info > Common options

📘 2 Common options

🚀 Quick Reference

Use CaseCommandDescription
📖 Display helpprogram --helpPrint usage message and exit
📌 Display versionprogram --versionPrint version number and exit
🔀 Delimit optionsprogram --Treat subsequent arguments as operands
💾 Make backupsprogram -bMake backup of files before overwriting
📏 Set block sizeprogram --block-size=SIZESet display block size
📊 Human-readable sizesprogram -hDisplay sizes in human-readable format
📡 Specify signalprogram -s SIGNALSend specified signal
🎲 Random sourceprogram --random-source=FILEUse FILE as source of random data
📂 Target directoryprogram -t DIRECTORYUse DIRECTORY as destination
🚫 No target directoryprogram -TDon't treat last operand as directory
🔚 Strip trailing slashesprogram --strip-trailing-slashesRemove trailing slashes from SOURCE
🔗 Traverse symlinksprogram -LTraverse symbolic links to directories
🛡️ Preserve rootprogram --preserve-rootDo not operate on '/'
⚠️ No preserve rootprogram --no-preserve-rootAllow operation on '/'
🔄 Multi-call invocationcoreutils --coreutils-prog=PROGRAMRun individual utility

📋 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.

ℹ️ 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.

📄 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.

📏 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.

📌 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 (http://www.bipm.org/en/publications/si-brochure/chapter3.html) for decimal multiples and with the ISO/IEC 80000-13 (formerly IEC 60027-2) prefixes (https://physics.nist.gov/cuu/Units/binary.html) 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.

📌 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. *Note (libc)Parsing of Floats::.

📌 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:

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

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

📡 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 F's 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:

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.

🔗 2.10 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.

⚠️ 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.

ℹ️ 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.

ℹ️ 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 ...

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

Common options □✕
📘 2 Common options 🚀 Quick Reference
🔚 2.1 Exit status 💾 2.2 Backup options 📏 2.3 Block size 🔢 2.4 Floating point numbers 📡 2.5 Signal specifications 👤 2.6 chown, chgrp, chroot, id: Disambiguating user names and IDs 🎲 2.7 Sources of random data 📂 2.8 Target directory 🔚 2.9 Trailing slashes 🔗 2.10 Traversing symlinks ⚠️ 2.11 Treating '/' specially ⚙️ 2.12 Special built-in utilities ✅ 2.13 Standards conformance 🔄 2.14 'coreutils': Multi-call program

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