used to find files of a certain size, du’s --threshold option can be used to also filter directories based on a given size.

Please note that the --threshold option can be combined with the --apparent-size option, and in this case would elide entries based on its apparent size.

Please note that the --threshold option can be combined with the --inodes option, and in this case would elide entries based on its inodes count.

Here’s how you would use --threshold to find directories with a size greater than or equal to 200 megabytes:

du --threshold=200MB

Here’s how you would use --threshold to find directories and files - note the -a - with an apparent size smaller than or equal to 500 bytes:

du -a -t -500 --apparent-size

Here’s how you would use --threshold to find directories on the root file system with more than 20000 inodes used in the directory tree below:

du --inodes -x --threshold=20000 /
--time

Show the most recent modification timestamp (mtime) of any file in the directory, or any of its subdirectories. See File timestamps.

--time=ctime
--time=status
--time=use

Show the most recent status change timestamp (ctime) of any file in the directory, or any of its subdirectories. See File timestamps.

--time=atime
--time=access

Show the most recent access timestamp (atime) of any file in the directory, or any of its subdirectories. See File timestamps.

--time-style=style

List timestamps in style style. This option has an effect only if the --time option is also specified. The style should be one of the following:

+format

List timestamps using format, where format is interpreted like the format argument of date (see date: Print or set system date and time). For example, --time-style="+%Y-%m-%d %H:%M:%S" causes du to list timestamps like ‘2020-07-21 23:45:56’. As with date, format’s interpretation is affected by the LC_TIME locale category.

full-iso

List timestamps in full using ISO 8601-like date, time, and time zone components with nanosecond precision, e.g., ‘2020-07-21 23:45:56.477817180 -0400’. This style is equivalent to ‘+%Y-%m-%d %H:%M:%S.%N %z’.

long-iso

List ISO 8601 date and time components with minute precision, e.g., ‘2020-07-21 23:45’. These timestamps are shorter than ‘full-iso’ timestamps, and are usually good enough for everyday work. This style is equivalent to ‘+%Y-%m-%d %H:%M’.

iso

List ISO 8601 dates for timestamps, e.g., ‘2020-07-21’. This style is equivalent to ‘+%Y-%m-%d’.

You can specify the default value of the --time-style option with the environment variable TIME_STYLE; if TIME_STYLE is not set the default style is ‘long-iso’. For compatibility with ls, if TIME_STYLE begins with ‘+’ and contains a newline, the newline and any later characters are ignored; if TIME_STYLE begins with ‘posix-’ the ‘posix-’ is ignored; and if TIME_STYLE is ‘locale’ it is ignored.

-X file
--exclude-from=file

Like --exclude, except take the patterns to exclude from file, one per line. If file is ‘-’, take the patterns from standard input.

--exclude=pattern

When recursing, skip subdirectories or files matching pattern. For example, du --exclude='*.o' excludes files whose names end in ‘.o’.

-x
--one-file-system

Skip directories that are on different file systems from the one that the argument being processed is on.

On BSD systems, du reports sizes that are half the correct values for files that are NFS-mounted from HP-UX systems. On HP-UX systems, it reports sizes that are twice the correct values for files that are NFS-mounted from BSD systems. This is due to a flaw in HP-UX; it also affects the HP-UX du program.

An exit status of zero indicates success, and a nonzero value indicates failure.


14.3 stat: Report file or file system status

stat displays information about the specified file(s). Synopsis:

stat [option]… [file]…

With no option, stat reports all information about the given files. But it also can be used to report the information of the file systems the given files are located on. If the files are links, stat can also give information about the files the links point to.

Due to shell aliases and built-in stat functions, using an unadorned stat interactively or in a script may get you different functionality than that described here. Invoke it via env (i.e., env stat …) to avoid interference from the shell.

-L
--dereference

Change how stat treats symbolic links. With this option, stat acts on the file referenced by each symbolic link argument. Without it, stat acts on any symbolic link argument directly.

-f
--file-system

Report information about the file systems where the given files are located instead of information about the files themselves. This option implies the -L option.

--cached=mode

Control how attributes are read from the file system; if supported by the system. This allows one to control the trade-off between freshness and efficiency of attribute access, especially useful with remote file systems. mode can be:

always

Always read the already cached attributes if available.

never

Always sychronize with the latest file system attributes. This also mounts automounted files.

default

Leave the caching behavior to the underlying file system.

-c
--format=format

Use format rather than the default format. format is automatically newline-terminated, so running a command like the following with two or more file operands produces a line of output for each operand:

$ stat --format=%d:%i / /usr
2050:2
2057:2
--printf=format

Use format rather than the default format. Like --format, but interpret backslash escapes, and do not output a mandatory trailing newline. If you want a newline, include ‘\n’ in the format. Here’s how you would use --printf to print the device and inode numbers of / and /usr:

$ stat --printf='%d:%i\n' / /usr
2050:2
2057:2
-t
--terse

Print the information in terse form, suitable for parsing by other programs.

The output of the following commands are identical and the --format also identifies the items printed (in fuller form) in the default format. Note the format string would include another ‘%C’ at the end with an active SELinux security context.

$ stat --format="%n %s %b %f %u %g %D %i %h %t %T %X %Y %Z %W %o" ...
$ stat --terse ...

The same illustrating terse output in --file-system mode:

$ stat -f --format="%n %i %l %t %s %S %b %f %a %c %d" ...
$ stat -f --terse ...

The valid format directives for files with --format and --printf are:

  • %a - Permission bits in octal (note ‘#’ and ‘0’ printf flags)
  • %A - Permission bits in symbolic form (similar to ls -ld)
  • %b - Number of blocks allocated (see ‘%B’)
  • %B - The size in bytes of each block reported by ‘%b
  • %C - The SELinux security context of a file, if available
  • %d - Device number in decimal (st_dev)
  • %D - Device number in hex (st_dev)
  • %Hd - Major device number in decimal
  • %Ld - Minor device number in decimal
  • %f - Raw mode in hex
  • %F - File type
  • %g - Group ID of owner
  • %G - Group name of owner
  • %h - Number of hard links
  • %i - Inode number
  • %m - Mount point (See note below)
  • %n - File name
  • %N - Quoted file name with dereference if symbolic link (see below)
  • %o - Optimal I/O transfer size hint
  • %s - Total size, in bytes
  • %r - Device type in decimal (st_rdev)
  • %R - Device type in hex (st_rdev)
  • %Hr - Major device type in decimal (see below)
  • %Lr - Minor device type in decimal (see below)
  • %t - Major device type in hex (see below)
  • %T - Minor device type in hex (see below)
  • %u - User ID of owner
  • %U - User name of owner
  • %w - Time of file birth, or ‘-’ if unknown
  • %W - Time of file birth as seconds since Epoch, or ‘0
  • %x - Time of last access
  • %X - Time of last access as seconds since Epoch
  • %y - Time of last data modification
  • %Y - Time of last data modification as seconds since Epoch
  • %z - Time of last status change
  • %Z - Time of last status change as seconds since Epoch

The ‘%a’ format prints the octal mode, and so it is useful to control the zero padding of the output with the ‘#’ and ‘0’ printf flags. For example to pad to at least 3 wide while making larger numbers unambiguously octal, you can use ‘%#03a’.

The ‘%N’ format can be set with the environment variable QUOTING_STYLE. If that environment variable is not set, the default value is ‘shell-escape-always’. Valid quoting styles are:

literal

Output strings as-is; this is the same as the --literal (-N) option.

shell

Quote strings for the shell if they contain shell metacharacters or would cause ambiguous output. The quoting is suitable for POSIX-compatible shells like bash, but it does not always work for incompatible shells like csh.

shell-always

Quote strings for the shell, even if they would normally not require quoting.

shell-escape

Like ‘shell’, but also quoting non-printable characters using the POSIX proposed ‘$''’ syntax suitable for most shells.

shell-escape-always

Like ‘shell-escape’, but quote strings even if they would normally not require quoting.

c

Quote strings as for C character string literals, including the surrounding double-quote characters; this is the same as the --quote-name (-Q) option.

escape

Quote strings as for C character string literals, except omit the surrounding double-quote characters; this is the same as the --escape (-b) option.

clocale

Quote strings as for C character string literals, except use surrounding quotation marks appropriate for the locale.

locale

Quote strings as for C character string literals, except use surrounding quotation marks appropriate for the locale, and quote 'like this' instead of "like this" in the default C locale. This looks nicer on many displays.

The ‘r’, ‘R’, ‘%t’, and ‘%T’ formats operate on the st_rdev member of the stat(2) structure, i.e., the represented device rather than the containing device, and so are only defined for character and block special files. On some systems or file types, st_rdev may be used to represent other quantities.

The ‘%W’, ‘%X’, ‘%Y’, and ‘%Z’ formats accept a precision preceded by a period to specify the number of digits to print after the decimal point. For example, ‘%.3X’ outputs the access timestamp to millisecond precision. If a period is given but no precision, stat uses 9 digits, so ‘%.X’ is equivalent to ‘%.9X’. When discarding excess precision, timestamps are truncated toward minus infinity.

zero pad:
  $ stat -c '[%015Y]' /usr
  [000001288929712]
space align:
  $ stat -c '[%15Y]' /usr
  [     1288929712]
  $ stat -c '[%-15Y]' /usr
  [1288929712     ]
precision:
  $ stat -c '[%.3Y]' /usr
  [1288929712.114]
  $ stat -c '[%.Y]' /usr
  [1288929712.114951834]

The mount point printed by ‘%m’ is similar to that output by df, except that:

  • stat does not dereference symlinks by default (unless -L is specified)
  • stat does not search for specified device nodes in the file system list, instead operating on them directly
  • stat outputs the alias for a bind mounted file, rather than the initial mount point of its backing device. One can recursively call stat until there is no change in output, to get the current base mount point

When listing file system information (--file-system (-f)), you must use a different set of format directives:

  • %a - Free blocks available to non-super-user
  • %b - Total data blocks in file system
  • %c - Total file nodes in file system
  • %d - Free file nodes in file system
  • %f - Free blocks in file system
  • %i - File System ID in hex
  • %l - Maximum length of file names
  • %n - File name
  • %s - Block size (for faster transfers)
  • %S - Fundamental block size (for block counts)
  • %t - Type in hex
  • %T - Type in human readable form

Timestamps are listed according to the time zone rules specified by the TZ environment variable, or by the system default rules if TZ is not set. See Specifying the Time Zone with TZ in The GNU C Library Reference Manual.

An exit status of zero indicates success, and a nonzero value indicates failure.


14.4 sync: Synchronize cached writes to persistent storage

sync synchronizes in memory files or file systems to persistent storage. Synopsis:

sync [option] [file]…

sync writes any data buffered in memory out to the storage device. This can include (but is not limited to) modified superblocks, modified inodes, and delayed reads and writes. This must be implemented by the kernel; The sync program does nothing but exercise the sync, syncfs, fsync, and fdatasync system calls.

The kernel keeps data in memory to avoid doing (relatively slow) device reads and writes. This improves performance, but if the computer crashes, data may be lost or the file system corrupted as a result. The sync command instructs the kernel to write data in memory to persistent storage.

If any argument is specified then only those files will be synchronized using the fsync(2) syscall by default.

If at least one file is specified, it is possible to change the synchronization method with the following options. Also see Common options.

-d
--data

Use fdatasync(2) to sync only the data for the file, and any metadata required to maintain file system consistency.

-f
--file-system

Synchronize all the I/O waiting for the file systems that contain the file, using the syscall syncfs(2). Note you would usually not specify this option if passing a device node like ‘/dev/sda’ for example, as that would sync the containing file system rather than the referenced one. Note also that depending on the system, passing individual device nodes or files may have different sync characteristics than using no arguments. I.e., arguments passed to fsync(2) may provide greater guarantees through write barriers, than a global sync(2) used when no arguments are provided.

An exit status of zero indicates success, and a nonzero value indicates failure.


14.5 truncate: Shrink or extend the size of a file

truncate shrinks or extends the size of each file to the specified size. Synopsis:

truncate optionfile

Any file that does not exist is created.

If a file is larger than the specified size, the extra data is lost. If a file is shorter, it is extended and the sparse extended part (or hole) reads as zero bytes.

The program accepts the following options. Also see Common options.

-c
--no-create

Do not create files that do not exist.

-o
--io-blocks

Treat size as number of I/O blocks of the file rather than bytes.

-r rfile
--reference=rfile

Base the size of each file on the size of rfile.

-s size
--size=size

Set or adjust the size of each file according to size. size is in bytes unless --io-blocks is specified. size may be, or may be an integer optionally followed by, one of the following multiplicative suffixes:

KB’ =>           1000 (KiloBytes)
‘K’  =>           1024 (KibiBytes)
‘MB’ =>      1000*1000 (MegaBytes)
‘M’  =>      1024*1024 (MebiBytes)
‘GB’ => 1000*1000*1000 (GigaBytes)
‘G’  => 1024*1024*1024 (GibiBytes)

and so on for ‘T’, ‘P’, ‘E’, ‘Z’, and ‘Y’. Binary prefixes can be used, too: ‘KiB’=‘K’, ‘MiB’=‘M’, and so on.

size may also be prefixed by one of the following to adjust the size of each file based on its current size:

+’  => extend by
‘-’  => reduce by
‘<’  => at most
‘>’  => at least
‘/’  => round down to multiple of
‘%’  => round up to multiple of

An exit status of zero indicates success, and a nonzero value indicates failure.


15 Printing text

This section describes commands that display text strings.


15.1 echo: Print a line of text

echo writes each given string to standard output, with a space between each and a newline after the last one. Synopsis:

echo [option]… [string]…

Due to shell aliases and built-in echo functions, using an unadorned echo interactively or in a script may get you different functionality than that described here. Invoke it via env (i.e., env echo …) to avoid interference from the shell.

Due to historical and backwards compatibility reasons, certain bare option-like strings cannot be passed to echo as non-option arguments. It is therefore not advisable to use echo for printing unknown or variable arguments. The printf command is recommended as a more portable and flexible replacement for tasks historically performed by echo. See printf: Format and print data.

The program accepts the following options. Also see Common options. Options must precede operands, and the normally-special argument ‘--’ has no special meaning and is treated like any other string.

-n

Do not output the trailing newline.

-e

Enable interpretation of the following backslash-escaped characters in each string:

\a

alert (bell)

\b

backspace

\c

produce no further output

\e

escape

\f

form feed

\n

newline

\r

carriage return

\t

horizontal tab

\v

vertical tab

\\

backslash

\0nnn

the eight-bit value that is the octal number nnn (zero to three octal digits), if nnn is a nine-bit value, the ninth bit is ignored

\nnn

the eight-bit value that is the octal number nnn (one to three octal digits), if nnn is a nine-bit value, the ninth bit is ignored

\xhh

the eight-bit value that is the hexadecimal number hh (one or two hexadecimal digits)

-E

Disable interpretation of backslash escapes in each string. This is the default. If -e and -E are both specified, the last one given takes effect.

If the POSIXLY_CORRECT environment variable is set, then when echo’s first argument is not -n it outputs option-like arguments instead of treating them as options. For example, echo -ne hello outputs ‘-ne hello’ instead of plain ‘hello’. Also backslash escapes are always enabled. Note to echo the string ‘-n’, one of the characters can be escaped in either octal or hexadecimal representation. For example, echo -e '\x2dn'.

POSIX does not require support for any options, and says that the behavior of echo is implementation-defined if any string contains a backslash or if the first argument is -n. Portable programs should use the printf command instead. See printf: Format and print data.

An exit status of zero indicates success, and a nonzero value indicates failure.


15.2 printf: Format and print data

printf does formatted printing of text. Synopsis:

printf format [argument]…

printf prints the format string, interpreting ‘%’ directives and ‘\’ escapes to format numeric and string arguments in a way that is mostly similar to the C ‘printf’ function. See printf format directives in The GNU C Library Reference Manual, for details. The differences are listed below.

Due to shell aliases and built-in printf functions, using an unadorned printf interactively or in a script may get you different functionality than that described here. Invoke it via env (i.e., env printf …) to avoid interference from the shell.

  • The format argument is reused as necessary to convert all the given arguments. For example, the command ‘printf %s a b’ outputs ‘ab’.
  • Missing arguments are treated as null strings or as zeros, depending on whether the context expects a string or a number. For example, the command ‘printf %sx%d’ prints ‘x0’.
  • An additional escape, ‘\c’, causes printf to produce no further output. For example, the command ‘printf 'A%sC\cD%sF' B E’ prints ‘ABC’.
  • The hexadecimal escape sequence ‘\xhh’ has at most two digits, as opposed to C where it can have an unlimited number of digits. For example, the command ‘printf '\x07e'’ prints two bytes, whereas the C statement ‘printf ("\x07e")’ prints just one.
  • An additional directive ‘%b’, prints its argument string with ‘\’ escapes interpreted in the same way as in the format string, except that octal escapes are of the form ‘\0ooo’ where ooo is 0 to 3 octal digits. If ‘\ooo’ is nine-bit value, ignore the ninth bit. If a precision is also given, it limits the number of bytes printed from the converted string.
  • An additional directive ‘%q’, prints its argument string in a format that can be reused as input by most shells. Non-printable characters are escaped with the POSIX proposed ‘$''’ syntax, and shell metacharacters are quoted appropriately. This is an equivalent format to ls --quoting=shell-escape output.
  • Numeric arguments must be single C constants, possibly with leading ‘+’ or ‘-’. For example, ‘printf %.4d -3’ outputs ‘-0003’.
  • If the leading character of a numeric argument is ‘"’ or ‘'’ then its value is the numeric value of the immediately following character. Any remaining characters are silently ignored if the POSIXLY_CORRECT environment variable is set; otherwise, a warning is printed. For example, ‘printf "%d" "'a"’ outputs ‘97’ on hosts that use the ASCII character set, since ‘a’ has the numeric value 97 in ASCII.

A floating point argument is interpreted according to the LC_NUMERIC category of either the current or the C locale, and is printed according to the current locale. For example, in a locale whose decimal point character is a comma, the command ‘printf '%g %g' 2,5 2.5’ outputs ‘2,5 2,5’. See Floating point numbers.

printf interprets ‘\ooo’ in format as an octal number (if ooo is 1 to 3 octal digits) specifying a byte to print, and ‘\xhh’ as a hexadecimal number (if hh is 1 to 2 hex digits) specifying a character to print. Note however that when ‘\ooo’ specifies a number larger than 255, printf ignores the ninth bit. For example, ‘printf '\400'’ is equivalent to ‘printf '\0'’.

printf interprets two character syntaxes introduced in ISO C 99: ‘\u’ for 16-bit Unicode (ISO/IEC 10646) characters, specified as four hexadecimal digits hhhh, and ‘\U’ for 32-bit Unicode characters, specified as eight hexadecimal digits hhhhhhhh. printf outputs the Unicode characters according to the LC_CTYPE locale. Unicode characters in the ranges U+0000…U+009F, U+D800…U+DFFF cannot be specified by this syntax, except for U+0024 ($), U+0040 (@), and U+0060 ()̀.

The processing of ‘\u’ and ‘\U’ requires a full-featured iconv facility. It is activated on systems with glibc 2.2 (or newer), or when libiconv is installed prior to this package. Otherwise ‘\u’ and ‘\U’ will print as-is.

The only options are a lone --help or --version. See Common options. Options must precede operands.

The Unicode character syntaxes are useful for writing strings in a locale independent way. For example, a string containing the Euro currency symbol

$ env printf '\u20AC 14.95'

will be output correctly in all locales supporting the Euro symbol (ISO-8859-15, UTF-8, and others). Similarly, a Chinese string

$ env printf '\u4e2d\u6587'

will be output correctly in all Chinese locales (GB2312, BIG5, UTF-8, etc).

Note that in these examples, the printf command has been invoked via env to ensure that we run the program found via your shell’s search path, and not a shell alias or a built-in function.

For larger strings, you don’t need to look up the hexadecimal code values of each character one by one. ASCII characters mixed with \u escape sequences is also known as the JAVA source file encoding. You can use GNU recode 3.5c (or newer) to convert strings to this encoding. Here is how to convert a piece of text into a shell script which will output this text in a locale-independent way:

$ LC_CTYPE=zh_CN.big5 /usr/local/bin/printf \
    '\u4e2d\u6587\n' > sample.txt
$ recode BIG5..JAVA < sample.txt \
    | sed -e "s|^|/usr/local/bin/printf '|" -e "s|$|\\\\n'|" \
    > sample.sh

An exit status of zero indicates success, and a nonzero value indicates failure.


15.3 yes: Print a string until interrupted

yes prints the command line arguments, separated by spaces and followed by a newline, forever until it is killed. If no arguments are given, it prints ‘y’ followed by a newline forever until killed.

Upon a write error, yes exits with status ‘1’.

The only options are a lone --help or --version. To output an argument that begins with ‘-’, precede it with --, e.g., ‘yes -- --help’. See Common options.


16 Conditions

This section describes commands that are primarily useful for their exit status, rather than their output. Thus, they are often used as the condition of shell if statements, or as the last command in a pipeline.


16.1 false: Do nothing, unsuccessfully

false does nothing except return an exit status of 1, meaning failure. It can be used as a place holder in shell scripts where an unsuccessful command is needed. In most modern shells, false is a built-in command, so when you use ‘false’ in a script, you’re probably using the built-in command, not the one documented here.

false honors the --help and --version options.

This version of false is implemented as a C program, and is thus more secure and faster than a shell script implementation, and may safely be used as a dummy shell for the purpose of disabling accounts.

Note that false (unlike all other programs documented herein) exits unsuccessfully, even when invoked with --help or --version.

Portable programs should not assume that the exit status of false is 1, as it is greater than 1 on some non-GNU hosts.


16.2 true: Do nothing, successfully

true does nothing except return an exit status of 0, meaning success. It can be used as a place holder in shell scripts where a successful command is needed, although the shell built-in command : (colon) may do the same thing faster. In most modern shells, true is a built-in command, so when you use ‘true’ in a script, you’re probably using the built-in command, not the one documented here.

true honors the --help and --version options.

Note, however, that it is possible to cause true to exit with nonzero status: with the --help or --version option, and with standard output already closed or redirected to a file that evokes an I/O error. For example, using a Bourne-compatible shell:

$ ./true --version >&-
./true: write error: Bad file number
$ ./true --version > /dev/full
./true: write error: No space left on device

This version of true is implemented as a C program, and is thus more secure and faster than a shell script implementation, and may safely be used as a dummy shell for the purpose of disabling accounts.


16.3 test: Check file types and compare values

test returns a status of 0 (true) or 1 (false) depending on the evaluation of the conditional expression expr. Each part of the expression must be a separate argument.

test has file status checks, string operators, and numeric comparison operators.

test has an alternate form that uses opening and closing square brackets instead a leading ‘test’. For example, instead of ‘test -d /’, you can write ‘[ -d / ]’. The square brackets must be separate arguments; for example, ‘[-d /]’ does not have the desired effect. Since ‘test expr’ and ‘[ expr ]’ have the same meaning, only the former form is discussed below.

Synopses:

test expression
test
[ expression ]
[ ]
[ option

Due to shell aliases and built-in test functions, using an unadorned test interactively or in a script may get you different functionality than that described here. Invoke it via env (i.e., env test …) to avoid interference from the shell.

If expression is omitted, test returns false. If expression is a single argument, test returns false if the argument is null and true otherwise. The argument can be any string, including strings like ‘-d’, ‘-1’, ‘--’, ‘--help’, and ‘--version’ that most other programs would treat as options. To get help and version information, invoke the commands ‘[ --help’ and ‘[ --version’, without the usual closing brackets. See Common options.

Exit status:

0 if the expression is true,
1 if the expression is false,
2 if an error occurred.

16.3.1 File type tests

These options test for particular types of files. (Everything’s a file, but not all files are the same!)

-b file

True if file exists and is a block special device.

-c file

True if file exists and is a character special device.

-d file

True if file exists and is a directory.

-f file

True if file exists and is a regular file.

-h file
-L file

True if file exists and is a symbolic link. Unlike all other file-related tests, this test does not dereference file if it is a symbolic link.

-p file

True if file exists and is a named pipe.

-S file

True if file exists and is a socket.

-t fd

True if fd is a file descriptor that is associated with a terminal.


16.3.2 Access permission tests

These options test for particular access permissions.

-g file

True if file exists and has its set-group-ID bit set.

-k file

True if file exists and has its sticky bit set.

-r file

True if file exists and the user has read access.

-u file

True if file exists and has its set-user-ID bit set.

-w file

True if file exists and the user has write access.

-x file

True if file exists and the user has execute access (or search permission, if it is a directory).

-O file

True if file exists and is owned by the current effective user ID.

-G file

True if file exists and is owned by the current effective group ID.


16.3.3 File characteristic tests

These options test other file characteristics.

-e file

True if file exists.

-s file

True if file exists and has a size greater than zero.

file1 -nt file2

True if file1 is newer (according to modification date) than file2, or if file1 exists and file2 does not.

file1 -ot file2

True if file1 is older (according to modification date) than file2, or if file2 exists and file1 does not.

file1 -ef file2

True if file1 and file2 have the same device and inode numbers, i.e., if they are hard links to each other.

-N file

True if file exists and has been modified (mtime) since it was last read (atime).


16.3.4 String tests

These options test string characteristics. You may need to quote string arguments for the shell. For example:

test -n "$V"

The quotes here prevent the wrong arguments from being passed to test if ‘$V’ is empty or contains special characters.

-z string

True if the length of string is zero.

-n string
string

True if the length of string is nonzero.

string1 = string2

True if the strings are equal.

string1 == string2

True if the strings are equal (synonym for =). Note this form is not as portable to other shells and systems.

string1 != string2

True if the strings are not equal.


16.3.5 Numeric tests

Numeric relational operators. The arguments must be entirely numeric (possibly negative), or the special expression -l string, which evaluates to the length of string.

arg1 -eq arg2
arg1 -ne arg2
arg1 -lt arg2
arg1 -le arg2
arg1 -gt arg2
arg1 -ge arg2

These arithmetic binary operators return true if arg1 is equal, not-equal, less-than, less-than-or-equal, greater-than, or greater-than-or-equal than arg2, respectively.

For example:

test -1 -gt -2 && echo yes
⇒ yes
test -l abc -gt 1 && echo yes
⇒ yes
test 0x100 -eq 1
error→ test: integer expression expected before -eq

16.3.6 Connectives for test

Note it’s preferred to use shell logical primitives rather than these logical connectives internal to test, because an expression may become ambiguous depending on the expansion of its parameters.

For example, this becomes ambiguous when ‘$1’ is set to ‘'!'’ and ‘$2’ to the empty string ‘''’:

test "$1" -a "$2"

and should be written as:

test "$1" && test "$2"

Note the shell logical primitives also benefit from short circuit operation, which can be significant for file attribute tests.

! expr

True if expr is false. ‘!’ has lower precedence than all parts of expr. Note ‘!’ needs to be specified to the left of a binary expression, I.e., ‘'!' 1 -gt 2’ rather than ‘1 '!' -gt 2’. Also ‘!’ is often a shell special character and is best used quoted.

expr1 -a expr2

True if both expr1 and expr2 are true. ‘-a’ is left associative, and has a higher precedence than ‘-o’.

expr1 -o expr2

True if either expr1 or expr2 is true. ‘-o’ is left associative.


16.4 expr: Evaluate expressions

expr evaluates an expression and writes the result on standard output. Each token of the expression must be a separate argument.

Operands are either integers or strings. Integers consist of one or more decimal digits, with an optional leading ‘-’. expr converts anything appearing in an operand position to an integer or a string depending on the operation being applied to it.

Strings are not quoted for expr itself, though you may need to quote them to protect characters with special meaning to the shell, e.g., spaces. However, regardless of whether it is quoted, a string operand should not be a parenthesis or any of expr’s operators like +, so you cannot safely pass an arbitrary string $str to expr merely by quoting it to the shell. One way to work around this is to use the GNU extension +, (e.g., + "$str" = foo); a more portable way is to use " $str" and to adjust the rest of the expression to take the leading space into account (e.g., " $str" = " foo").

You should not pass a negative integer or a string with leading ‘-’ as expr’s first argument, as it might be misinterpreted as an option; this can be avoided by parenthesization. Also, portable scripts should not use a string operand that happens to take the form of an integer; this can be worked around by inserting leading spaces as mentioned above.

Operators may be given as infix symbols or prefix keywords. Parentheses may be used for grouping in the usual manner. You must quote parentheses and many operators to avoid the shell evaluating them, however.

Because expr uses multiple-precision arithmetic, it works with integers wider than those of machine registers.

The only options are --help and --version. See Common options. Options must precede operands.

Exit status:

0 if the expression is neither null nor 0,
1 if the expression is null or 0,
2 if the expression is invalid,
3 if an internal error occurred (e.g., arithmetic overflow).

16.4.1 String expressions

expr supports pattern matching and other string operators. These have higher precedence than both the numeric and relational operators (in the next sections).

string : regex

Perform pattern matching. The arguments are converted to strings and the second is considered to be a (basic, a la GNU grep) regular expression, with a ^ implicitly prepended. The first argument is then matched against this regular expression.

If regex does not use ‘\(’ and ‘\)’, the : expression returns the number of characters matched, or 0 if the match fails.

If regex uses ‘\(’ and ‘\)’, the : expression returns the part of string that matched the subexpression, or the null string if the match failed or the subexpression did not contribute to the match.

Only the first ‘\( … \)’ pair is relevant to the return value; additional pairs are meaningful only for grouping the regular expression operators.

In the regular expression, \+, \?, and \| are operators which respectively match one or more, zero or one, or separate alternatives. These operators are GNU extensions. See Regular Expressions in The GNU Grep Manual, for details of regular expression syntax. Some examples are in Examples of using expr.

match string regex

An alternative way to do pattern matching. This is the same as ‘string : regex.

substr string position length

Returns the substring of string beginning at position with length at most length. If either position or length is negative, zero, or non-numeric, returns the null string.

index string charset

Returns the first position in string where the first character in charset was found. If no character in charset is found in string, return 0.

length string

Returns the length of string.

+ token

Interpret token as a string, even if it is a keyword like match or an operator like /. This makes it possible to test expr length + "$x" or expr + "$x" : '.*/\(.\)' and have it do the right thing even if the value of $x happens to be (for example) / or index. This operator is a GNU extension. Portable shell scripts should use " $token" : ' \(.*\)' instead of + "$token".

To make expr interpret keywords as strings, you must use the quote operator.


16.4.2 Numeric expressions

expr supports the usual numeric operators, in order of increasing precedence. These numeric operators have lower precedence than the string operators described in the previous section, and higher precedence than the connectives (next section).

+ -

Addition and subtraction. Both arguments are converted to integers; an error occurs if this cannot be done.

* / %

Multiplication, division, remainder. Both arguments are converted to integers; an error occurs if this cannot be done.


16.4.3 Relations for expr

expr supports the usual logical connectives and relations. These have lower precedence than the string and numeric operators (previous sections). Here is the list, lowest-precedence operator first.

|

Returns its first argument if that is neither null nor zero, otherwise its second argument if it is neither null nor zero, otherwise 0. It does not evaluate its second argument if its first argument is neither null nor zero.

&

Return its first argument if neither argument is null or zero, otherwise 0. It does not evaluate its second argument if its first argument is null or zero.

< <= = == != >= >

Compare the arguments and return 1 if the relation is true, 0 otherwise. == is a synonym for =. expr first tries to convert both arguments to integers and do a numeric comparison; if either conversion fails, it does a lexicographic comparison using the character collating sequence specified by the LC_COLLATE locale.


16.4.4 Examples of using expr

Here are a few examples, including quoting for shell metacharacters.

To add 1 to the shell variable foo, in Bourne-compatible shells:

foo=$(expr $foo + 1)

To print the non-directory part of the file name stored in $fname, which need not contain a /:

expr $fname : '.*/\(.*\)' '|' $fname

An example showing that \+ is an operator:

expr aaa : 'a\+'
⇒ 3
expr abc : 'a\(.\)c'
⇒ b
expr index abcdef cz
⇒ 3
expr index index a
error→ expr: syntax error
expr index + index a
⇒ 0

17 Redirection

Unix shells commonly provide several forms of redirection—ways to change the input source or output destination of a command. But one useful redirection is performed by a separate command, not by the shell; it’s described here.


17.1 tee: Redirect output to multiple files or processes

The tee command copies standard input to standard output and also to any files given as arguments. This is useful when you want not only to send some data down a pipe, but also to save a copy. Synopsis:

tee [option]… [file]…

If a file being written to does not already exist, it is created. If a file being written to already exists, the data it previously contained is overwritten unless the -a option is used.

In previous versions of GNU Coreutils (v5.3.0 – v8.23), a file of ‘-’ caused tee to send another copy of input to standard output. However, as the interleaved output was not very useful, tee now conforms to POSIX and treats ‘-’ as a file name.

The program accepts the following options. Also see Common options.

-a
--append

Append standard input to the given files rather than overwriting them.

-i
--ignore-interrupts

Ignore interrupt signals.

-p
--output-error[=mode]

Adjust the behavior with errors on the outputs, with the long form option supporting selection between the following modes:

warn

Warn on error opening or writing any output, including pipes. Writing is continued to still open files/pipes. Exit status indicates failure if any output has an error.

warn-nopipe

This is the default mode when not specified, or when the short form -p is used. Warn on error opening or writing any output, except pipes. Writing is continued to still open files/pipes. Exit status indicates failure if any non pipe output had an error.

exit

Exit on error opening or writing any output, including pipes.

exit-nopipe

Exit on error opening or writing any output, except pipes.

The tee command is useful when you happen to be transferring a large amount of data and also want to summarize that data without reading it a second time. For example, when you are downloading a DVD image, you often want to verify its signature or checksum right away. The inefficient way to do it is simply:

wget https://example.com/some.iso && sha1sum some.iso

One problem with the above is that it makes you wait for the download to complete before starting the time-consuming SHA1 computation. Perhaps even more importantly, the above requires reading the DVD image a second time (the first was from the network).

The efficient way to do it is to interleave the download and SHA1 computation. Then, you’ll get the checksum for free, because the entire process parallelizes so well:

# slightly contrived, to demonstrate process substitution
wget -O - https://example.com/dvd.iso \
  | tee >(sha1sum > dvd.sha1) > dvd.iso

That makes tee write not just to the expected output file, but also to a pipe running sha1sum and saving the final checksum in a file named dvd.sha1.

Note, however, that this example relies on a feature of modern shells called process substitution (the ‘>(command)’ syntax, above; See Process Substitution in The Bash Reference Manual.), so it works with zsh, bash, and ksh, but not with /bin/sh. So if you write code like this in a shell script, be sure to start the script with ‘#!/bin/bash’.

Note also that if any of the process substitutions (or piped standard output) might exit early without consuming all the data, the -p option is needed to allow tee to continue to process the input to any remaining outputs.

Since the above example writes to one file and one process, a more conventional and portable use of tee is even better:

wget -O - https://example.com/dvd.iso \
  | tee dvd.iso | sha1sum > dvd.sha1

You can extend this example to make tee write to two processes, computing MD5 and SHA1 checksums in parallel. In this case, process substitution is required:

wget -O - https://example.com/dvd.iso \
  | tee >(sha1sum > dvd.sha1) \
        >(md5sum > dvd.md5) \
  > dvd.iso

This technique is also useful when you want to make a compressed copy of the contents of a pipe. Consider a tool to graphically summarize file system usage data from ‘du -ak’. For a large hierarchy, ‘du -ak’ can run for a long time, and can easily produce terabytes of data, so you won’t want to rerun the command unnecessarily. Nor will you want to save the uncompressed output.

Doing it the inefficient way, you can’t even start the GUI until after you’ve compressed all of the du output:

du -ak | gzip -9 > /tmp/du.gz
gzip -d /tmp/du.gz | checkspace -a

With tee and process substitution, you start the GUI right away and eliminate the decompression completely:

du -ak | tee >(gzip -9 > /tmp/du.gz) | checkspace -a

Finally, if you regularly create more than one type of compressed tarball at once, for example when make dist creates both gzip-compressed and bzip2-compressed tarballs, there may be a better way. Typical automake-generated Makefile rules create the two compressed tar archives with commands in sequence, like this (slightly simplified):

tardir=your-pkg-M.N
tar chof - "$tardir" | gzip  -9 -c > your-pkg-M.N.tar.gz
tar chof - "$tardir" | bzip2 -9 -c > your-pkg-M.N.tar.bz2

However, if the hierarchy you are archiving and compressing is larger than a couple megabytes, and especially if you are using a multi-processor system with plenty of memory, then you can do much better by reading the directory contents only once and running the compression programs in parallel:

tardir=your-pkg-M.N
tar chof - "$tardir" \
  | tee >(gzip -9 -c > your-pkg-M.N.tar.gz) \
  | bzip2 -9 -c > your-pkg-M.N.tar.bz2

If you want to further process the output from process substitutions, and those processes write atomically (i.e., write less than the system’s PIPE_BUF size at a time), that’s possible with a construct like:

tardir=your-pkg-M.N
tar chof - "$tardir" \
  | tee >(md5sum --tag) > >(sha256sum --tag) \
  | sort | gpg --clearsign > your-pkg-M.N.tar.sig

An exit status of zero indicates success, and a nonzero value indicates failure.


18 File name manipulation

This section describes commands that manipulate file names.


18.1 basename: Strip directory and suffix from a file name

basename removes any leading directory components from name. Synopsis:

basename name [suffix]
basename optionname

If suffix is specified and is identical to the end of name, it is removed from name as well. Note that since trailing slashes are removed prior to suffix matching, suffix will do nothing if it contains slashes. basename prints the result on standard output.

Together, basename and dirname are designed such that if ‘ls "$name"’ succeeds, then the command sequence ‘cd "$(dirname "$name")"; ls "$(basename "$name")"’ will, too. This works for everything except file names containing a trailing newline.

POSIX allows the implementation to define the results if name is empty or ‘//’. In the former case, GNU basename returns the empty string. In the latter case, the result is ‘//’ on platforms where // is distinct from /, and ‘/’ on platforms where there is no difference.

The program accepts the following options. Also see Common options. Options must precede operands.

-a
--multiple

Support more than one argument. Treat every argument as a name. With this, an optional suffix must be specified using the -s option.

-s suffix
--suffix=suffix

Remove a trailing suffix. This option implies the -a option.

-z
--zero

Output a zero byte (ASCII NUL) at the end of each line, rather than a newline. This option enables other programs to parse the output even when that output would contain data with embedded newlines.

An exit status of zero indicates success, and a nonzero value indicates failure.

Examples:

# Output "sort".
basename /usr/bin/sort

# Output "stdio".
basename include/stdio.h .h

# Output "stdio".
basename -s .h include/stdio.h

# Output "stdio" followed by "stdlib"
basename -a -s .h include/stdio.h include/stdlib.h

18.2 dirname: Strip last file name component

dirname prints all but the final slash-delimited component of each name. Slashes on either side of the final component are also removed. If the string contains no slash, dirname prints ‘.’ (meaning the current directory). Synopsis:

dirname [option] name

name need not be a file name, but if it is, this operation effectively lists the directory that contains the final component, including the case when the final component is itself a directory.

Together, basename and dirname are designed such that if ‘ls "$name"’ succeeds, then the command sequence ‘cd "$(dirname "$name")"; ls "$(basename "$name")"’ will, too. This works for everything except file names containing a trailing newline.

POSIX allows the implementation to define the results if name is ‘//’. With GNU dirname, the result is ‘//’ on platforms where // is distinct from /, and ‘/’ on platforms where there is no difference.

The program accepts the following option. Also see Common options.

-z
--zero

Output a zero byte (ASCII NUL) at the end of each line, rather than a newline. This option enables other programs to parse the output even when that output would contain data with embedded newlines.

An exit status of zero indicates success, and a nonzero value indicates failure.

Examples:

# Output "/usr/bin".
dirname /usr/bin/sort
dirname /usr/bin//.//

# Output "dir1" followed by "dir2"
dirname dir1/str dir2/str

# Output ".".
dirname stdio.h

18.3 pathchk: Check file name validity and portability

pathchk checks validity and portability of file names. Synopsis:

pathchk [option]… name

For each name, pathchk prints an error message if any of these conditions is true:

  1. One of the existing directories in name does not have search (execute) permission,
  2. The length of name is larger than the maximum supported by the operating system.
  3. The length of one component of name is longer than its file system’s maximum.

A nonexistent name is not an error, so long as a file with that name could be created under the above conditions.

The program accepts the following options. Also see Common options. Options must precede operands.

-p

Instead of performing checks based on the underlying file system, print an error message if any of these conditions is true:

  1. A file name is empty.
  2. A file name contains a character outside the POSIX portable file name character set, namely, the ASCII letters and digits, ‘.’, ‘_’, ‘-’, and ‘/’.
  3. The length of a file name or one of its components exceeds the POSIX minimum limits for portability.
-P

Print an error message if a file name is empty, or if it contains a component that begins with ‘-’.

--portability

Print an error message if a file name is not portable to all POSIX hosts. This option is equivalent to ‘-p -P’.

Exit status:

0 if all specified file names passed all checks,
1 otherwise.

18.4 mktemp: Create temporary file or directory

mktemp manages the creation of temporary files and directories. Synopsis:

mktemp [option]… [template]

Safely create a temporary file or directory based on template, and print its name. If given, template must include at least three consecutive ‘X’s in the last component. If omitted, the template ‘tmp.XXXXXXXXXX’ is used, and option --tmpdir is implied. The final run of ‘X’s in the template will be replaced by alpha-numeric characters; thus, on a case-sensitive file system, and with a template including a run of n instances of ‘X’, there are ‘62**n’ potential file names.

Older scripts used to create temporary files by simply joining the name of the program with the process id (‘$$’) as a suffix. However, that naming scheme is easily predictable, and suffers from a race condition where the attacker can create an appropriately named symbolic link, such that when the script then opens a handle to what it thought was an unused file, it is instead modifying an existing file. Using the same scheme to create a directory is slightly safer, since the mkdir will fail if the target already exists, but it is still inferior because it allows for denial of service attacks. Therefore, modern scripts should use the mktemp command to guarantee that the generated name will be unpredictable, and that knowledge of the temporary file name implies that the file was created by the current script and cannot be modified by other users.

When creating a file, the resulting file has read and write permissions for the current user, but no permissions for the group or others; these permissions are reduced if the current umask is more restrictive.

Here are some examples (although note that if you repeat them, you will most likely get different file names):

  • Create a temporary file in the current directory.
    $ mktemp file.XXXX
    file.H47c
    
  • Create a temporary file with a known suffix.
    $ mktemp --suffix=.txt file-XXXX
    file-H08W.txt
    $ mktemp file-XXXX-XXXX.txt
    file-XXXX-eI9L.txt
    
  • Create a secure fifo relative to the user’s choice of TMPDIR, but falling back to the current directory rather than /tmp. Note that mktemp does not create fifos, but can create a secure directory in which the fifo can live. Exit the shell if the directory or fifo could not be created.
    $ dir=$(mktemp -p "${TMPDIR:-.}" -d dir-XXXX) || exit 1
    $ fifo=$dir/fifo
    $ mkfifo "$fifo" || { rmdir "$dir"; exit 1; }
    
  • Create and use a temporary file if possible, but ignore failure. The file will reside in the directory named by TMPDIR, if specified, or else in /tmp.
    $ file=$(mktemp -q) && {
    >   # Safe to use $file only within this block.  Use quotes,
    >   # since $TMPDIR, and thus $file, may contain whitespace.
    >   echo ... > "$file"
    >   rm "$file"
    > }
    
  • Act as a semi-random character generator (it is not fully random, since it is impacted by the contents of the current directory). To avoid security holes, do not use the resulting names to create a file.
    $ mktemp -u XXX
    Gb9
    $ mktemp -u XXX
    nzC
    

The program accepts the following options. Also see Common options.

-d
--directory

Create a directory rather than a file. The directory will have read, write, and search permissions for the current user, but no permissions for the group or others; these permissions are reduced if the current umask is more restrictive.

-q
--quiet

Suppress diagnostics about failure to create a file or directory. The exit status will still reflect whether a file was created.

-u
--dry-run

Generate a temporary name that does not name an existing file, without changing the file system contents. Using the output of this command to create a new file is inherently unsafe, as there is a window of time between generating the name and using it where another process can create an object by the same name.

-p dir
--tmpdir[=dir]

Treat template relative to the directory dir. If dir is not specified (only possible with the long option --tmpdir) or is the empty string, use the value of TMPDIR if available, otherwise use ‘/tmp’. If this is specified, template must not be absolute. However, template can still contain slashes, although intermediate directories must already exist.

--suffix=suffix

Append suffix to the template. suffix must not contain slash. If --suffix is specified, template must end in ‘X’; if it is not specified, then an appropriate --suffix is inferred by finding the last ‘X’ in template. This option exists for use with the default template and for the creation of a suffix that starts with ‘X’.

-t

Treat template as a single file relative to the value of TMPDIR if available, or to the directory specified by -p, otherwise to ‘/tmp’. template must not contain slashes. This option is deprecated; the use of -p without -t offers better defaults (by favoring the command line over TMPDIR) and more flexibility (by allowing intermediate directories).

Exit status:

0 if the file was created,
1 otherwise.

18.5 realpath: Print the resolved file name.

realpath expands all symbolic links and resolves references to ‘/./’, ‘/../’ and extra ‘/’ characters. By default, all but the last component of the specified files must exist. Synopsis:

realpath [option]… file

The file name canonicalization functionality overlaps with that of the readlink command. This is the preferred command for canonicalization as it’s a more suitable and standard name. In addition this command supports relative file name processing functionality.

The program accepts the following options. Also see Common options.

-e
--canonicalize-existing

Ensure that all components of the specified file names exist. If any component is missing or unavailable, realpath will output a diagnostic unless the -q option is specified, and exit with a nonzero exit code. A trailing slash requires that the name resolve to a directory.

-m
--canonicalize-missing

If any component of a specified file name is missing or unavailable, treat it as a directory.

-L
--logical

Symbolic links are resolved in the specified file names, but they are resolved after any subsequent ‘..’ components are processed.

-P
--physical

Symbolic links are resolved in the specified file names, and they are resolved before any subsequent ‘..’ components are processed. This is the default mode of operation.

-q
--quiet

Suppress diagnostic messages for specified file names.

--relative-to=dir

Print the resolved file names relative to the specified directory. Note this option honors the -m and -e options pertaining to file existence.

--relative-base=dir

Print the resolved file names as relative if the files are descendants of dir. Otherwise, print the resolved file names as absolute. Note this option honors the -m and -e options pertaining to file existence. For details about combining --relative-to and --relative-base, see Realpath usage examples.

-s
--strip
--no-symlinks

Do not resolve symbolic links. Only resolve references to ‘/./’, ‘/../’ and remove extra ‘/’ characters. When combined with the -m option, realpath operates only on the file name, and does not touch any actual file.

-z
--zero

Output a zero byte (ASCII NUL) at the end of each line, rather than a newline. This option enables other programs to parse the output even when that output would contain data with embedded newlines.

Exit status:

0 if all file names were printed without issue.
1 otherwise.

18.5.1 Realpath usage examples

By default, realpath prints the absolute file name of given files (symlinks are resolved, words is resolved to american-english):

cd /home/user
realpath /usr/bin/sort /tmp/foo /usr/share/dict/words 1.txt
⇒ /usr/bin/sort
⇒ /tmp/foo
⇒ /usr/share/dict/american-english
⇒ /home/user/1.txt

With --relative-to, file names are printed relative to the given directory:

realpath --relative-to=/usr/bin \
         /usr/bin/sort /tmp/foo /usr/share/dict/words 1.txt
⇒ sort
⇒ ../../tmp/foo
⇒ ../share/dict/american-english
⇒ ../../home/user/1.txt

With --relative-base, relative file names are printed if the resolved file name is below the given base directory. For files outside the base directory absolute file names are printed:

realpath --relative-base=/usr \
         /usr/bin/sort /tmp/foo /usr/share/dict/words 1.txt
⇒ bin/sort
⇒ /tmp/foo
⇒ share/dict/american-english
⇒ /home/user/1.txt

When both --relative-to=DIR1 and --relative-base=DIR2 are used, file names are printed relative to dir1 if they are located below dir2. If the files are not below dir2, they are printed as absolute file names:

realpath --relative-to=/usr/bin --relative-base=/usr \
         /usr/bin/sort /tmp/foo /usr/share/dict/words 1.txt
⇒ sort
⇒ /tmp/foo
⇒ ../share/dict/american-english
⇒ /home/user/1.txt

When both --relative-to=DIR1 and --relative-base=DIR2 are used, dir1 must be a subdirectory of dir2. Otherwise, realpath prints absolutes file names.


19 Working context

This section describes commands that display or alter the context in which you are working: the current directory, the terminal settings, and so forth. See also the user-related commands in the next section.


19.1 pwd: Print working directory

pwd prints the name of the current directory. Synopsis:

pwd [option]…

The program accepts the following options. Also see Common options.

-L
--logical

If the contents of the environment variable PWD provide an absolute name of the current directory with no ‘.’ or ‘..’ components, but possibly with symbolic links, then output those contents. Otherwise, fall back to default -P handling.

-P
--physical

Print a fully resolved name for the current directory. That is, all components of the printed name will be actual directory names—none will be symbolic links.

If -L and -P are both given, the last one takes precedence. If neither option is given, then this implementation uses -P as the default unless the POSIXLY_CORRECT environment variable is set.

Due to shell aliases and built-in pwd functions, using an unadorned pwd interactively or in a script may get you different functionality than that described here. Invoke it via env (i.e., env pwd …) to avoid interference from the shell.

An exit status of zero indicates success, and a nonzero value indicates failure.


19.2 stty: Print or change terminal characteristics

stty prints or changes terminal characteristics, such as baud rate. Synopses:

stty [option] [setting]…
stty [option]

If given no line settings, stty prints the baud rate, line discipline number (on systems that support it), and line settings that have been changed from the values set by ‘stty sane’. By default, mode reading and setting are performed on the tty line connected to standard input, although this can be modified by the --file option.

stty accepts many non-option arguments that change aspects of the terminal line operation, as described below.

The program accepts the following options. Also see Common options.

-a
--all

Print all current settings in human-readable form. This option may not be used in combination with any line settings.

-F device
--file=device

Set the line opened by the file name specified in device instead of the tty line connected to standard input. This option is necessary because opening a POSIX tty requires use of the O_NONDELAY flag to prevent a POSIX tty from blocking until the carrier detect line is high if the clocal flag is not set. Hence, it is not always possible to allow the shell to open the device in the traditional manner.

-g
--save

Print all current settings in a form that can be used as an argument to another stty command to restore the current settings. This option may not be used in combination with any line settings.

Many settings can be turned off by preceding them with a ‘-’. Such arguments are marked below with “May be negated” in their description. The descriptions themselves refer to the positive case, that is, when not negated (unless stated otherwise, of course).

Some settings are not available on all POSIX systems, since they use extensions. Such arguments are marked below with “Non-POSIX” in their description. On non-POSIX systems, those or other settings also may not be available, but it’s not feasible to document all the variations: just try it and see.

stty is installed only on platforms with the POSIX terminal interface, so portable scripts should not rely on its existence on non-POSIX platforms.

An exit status of zero indicates success, and a nonzero value indicates failure.


19.2.1 Control settings

Control settings:

parenb

Generate parity bit in output and expect parity bit in input. May be negated.

parodd

Set odd parity (even if negated). May be negated.

cmspar

Use "stick" (mark/space) parity. If parodd is set, the parity bit is always 1; if parodd is not set, the parity bit is always zero. Non-POSIX. May be negated.

cs5
cs6
cs7
cs8

Set character size to 5, 6, 7, or 8 bits.

hup
hupcl

Send a hangup signal when the last process closes the tty. May be negated.

cstopb

Use two stop bits per character (one if negated). May be negated.

cread

Allow input to be received. May be negated.

clocal

Disable modem control signals. May be negated.

crtscts

Enable RTS/CTS flow control. Non-POSIX. May be negated.

cdtrdsr

Enable DTR/DSR flow control. Non-POSIX. May be negated.


19.2.2 Input settings

These settings control operations on data received from the terminal.

ignbrk

Ignore break characters. May be negated.

brkint

Make breaks cause an interrupt signal. May be negated.

ignpar

Ignore characters with parity errors. May be negated.

parmrk

Mark parity errors (with a 255-0-character sequence). May be negated.

inpck

Enable input parity checking. May be negated.

istrip

Clear high (8th) bit of input characters. May be negated.

inlcr

Translate newline to carriage return. May be negated.

igncr

Ignore carriage return. May be negated.

icrnl

Translate carriage return to newline. May be negated.

iutf8

Assume input characters are UTF-8 encoded. May be negated.

ixon

Enable XON/XOFF flow control (that is, Ctrl-S/Ctrl-Q). May be negated.

ixoff
tandem

Enable sending of stop character when the system input buffer is almost full, and start character when it becomes almost empty again. May be negated.

iuclc

Translate uppercase characters to lowercase. Non-POSIX. May be negated. Note ilcuc is not implemented, as one would not be able to issue almost any (lowercase) Unix command, after invoking it.

ixany

Allow any character to restart output (only the start character if negated). Non-POSIX. May be negated.

imaxbel

Enable beeping and not flushing input buffer if a character arrives when the input buffer is full. Non-POSIX. May be negated.


19.2.3 Output settings

These settings control operations on data sent to the terminal.

opost

Postprocess output. May be negated.

olcuc

Translate lowercase characters to uppercase. Non-POSIX. May be negated. (Note ouclc is not currently implemented.)

ocrnl

Translate carriage return to newline. Non-POSIX. May be negated.

onlcr

Translate newline to carriage return-newline. Non-POSIX. May be negated.

onocr

Do not print carriage returns in the first column. Non-POSIX. May be negated.

onlret

Newline performs a carriage return. Non-POSIX. May be negated.

ofill

Use fill (padding) characters instead of timing for delays. Non-POSIX. May be negated.

ofdel

Use ASCII DEL characters for fill instead of ASCII NUL characters. Non-POSIX. May be negated.

nl1
nl0

Newline delay style. Non-POSIX.

cr3
cr2
cr1
cr0

Carriage return delay style. Non-POSIX.

tab3
tab2
tab1
tab0

Horizontal tab delay style. Non-POSIX.

bs1
bs0

Backspace delay style. Non-POSIX.

vt1
vt0

Vertical tab delay style. Non-POSIX.

ff1
ff0

Form feed delay style. Non-POSIX.


19.2.4 Local settings

isig

Enable interrupt, quit, and suspend special characters. May be negated.

icanon

Enable erase, kill, werase, and rprnt special characters. May be negated.

iexten

Enable non-POSIX special characters. May be negated.

echo

Echo input characters. May be negated.

echoe
crterase

Echo erase characters as backspace-space-backspace. May be negated.

echok

Echo a newline after a kill character. May be negated.

echonl

Echo newline even if not echoing other characters. May be negated.

noflsh

Disable flushing after interrupt and quit special characters. May be negated.

xcase

Enable input and output of uppercase characters by preceding their lowercase equivalents with ‘\’, when icanon is set. Non-POSIX. May be negated.

tostop

Stop background jobs that try to write to the terminal. Non-POSIX. May be negated.

echoprt
prterase

Echo erased characters backward, between ‘\’ and ‘/’. Non-POSIX. May be negated.

echoctl
ctlecho

Echo control characters in hat notation (‘^c’) instead of literally. Non-POSIX. May be negated.

echoke
crtkill

Echo the kill special character by erasing each character on the line as indicated by the echoprt and echoe settings, instead of by the echoctl and echok settings. Non-POSIX. May be negated.

extproc

Enable ‘LINEMODE’, which is used to avoid echoing each character over high latency links. See also Internet RFC 1116. Non-POSIX. May be negated.

flusho

Discard output. Note this setting is currently ignored on GNU/Linux systems. Non-POSIX. May be negated.


19.2.5 Combination settings

Combination settings:

evenp
parity

Same as parenb -parodd cs7. May be negated. If negated, same as -parenb cs8.

oddp

Same as parenb parodd cs7. May be negated. If negated, same as -parenb cs8.

nl

Same as -icrnl -onlcr. May be negated. If negated, same as icrnl -inlcr -igncr onlcr -ocrnl -onlret.

ek

Reset the erase and kill special characters to their default values.

sane

Same as:

cread -ignbrk brkint -inlcr -igncr icrnl
icanon iexten echo echoe echok -echonl -noflsh
-ixoff -iutf8 -iuclc -ixany imaxbel -xcase -olcuc -ocrnl
opost -ofill onlcr -onocr -onlret nl0 cr0 tab0 bs0 vt0 ff0
isig -tostop -ofdel -echoprt echoctl echoke -extproc

and also sets all special characters to their default values.

cooked

Same as brkint ignpar istrip icrnl ixon opost isig icanon, plus sets the eof and eol characters to their default values if they are the same as the min and time characters. May be negated. If negated, same as raw.

raw

Same as:

-ignbrk -brkint -ignpar -parmrk -inpck -istrip
-inlcr -igncr -icrnl -ixon -ixoff -icanon -opost
-isig -iuclc -ixany -imaxbel -xcase min 1 time 0

May be negated. If negated, same as cooked.

cbreak

Same as -icanon. May be negated. If negated, same as icanon.

pass8

Same as -parenb -istrip cs8. May be negated. If negated, same as parenb istrip cs7.

litout

Same as -parenb -istrip -opost cs8. May be negated. If negated, same as parenb istrip opost cs7.

decctlq

Same as -ixany. Non-POSIX. May be negated.

tabs

Same as tab0. Non-POSIX. May be negated. If negated, same as tab3.

lcase
LCASE

Same as xcase iuclc olcuc. Non-POSIX. May be negated. (Used for terminals with uppercase characters only.)

crt

Same as echoe echoctl echoke.

dec

Same as echoe echoctl echoke -ixany intr ^C erase ^? kill C-u.


19.2.6 Special characters

The special characters’ default values vary from system to system. They are set with the syntax ‘name value’, where the names are listed below and the value can be given either literally, in hat notation (‘^c’), or as an integer which may start with ‘0x’ to indicate hexadecimal, ‘0’ to indicate octal, or any other digit to indicate decimal.

For GNU stty, giving a value of ^- or undef disables that special character. (This is incompatible with Ultrix stty, which uses a value of ‘u’ to disable a special character. GNU stty treats a value ‘u’ like any other, namely to set that special character to U.)

intr

Send an interrupt signal.

quit

Send a quit signal.

erase

Erase the last character typed.

kill

Erase the current line.

eof

Send an end of file (terminate the input).

eol

End the line.

eol2

Alternate character to end the line. Non-POSIX.

discard

Alternate character to toggle discarding of output. Non-POSIX.

swtch

Switch to a different shell layer. Non-POSIX.

status

Send an info signal. Not currently supported on GNU/Linux. Non-POSIX.

start

Restart the output after stopping it.

stop

Stop the output.

susp

Send a terminal stop signal.

dsusp

Send a terminal stop signal after flushing the input. Non-POSIX.

rprnt

Redraw the current line. Non-POSIX.

werase

Erase the last word typed. Non-POSIX.

lnext

Enter the next character typed literally, even if it is a special character. Non-POSIX.


19.2.7 Special settings

min n

Set the minimum number of characters that will satisfy a read until the time value has expired, when -icanon is set.

time n

Set the number of tenths of a second before reads time out if the minimum number of characters have not been read, when -icanon is set.

ispeed n

Set the input speed to n.

ospeed n

Set the output speed to n.

rows n

Tell the tty kernel driver that the terminal has n rows. Non-POSIX.

cols n
columns n

Tell the kernel that the terminal has n columns. Non-POSIX.

drain

Apply settings after first waiting for pending output to be transmitted. This is enabled by default for GNU stty. It is useful to disable this option in cases where the system may be in a state where serial transmission is not possible. For example, if the system has received the ‘DC3’ character with ixon (software flow control) enabled, then stty would block without -drain being specified. May be negated. Non-POSIX.

size

Print the number of rows and columns that the kernel thinks the terminal has. (Systems that don’t support rows and columns in the kernel typically use the environment variables LINES and COLUMNS instead; however, GNU stty does not know anything about them.) Non-POSIX.

line n

Use line discipline n. Non-POSIX.

speed

Print the terminal speed.

n

Set the input and output speeds to n. n can be one of: 0 50 75 110 134 134.5 150 200 300 600 1200 1800 2400 4800 9600 19200 38400 exta extb. exta is the same as 19200; extb is the same as 38400. Many systems, including GNU/Linux, support higher speeds. The stty command includes support for speeds of 57600, 115200, 230400, 460800, 500000, 576000, 921600, 1000000, 1152000, 1500000, 2000000, 2500000, 3000000, 3500000, or 4000000 where the system supports these. 0 hangs up the line if -clocal is set.


19.3 printenv: Print all or some environment variables

printenv prints environment variable values. Synopsis:

printenv [option] [variable]…

If no variables are specified, printenv prints the value of every environment variable. Otherwise, it prints the value of each variable that is set, and nothing for those that are not set.

The program accepts the following option. Also see Common options.

-0
--null

Output a zero byte (ASCII NUL) at the end of each line, rather than a newline. This option enables other programs to parse the output even when that output would contain data with embedded newlines.

Exit status:

0 if all variables specified were found
1 if at least one specified variable was not found
2 if a write error occurred

19.4 tty: Print file name of terminal on standard input

tty prints the file name of the terminal connected to its standard input. It prints ‘not a tty’ if standard input is not a terminal. Synopsis:

tty [option]…

The program accepts the following option. Also see Common options.

-s
--silent
--quiet

Print nothing; only return an exit status.

Exit status:

0 if standard input is a terminal
1 if standard input is a non-terminal file
2 if given incorrect arguments
3 if a write error occurs

20 User information

This section describes commands that print user-related information: logins, groups, and so forth.


20.1 id: Print user identity

id prints information about the given user, or the process running it if no user is specified. Synopsis:

id [option]… [user]…

user can be either a user ID or a name, with name look-up taking precedence unless the ID is specified with a leading ‘+’. See chown, chgrp, chroot, id: Disambiguating user names and IDs.

By default, it prints the real user ID, real group ID, effective user ID if different from the real user ID, effective group ID if different from the real group ID, and supplemental group IDs. In addition, if SELinux is enabled and the POSIXLY_CORRECT environment variable is not set, then print ‘context=c’, where c is the security context.

Each of these numeric values is preceded by an identifying string and followed by the corresponding user or group name in parentheses.

The options cause id to print only part of the above information. Also see Common options.

-g
--group

Print only the group ID.

-G
--groups

Print only the group ID and the supplementary groups.

-n
--name

Print the user or group name instead of the ID number. Requires -u, -g, or -G.

-r
--real

Print the real, instead of effective, user or group ID. Requires -u, -g, or -G.

-u
--user

Print only the user ID.

-Z
--context

Print only the security context of the process, which is generally the user’s security context inherited from the parent process. If neither SELinux or SMACK is enabled then print a warning and set the exit status to 1.

-z
--zero

Delimit output items with ASCII NUL characters. This option is not permitted when using the default format. When multiple users are specified, and the --groups option is also in effect, groups are delimited with a single NUL character, while users are delimited with two NUL characters.

Example:

$ id -Gn --zero
users <NUL> devs <NUL>

Primary and supplementary groups for a process are normally inherited from its parent and are usually unchanged since login. This means that if you change the group database after logging in, id will not reflect your changes within your existing login session. Running id with a user argument causes the user and group database to be consulted afresh, and so will give a different result.

An exit status of zero indicates success, and a nonzero value indicates failure.


20.2 logname: Print current login name

logname prints the calling user’s name, as found in a system-maintained file (often /var/run/utmp or /etc/utmp), and exits with a status of 0. If there is no entry for the calling process, logname prints an error message and exits with a status of 1.

The only options are --help and --version. See Common options.

An exit status of zero indicates success, and a nonzero value indicates failure.


20.3 whoami: Print effective user name

whoami prints the user name associated with the current effective user ID. It is equivalent to the command ‘id -un’.

The only options are --help and --version. See Common options.

An exit status of zero indicates success, and a nonzero value indicates failure.


20.4 groups: Print group names a user is in

groups prints the names of the primary and any supplementary groups for each given username, or the current process if no names are given. If more than one name is given, the name of each user is printed before the list of that user’s groups and the user name is separated from the group list by a colon. Synopsis:

groups [username]…

The group lists are equivalent to the output of the command ‘id -Gn’.

The only options are --help and --version. See Common options.

Primary and supplementary groups for a process are normally inherited from its parent and are usually unchanged since login. This means that if you change the group database after logging in, groups will not reflect your changes within your existing login session. Running groups with a list of users causes the user and group database to be consulted afresh, and so will give a different result.

An exit status of zero indicates success, and a nonzero value indicates failure.


20.5 users: Print login names of users currently logged in

users prints on a single line a blank-separated list of user names of users currently logged in to the current host. Each user name corresponds to a login session, so if a user has more than one login session, that user’s name will appear the same number of times in the output. Synopsis:

users [file]

With no file argument, users extracts its information from a system-maintained file (often /var/run/utmp or /etc/utmp). If a file argument is given, users uses that file instead. A common choice is /var/log/wtmp.

The only options are --help and --version. See Common options.

The users command is installed only on platforms with the POSIX <utmpx.h> include file or equivalent, so portable scripts should not rely on its existence on non-POSIX platforms.

An exit status of zero indicates success, and a nonzero value indicates failure.


20.6 who: Print who is currently logged in

who prints information about users who are currently logged on. Synopsis:

who [option] [file] [am i]

If given no non-option arguments, who prints the following information for each user currently logged on: login name, terminal line, login time, and remote hostname or X display.

If given one non-option argument, who uses that instead of a default system-maintained file (often /var/run/utmp or /etc/utmp) as the name of the file containing the record of users logged on. /var/log/wtmp is commonly given as an argument to who to look at who has previously logged on.

If given two non-option arguments, who prints only the entry for the user running it (determined from its standard input), preceded by the hostname. Traditionally, the two arguments given are ‘am i’, as in ‘who am i’.

Timestamps are listed according to the time zone rules specified by the TZ environment variable, or by the system default rules if TZ is not set. See Specifying the Time Zone with TZ in The GNU C Library Reference Manual.

The program accepts the following options. Also see Common options.

-a
--all

Same as ‘-b -d --login -p -r -t -T -u’.

-b
--boot

Print the date and time of last system boot.

-d
--dead

Print information corresponding to dead processes.

-H
--heading

Print a line of column headings.

-l
--login

List only the entries that correspond to processes via which the system is waiting for a user to login. The user name is always ‘LOGIN’.

--lookup

Attempt to canonicalize hostnames found in utmp through a DNS lookup. This is not the default because it can cause significant delays on systems with automatic dial-up internet access.

-m

Same as ‘who am i’.

-p
--process

List active processes spawned by init.

-q
--count

Print only the login names and the number of users logged on. Overrides all other options.

-r
--runlevel

Print the current (and maybe previous) run-level of the init process.

-s

Ignored; for compatibility with other versions of who.

-t
--time

Print last system clock change.

-u

After the login time, print the number of hours and minutes that the user has been idle. ‘.’ means the user was active in the last minute. ‘old’ means the user has been idle for more than 24 hours.

-w
-T
--mesg
--message
--writable

After each login name print a character indicating the user’s message status:

+’ allowing write messages
‘-’ disallowing write messages
‘?’ cannot find terminal device

The who command is installed only on platforms with the POSIX <utmpx.h> include file or equivalent, so portable scripts should not rely on its existence on non-POSIX platforms.

An exit status of zero indicates success, and a nonzero value indicates failure.


21 System context

This section describes commands that print or change system-wide information.


21.1 date: Print or set system date and time

Synopses:

date [option]… [+format]
date [-u|--utc|--universal] [ MMDDhhmm[[CC]YY][.ss] ]

The date command displays the date and time. With the --set (-s) option, or with ‘MMDDhhmm[[CC]YY][.ss]’, it sets the date and time.

Invoking date with no format argument is equivalent to invoking it with a default format that depends on the LC_TIME locale category. In the default C locale, this format is ‘'+%a %b %e %H:%M:%S %Z %Y'’, so the output looks like ‘Thu Jul  9 17:00:00 EDT 2020’.

Normally, date uses the time zone rules indicated by the TZ environment variable, or the system default rules if TZ is not set. See Specifying the Time Zone with TZ in The GNU C Library Reference Manual.

If given an argument that starts with a ‘+’, date prints the current date and time (or the date and time specified by the --date option, see below) in the format defined by that argument, which is similar to that of the strftime function. Except for conversion specifiers, which start with ‘%’, characters in the format string are printed unchanged. The conversion specifiers are described below.

An exit status of zero indicates success, and a nonzero value indicates failure.


21.1.1 Time conversion specifiers

date conversion specifiers related to times.

%H

hour (‘00’…‘23’)

%I

hour (‘01’…‘12’)

%k

hour, space padded (‘ 0’…‘23’); equivalent to ‘%_H’. This is a GNU extension.

%l

hour, space padded (‘ 1’…‘12’); equivalent to ‘%_I’. This is a GNU extension.

%M

minute (‘00’…‘59’)

%N

nanoseconds (‘000000000’…‘999999999’). This is a GNU extension.

%p

locale’s equivalent of either ‘AM’ or ‘PM’; blank in many locales. Noon is treated as ‘PM’ and midnight as ‘AM’.

%P

like ‘%p’, except lower case. This is a GNU extension.

%r

locale’s 12-hour clock time (e.g., ‘11:11:04 PM’)

%R

24-hour hour and minute. Same as ‘%H:%M’.

%s

seconds since the Epoch, i.e., since 1970-01-01 00:00 UTC. Leap seconds are not counted unless leap second support is available. See %s-examples, for examples. This is a GNU extension.

%S

second (‘00’…‘60’). This may be ‘60’ if leap seconds are supported.

%T

24-hour hour, minute, and second. Same as ‘%H:%M:%S’.

%X

locale’s time representation (e.g., ‘23:13:48’)

%z

Four-digit numeric time zone, e.g., ‘-0600’ or ‘+0530’, or ‘-0000’ if no time zone is determinable. This value reflects the numeric time zone appropriate for the current time, using the time zone rules specified by the TZ environment variable. A time zone is not determinable if its numeric offset is zero and its abbreviation begins with ‘-’. The time (and optionally, the time zone rules) can be overridden by the --date option.

%:z

Numeric time zone with ‘:’, e.g., ‘-06:00’ or ‘+05:30’), or ‘-00:00’ if no time zone is determinable. This is a GNU extension.

%::z

Numeric time zone to the nearest second with ‘:’ (e.g., ‘-06:00:00’ or ‘+05:30:00’), or ‘-00:00:00’ if no time zone is determinable. This is a GNU extension.

%:::z

Numeric time zone with ‘:’ using the minimum necessary precision (e.g., ‘-06’, ‘+05:30’, or ‘-04:56:02’), or ‘-00’ if no time zone is determinable. This is a GNU extension.

%Z

alphabetic time zone abbreviation (e.g., ‘EDT’), or nothing if no time zone is determinable. See ‘%z’ for how it is determined.


21.1.2 Date conversion specifiers

date conversion specifiers related to dates.

%a

locale’s abbreviated weekday name (e.g., ‘Sun’)

%A

locale’s full weekday name, variable length (e.g., ‘Sunday’)

%b

locale’s abbreviated month name (e.g., ‘Jan’)

%B

locale’s full month name, variable length (e.g., ‘January’)

%c

locale’s date and time (e.g., ‘Thu Mar  3 23:05:25 2020’)

%C

century. This is like ‘%Y’, except the last two digits are omitted. For example, it is ‘20’ if ‘%Y’ is ‘2019’, and is ‘-0’ if ‘%Y’ is ‘-001’. It is normally at least two characters, but it may be more.

%d

day of month (e.g., ‘01’)

%D

date; same as ‘%m/%d/%y

%e

day of month, space padded; same as ‘%_d

%F

full date in ISO 8601 format; like ‘%+4Y-%m-%d’ except that any flags or field width override the ‘+’ and (after subtracting 6) the ‘4’. This is a good choice for a date format, as it is standard and is easy to sort in the usual case where years are in the range 0000…9999.

%g

year corresponding to the ISO week number, but without the century (range ‘00’ through ‘99’). This has the same format and value as ‘%y’, except that if the ISO week number (see ‘%V’) belongs to the previous or next year, that year is used instead.

%G

year corresponding to the ISO week number. This has the same format and value as ‘%Y’, except that if the ISO week number (see ‘%V’) belongs to the previous or next year, that year is used instead. It is normally useful only if ‘%V’ is also used; for example, the format ‘%G-%m-%d’ is probably a mistake, since it combines the ISO week number year with the conventional month and day.

%h

same as ‘%b

%j

day of year (‘001’…‘366’)

%m

month (‘01’…‘12’)

%q

quarter of year (‘1’…‘4’)

%u

day of week (‘1’…‘7’) with ‘1’ corresponding to Monday

%U

week number of year, with Sunday as the first day of the week (‘00’…‘53’). Days in a new year preceding the first Sunday are in week zero.

%V

ISO week number, that is, the week number of year, with Monday as the first day of the week (‘01’…‘53’). If the week containing January 1 has four or more days in the new year, then it is considered week 1; otherwise, it is week 53 of the previous year, and the next week is week 1. (See the ISO 8601 standard.)

%w

day of week (‘0’…‘6’) with 0 corresponding to Sunday

%W

week number of year, with Monday as first day of week (‘00’…‘53’). Days in a new year preceding the first Monday are in week zero.

%x

locale’s date representation (e.g., ‘12/31/99’)

%y

last two digits of year (‘00’…‘99’)

%Y

year. This is normally at least four characters, but it may be more. Year ‘0000’ precedes year ‘0001’, and year ‘-001’ precedes year ‘0000’.


21.1.3 Literal conversion specifiers

date conversion specifiers that produce literal strings.

%%

a literal %

%n

a newline

%t

a horizontal tab


21.1.4 Padding and other flags

Unless otherwise specified, date normally pads numeric fields with zeros, so that, for example, numeric months are always output as two digits. Most numeric fields are padded on the left. However, nanoseconds are padded on the right since they are commonly used after decimal points in formats like ‘%s.%-N’. Also, seconds since the Epoch are not padded since there is no natural width for them.

The following optional flags can appear after the ‘%’:

-

(hyphen) Do not pad the field; useful if the output is intended for human consumption. This is a GNU extension. As a special case, ‘%-N’ outputs only enough trailing digits to not lose information, assuming that the timestamp’s resolution is the same as the current hardware clock. For example, if the hardware clock resolution is 1 microsecond, ‘%s.%-N’ outputs something like ‘1640890100.395710’.

_

(underscore) Pad with spaces; useful if you need a fixed number of characters in the output, but zeros are too distracting. This is a GNU extension.

0

(zero) Pad with zeros even if the conversion specifier would normally pad with spaces.

+

Pad with zeros, like ‘0’. In addition, precede any year number with ‘+’ if it exceeds 9999 or if its field width exceeds 4; similarly, precede any century number with ‘+’ if it exceeds 99 or if its field width exceeds 2. This supports ISO 8601 formats for dates far in the future; for example, the command date --date=12019-02-25 +%+13F outputs the string ‘+012019-02-25’.

^

Use upper case characters if possible. This is a GNU extension.

#

Use opposite case characters if possible. A field that is normally upper case becomes lower case, and vice versa. This is a GNU extension.

Here are some examples of padding:

date +%d/%m -d "Feb 1"
⇒ 01/02
date +%-d/%-m -d "Feb 1"
⇒ 1/2
date +%_d/%_m -d "Feb 1"
⇒  1/ 2

You can optionally specify the field width (after any flag, if present) as a decimal number. If the natural size of the output of the field has less than the specified number of characters, the result is normally written right adjusted and padded to the given size. For example, ‘%9B’ prints the right adjusted month name in a field of width 9. Nanoseconds are left adjusted, and are truncated or padded to the field width.

An optional modifier can follow the optional flag and width specification. The modifiers are:

E

Use the locale’s alternate representation for date and time. This modifier applies to the ‘%c’, ‘%C’, ‘%x’, ‘%X’, ‘%y’ and ‘%Y’ conversion specifiers. In a Japanese locale, for example, ‘%Ex’ might yield a date format based on the Japanese Emperors’ reigns.

O

Use the locale’s alternate numeric symbols for numbers. This modifier applies only to numeric conversion specifiers.

If the format supports the modifier but no alternate representation is available, it is ignored.

POSIX specifies the behavior of flags and field widths only for ‘%C’, ‘%F’, ‘%G’, and ‘%Y’ (all without modifiers), and requires a flag to be present if and only if a field width is also present. Other combinations of flags, field widths and modifiers are GNU extensions.


21.1.5 Setting the time

You must have appropriate privileges to set the system clock. For changes to persist across a reboot, the hardware clock may need to be updated from the system clock, which might not happen automatically on your system.

To set the clock, you can use the --set (-s) option (see Options for date). To set the clock without using GNU extensions, you can give date an argument of the form ‘MMDDhhmm[[CC]YY][.ss]’ where each two-letter component stands for two digits with the following meanings:

MM

month

DD

day within month

hh

hour

mm

minute

CC

first two digits of year (optional)

YY

last two digits of year (optional)

ss

second (optional)

Note, the --date and --set options may not be used with an argument in the above format. The --universal option may be used with such an argument to indicate that the specified date and time are relative to Universal Time rather than to the local time zone.


21.1.6 Options for date

The program accepts the following options. Also see Common options. Except for -u, these options are all GNU extensions to POSIX.

-d datestr
--date=datestr

Display the date and time specified in datestr instead of the current date and time. datestr can be in almost any common format. It can contain month names, time zones, ‘am’ and ‘pm’, ‘yesterday’, etc. For example, --date="2020-07-21 14:19:13.489392193 +0530" specifies the instant of time that is 489,392,193 nanoseconds after July 21, 2020 at 2:19:13 PM in a time zone that is 5 hours and 30 minutes east of UTC.
Note: input currently must be in locale independent format. E.g., the LC_TIME=C below is needed to print back the correct date in many locales:

date -d "$(LC_TIME=C date)"

See Date input formats.

--debug

Annotate the parsed date, display the effective time zone, and warn about potential misuse.

-f datefile
--file=datefile

Parse each line in datefile as with -d and display the resulting date and time. If datefile is ‘-’, use standard input. This is useful when you have many dates to process, because the system overhead of starting up the date executable many times can be considerable.

-I[timespec]
--iso-8601[=timespec]

Display the date using an ISO 8601 format, ‘%Y-%m-%d’.

The argument timespec specifies the number of additional terms of the time to include. It can be one of the following:

auto

Print just the date. This is the default if timespec is omitted.

hours

Append the hour of the day to the date.

minutes

Append the hours and minutes.

seconds

Append the hours, minutes and seconds.

ns

Append the hours, minutes, seconds and nanoseconds.

If showing any time terms, then include the time zone using the format ‘%:z’. This format is always suitable as input for the --date (-d) and --file (-f) options, regardless of the current locale.

-r file
--reference=file

Display the date and time of the last modification of file, instead of the current date and time.

--resolution

Display the timestamp resolution instead of the time. Current clock timestamps that are output by date are integer multiples of the timestamp resolution. With this option, the format defaults to ‘%s.%N’. For example, if the clock resolution is 1 millsecond, the output is:

0.001000000
-R
--rfc-email

Display the date and time using the format ‘%a, %d %b %Y %H:%M:%S %z’, evaluated in the C locale so abbreviations are always in English. For example:

Mon, 09 Jul 2020 17:00:00 -0400

This format conforms to Internet RFCs 5322, 2822 and 822, the current and previous standards for Internet email. For compatibility with older versions of date, --rfc-2822 and --rfc-822 are aliases for --rfc-email.

--rfc-3339=timespec

Display the date using a format specified by Internet RFC 3339. This is like --iso-8601, except that a space rather than a ‘T’ separates dates from times, and a period rather than a comma separates seconds from subseconds. This format is always suitable as input for the --date (-d) and --file (-f) options, regardless of the current locale.

The argument timespec specifies how much of the time to include. It can be one of the following:

date

Print just the full-date, e.g., ‘2020-07-21’. This is equivalent to the format ‘%Y-%m-%d’.

seconds

Print the full-date and full-time separated by a space, e.g., ‘2020-07-21 04:30:37+05:30’. The output ends with a numeric time-offset; here the ‘+05:30’ means that local time is five hours and thirty minutes east of UTC. This is equivalent to the format ‘%Y-%m-%d %H:%M:%S%:z’.

ns

Like ‘seconds’, but also print nanoseconds, e.g., ‘2020-07-21 04:30:37.998458565+05:30’. This is equivalent to the format ‘%Y-%m-%d %H:%M:%S.%N%:z’.

-s datestr
--set=datestr

Set the date and time to datestr. See -d above. See also Setting the time.

-u
--utc
--universal

Use Universal Time by operating as if the TZ environment variable were set to the string ‘UTC0’. UTC stands for Coordinated Universal Time, established in 1960. Universal Time is often called “Greenwich Mean Time” (GMT) for historical reasons. Typically, systems ignore leap seconds and thus implement an approximation to UTC rather than true UTC.


21.1.7 Examples of date

Here are a few examples. Also see the documentation for the -d option in the previous section.

  • To print the date of the day before yesterday:
    date --date='2 days ago'
    
  • To print the date of the day three months and one day hence:
    date --date='3 months 1 day'
    
  • To print the day of year of Christmas in the current year:
    date --date='25 Dec' +%j
    
  • To print the current full month name and the day of the month:
    date '+%B %d'
    

    But this may not be what you want because for the first nine days of the month, the ‘%d’ expands to a zero-padded two-digit field, for example ‘date -d 1may '+%B %d'’ will print ‘May 01’.

  • To print a date without the leading zero for one-digit days of the month, you can use the (GNU extension) ‘-’ flag to suppress the padding altogether:
    date -d 1may '+%B %-d'
    
  • To print the current date and time in the format required by many non-GNU versions of date when setting the system clock:
    date +%m%d%H%M%Y.%S
    
  • To set the system clock forward by two minutes:
    date --set='+2 minutes'
    
  • To print the date in Internet RFC 5322 format, use ‘date --rfc-email’. Here is some example output:
    Tue, 09 Jul 2020 19:00:37 -0400
    
  • To convert a date string to the number of seconds since the Epoch (which is 1970-01-01 00:00 UTC), use the --date option with the ‘%s’ format. That can be useful in sorting and/or graphing and/or comparing data by date. The following command outputs the number of the seconds since the Epoch for the time two minutes after the Epoch:
    date --date='1970-01-01 00:02:00 +0000' +%s
    120
    

    If you do not specify time zone information in the date string, date uses your computer’s idea of the time zone when interpreting the string. For example, if your computer’s time zone is that of Cambridge, Massachusetts, which was then 5 hours (i.e., 18,000 seconds) behind UTC:

    # local time zone used
    date --date='1970-01-01 00:02:00' +%s
    18120
    
  • If you’re sorting or graphing dated data, your raw date values may be represented as seconds since the Epoch. But few people can look at the date ‘1577836800’ and casually note “Oh, that’s the first second of the year 2020 in Greenwich, England.”
    date --date='2020-01-01 UTC' +%s
    1577836800
    

    An alternative is to use the --utc (-u) option. Then you may omit ‘UTC’ from the date string. Although this produces the same result for ‘%s’ and many other format sequences, with a time zone offset different from zero, it would give a different result for zone-dependent formats like ‘%z’.

    date -u --date=2020-07-21 +%s
    1595289600
    

    To convert such an unwieldy number of seconds back to a more readable form, use a command like this:

    date -d @1595289600 +"%F %T %z"
    2020-07-20 20:00:00 -0400
    

    Often it is better to output UTC-relative date and time:

    date -u -d @1595289600 +"%F %T %z"
    2020-07-21 00:00:00 +0000
    
  • Typically the seconds count omits leap seconds, but some systems are exceptions. Because leap seconds are not predictable, the mapping between the seconds count and a future timestamp is not reliable on the atypical systems that include leap seconds in their counts.

    Here is how the two kinds of systems handle the leap second at the end of the year 2016:

    # Typical systems ignore leap seconds:
    date --date='2016-12-31 23:59:59 +0000' +%s
    1483228799
    date --date='2016-12-31 23:59:60 +0000' +%s
    date: invalid date '2016-12-31 23:59:60 +0000'
    date --date='2017-01-01 00:00:00 +0000' +%s
    1483228800
    
    # Atypical systems count leap seconds:
    date --date='2016-12-31 23:59:59 +0000' +%s
    1483228825
    date --date='2016-12-31 23:59:60 +0000' +%s
    1483228826
    date --date='2017-01-01 00:00:00 +0000' +%s
    1483228827
    

21.2 arch: Print machine hardware name

arch prints the machine hardware name, and is equivalent to ‘uname -m’. Synopsis:

arch [option]

The program accepts the Common options only.

arch is not installed by default, so portable scripts should not rely on its existence.

An exit status of zero indicates success, and a nonzero value indicates failure.


21.3 nproc: Print the number of available processors

Print the number of processing units available to the current process, which may be less than the number of online processors. If this information is not accessible, then print the number of processors installed. If the OMP_NUM_THREADS or OMP_THREAD_LIMIT environment variables are set, then they will determine the minimum and maximum returned value respectively. The result is guaranteed to be greater than zero. Synopsis:

nproc [option]

The program accepts the following options. Also see Common options.

--all

Print the number of installed processors on the system, which may be greater than the number online or available to the current process. The OMP_NUM_THREADS or OMP_THREAD_LIMIT environment variables are not honored in this case.

--ignore=number

If possible, exclude this number of processing units.

An exit status of zero indicates success, and a nonzero value indicates failure.


21.4 uname: Print system information

uname prints information about the machine and operating system it is run on. If no options are given, uname acts as if the -s option were given. Synopsis:

uname [option]…

If multiple options or -a are given, the selected information is printed in this order:

kernel-name nodename kernel-release kernel-version
machine processor hardware-platform operating-system

The information may contain internal spaces, so such output cannot be parsed reliably. In the following example, kernel-version is ‘#1 SMP Fri Jul 17 17:18:38 UTC 2020’:

uname -a
⇒ Linux dumdum.example.org 5.9.16-200.fc33.x86_64 #1 SMP Mon Dec 21 14:08:22 UTC 2020 x86_64 x86_64 x86_64 GNU/Linux

The program accepts the following options. Also see Common options.

-a
--all

Print all of the below information, except omit the processor type and the hardware platform name if they are unknown.

-i
--hardware-platform

Print the hardware platform name (sometimes called the hardware implementation). Print ‘unknown’ if this information is not available. Note this is non-portable (even across GNU/Linux distributions).

-m
--machine

Print the machine hardware name (sometimes called the hardware class or hardware type).

-n
--nodename

Print the network node hostname.

-p
--processor

Print the processor type (sometimes called the instruction set architecture or ISA). Print ‘unknown’ if this information is not available. Note this is non-portable (even across GNU/Linux distributions).

-o
--operating-system

Print the name of the operating system.

-r
--kernel-release

Print the kernel release.

-s
--kernel-name

Print the kernel name. POSIX 1003.1-2001 (see Standards conformance) calls this “the implementation of the operating system”, because the POSIX specification itself has no notion of “kernel”. The kernel name might be the same as the operating system name printed by the -o or --operating-system option, but it might differ. Some operating systems (e.g., FreeBSD, HP-UX) have the same name as their underlying kernels; others (e.g., GNU/Linux, Solaris) do not.

-v
--kernel-version

Print the kernel version.

An exit status of zero indicates success, and a nonzero value indicates failure.


21.5 hostname: Print or set system name

With no arguments, hostname prints the name of the current host system. With one argument, it sets the current host name to the specified string. You must have appropriate privileges to set the host name. Synopsis:

hostname [name]

The only options are --help and --version. See Common options.

hostname is not installed by default, and other packages also supply a hostname command, so portable scripts should not rely on its existence or on the exact behavior documented above.

An exit status of zero indicates success, and a nonzero value indicates failure.


21.6 hostid: Print numeric host identifier

hostid prints the numeric identifier of the current host in hexadecimal. This command accepts no arguments. The only options are --help and --version. See Common options.

For example, here’s what it prints on one system I use:

$ hostid
1bac013d

On that system, the 32-bit quantity happens to be closely related to the system’s Internet address, but that isn’t always the case.

hostid is installed only on systems that have the gethostid function, so portable scripts should not rely on its existence.

An exit status of zero indicates success, and a nonzero value indicates failure.


21.7 uptime: Print system uptime and load

uptime prints the current time, the system’s uptime, the number of logged-in users and the current load average.

If an argument is specified, it is used as the file to be read to discover how many users are logged in. If no argument is specified, a system default is used (uptime --help indicates the default setting).

The only options are --help and --version. See Common options.

For example, here’s what it prints right now on one system I use:

$ uptime
 14:07  up   3:35,  3 users,  load average: 1.39, 1.15, 1.04

The precise method of calculation of load average varies somewhat between systems. Some systems calculate it as the average number of runnable processes over the last 1, 5 and 15 minutes, but some systems also include processes in the uninterruptible sleep state (that is, those processes which are waiting for device I/O). The Linux kernel includes uninterruptible processes.

uptime is installed only on platforms with infrastructure for obtaining the boot time, and other packages also supply an uptime command, so portable scripts should not rely on its existence or on the exact behavior documented above.

An exit status of zero indicates success, and a nonzero value indicates failure.


22 SELinux context

This section describes commands for operations with SELinux contexts.


22.1 chcon: Change SELinux context of file

chcon changes the SELinux security context of the selected files. Synopses:

chcon [option]… context file…
chcon [option]… [-u user] [-r role] [-l range] [-t type] file…
chcon [option]… --reference=rfile file

Change the SELinux security context of each file to context. With --reference, change the security context of each file to that of rfile.

The program accepts the following options. Also see Common options.

--dereference

Do not affect symbolic links but what they refer to; this is the default.

-h
--no-dereference

Affect the symbolic links themselves instead of any referenced file.

--reference=rfile

Use rfile’s security context rather than specifying a context value.

-R
--recursive

Operate on files and directories recursively.

--preserve-root

Refuse to operate recursively on the root directory, /, when used together with the --recursive option. See Treating / specially.

--no-preserve-root

Do not treat the root directory, /, specially when operating recursively; this is the default. See Treating / specially.

-H

If --recursive (-R) is specified and a command line argument is a symbolic link to a directory, traverse it. See Traversing symlinks.

-L

In a recursive traversal, traverse every symbolic link to a directory that is encountered. See Traversing symlinks.

-P

Do not traverse any symbolic links. This is the default if none of -H, -L, or -P is specified. See Traversing symlinks.

-v
--verbose

Output a diagnostic for every file processed.

-u user
--user=user

Set user user in the target security context.

-r role
--role=role