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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 standard input, or for standard output if that is clear from the context. For example, `sort -' reads from standard input, and is equivalent to plain `sort', and `tee -' writes an extra copy of its input to standard output. Unless otherwise specified, `-' can appear as any operand that requires a file name.


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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, printenv, sort, stdbuf, su, test, timeout, tty.


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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.


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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 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 IEC 60027-2 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. (`Zi' is a GNU extension to IEC 60027-2.)

`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. (`Yi' is a GNU extension to IEC 60027-2.)

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'.


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2.4 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'.


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2.5 chown and chgrp: Disambiguating user names and IDs

Since the owner and group arguments to chown and chgrp 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 chown and chgrp 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 `F's owner ID to 1000--not what you intended.

GNU chown and chgrp 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 /

GNU chown and chgrp skip the name look-up process 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.


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2.6 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 pseudorandom 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 pseudorandom 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.


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2.7 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.


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2.8 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.


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2.9 Traversing symlinks

The following options modify how chown and chgrp traverse a hierarchy when the `--recursive' (`-R') option is also specified. If more than one of the following options is specified, only the final one takes effect. These options specify whether processing a symbolic link to a directory entails operating on just the symbolic link or on all files in the hierarchy rooted at that directory.

These options are independent of `--dereference' and `--no-dereference' (`-h'), which control whether to modify a symlink or its referent.

`-H'

If `--recursive' (`-R') is specified and a command line argument is a symbolic link to a directory, traverse it.

`-L'

In a recursive traversal, traverse every symbolic link to a directory that is encountered.

`-P'

Do not traverse any symbolic links. This is the default if none of `-H', `-L', or `-P' is specified.


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2.10 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-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 `/'.


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2.11 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.


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2.12 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 starting with 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.

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. Two values are currently supported for _POSIX2_VERSION: `199209' stands for POSIX 1003.2-1992, and `200112' stands for POSIX 1003.1-2001. For example, if you have a newer system but are running software that assumes an older version of POSIX and uses `sort +1' or `tail +10', you can work around any compatibility problems by setting `_POSIX2_VERSION=199209' in your environment.


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