xargs/man – Foxwiki

xargs - build and execute command lines from standard input

SYNOPSIS

xargs [options] [command [initial-arguments]]

DESCRIPTION

This manual page documents the GNU version of xargs

xargs reads items from the standard input, delimited by blanks (which can be protected with double or single quotes or a backslash) or newlines, and executes the command (default is echo) one or more times with any initial-arguments followed by items read from standard input
Blank lines on the standard input are ignored

The command line for command is built up until it reaches a system-defined limit (unless the -n and -L options are used)

The specified command will be invoked as many times as necessary to use up the list of input items
In general, there will be many fewer invocations of command than there were items in the input
This will normally have significant performance benefits
Some commands can usefully be executed in parallel too; see the -P option

Because Unix filenames can contain blanks and newlines, this default behaviour is often problematic; filenames containing blanks and/or newlines are incorrectly processed by xargs

In these situations it is better to use the -0 option, which prevents such problems
When using this option you will need to ensure that the program which produces the input for xargs also uses a null character as a separator
If that program is GNU find for example, the -print0 option does this for you

If any invocation of the command exits with a status of 255, xargs will stop immediately without reading any further input

An error message is issued on stderr when this happens

OPTIONS

Unix	GNU	Parameter	Beschreibung
-0	--nulln		Input items are terminated by a null character instead of by whitespace, and the quotes and backslash are not special (every character is taken literally) Disables the end-of-file string, which is treated like any other argument Useful when input items might contain white space, quote marks, or backslashes The GNU find -print0 option produces input suitable for this mode
-a	--arg-file=	file	Read items from file instead of standard input If you use this option, stdin remains unchanged when commands are run Otherwise, stdin is redirected from /dev/null delimiter=delim, -d delim Input items are terminated by the specified character The specified delimiter may be a single character, a C-style character escape such as \n, or an octal or hexadecimal escape code Octal and hexadecimal escape codes are understood as for the printf command Multibyte characters are not supported When processing the input, quotes and backslash are not special; every character in the input is taken literally The -d option disables any end-of-file string, which is treated like any other argument You can use this option when the input consists of simply newline-separated items, although it is almost always better to design your program to use --null where this is possible
-E		eof-str	Set the end-of-file string to eof-str If the end-of-file string occurs as a line of input, the rest of the input is ignored If neither -E nor -e is used, no end-of-file string is used
-e	--eof=	[eof-str]	This option is a synonym for the -E option Use -E instead, because it is POSIX compliant while this option is not If eof-str is omitted, there is no end-of-file string If neither -E nor -e is used, no end-of-file string is used
-I			replace-str Replace occurrences of replace-str in the initial-arguments with names read from standard input Also, unquoted blanks do not terminate input items; instead the separator is the newline character Implies -x and -L 1
-i	--replace=	[replace-str]	This option is a synonym for -Ireplace-str if replace-str is specified If the replace-str argument is missing, the effect is the same as -I{} The -i option is deprecated; use -I instead
-L		max-lines	Use at most max-lines nonblank input lines per command line Trailing blanks cause an input line to be logically continued on the next input line Implies -x
-l	--max-lines=	[max-lines]	Synonym for the -L option Unlike -L, the max-lines argument is optional If max-lines is not specified, it defaults to one The -l option is deprecated since the POSIX standard specifies -L instead
-n	--max-args=	max-args	Use at most max-args arguments per command line Fewer than max-args arguments will be used if the size (see the -s option) is exceeded, unless the -x option is given, in which case xargs will exit
-P	--max-procs=	max-procs	Run up to max-procs processes at a time; the default is 1 If max-procs is 0, xargs will run as many processes as possible at a time Use the -n option or the -L option with -P; otherwise chances are that only one exec will be done While xargs is running, you can send its process a SIGUSR1 signal to increase the number of commands to run simultaneously, or a SIGUSR2 to decrease the number You cannot increase it above an implementation-defined limit (which is shown with --show-limits) You cannot decrease it below 1 xargs never terminates its commands; when asked to decrease, it merely waits for more than one existing command to terminate before starting another xargs always waits for all child processes to exit before exiting itself (but see BUGS) If you do not use the -P option, xargs will not handle the SIGUSR1 and SIGUSR2 signals, meaning that they will terminate the program (unless they were blocked in the parent process before xargs was started) Please note that it is up to the called processes to properly manage parallel access to shared resources For example, if more than one of them tries to print to stdout, the output will be produced in an indeterminate order (and very likely mixed up) unless the processes collaborate in some way to prevent this Using some kind of locking scheme is one way to prevent such problems In general, using a locking scheme will help ensure correct output but reduce performance If you don't want to tolerate the performance difference, simply arrange for each process to produce a separate output file (or otherwise use separate resources)
-o	--open-tty		Reopen stdin as /dev/tty in the child process before executing the command This is useful if you want xargs to run an interactive application
-p	--interactive		Prompt the user about whether to run each command line and read a line from the terminal Only run the command line if the response starts with `y' or `Y' Implies -t process-slot-var=name Set the environment variable name to a unique value in each running child process Values are reused once child processes exit This can be used in a rudimentary load distribution scheme, for example
-r	--no-run-if-empty		If the standard input does not contain any nonblanks, do not run the command Normally, the command is run once even if there is no input This option is a GNU extension
-s	--max-chars=	max-chars	Use at most max-chars characters per command line, including the command and initial-arguments and the terminating nulls at the ends of the argument strings The largest allowed value is system-dependent, and is calculated as the argument length limit for exec, less the size of your environment, less 2048 bytes of headroom If this value is more than 128 KiB, 128 KiB is used as the default value; otherwise, the default value is the maximum. 1 KiB is 1024 bytes xargs automatically adapts to tighter constraints show-limits Display the limits on the command-line length which are imposed by the operating system, xargs' choice of buffer size and the -s option Pipe the input from /dev/null (and perhaps specify no-run-if-empty) if you don't want xargs to do anything
-t	--verbose		Print the command line on the standard error output before executing it
-x	--exit		Exit if the size (see the -s option) is exceeded Delimit the option list Later arguments, if any, are treated as operands even if they begin with - For example, xargs -- --help runs the command --help (found in PATH) instead of printing the usage text, and xargs -- --mycommand runs the command --mycommand instead of rejecting this as unrecognized option

help Print a summary of the options to xargs and exit

version Print the version number of xargs and exit

The options --max-lines (-L, -l) || --replace (-I, -i) and --max-args (-n) are mutually exclusive

If some of them are specified at the same time, then xargs will generally use the option specified last on the command line, i.e., it will reset the value of the offending option (given before) to its default value
Additionally, xargs will issue a warning diagnostic on stderr
The exception to this rule is that the special max-args value 1 ('-n1') is ignored after the --replace option and its aliases -I and -i, because it would not actually conflict

EXAMPLES

find /tmp -name core -type f -print | xargs /bin/rm -f

Find files named core in or below the directory /tmp and delete them

Note that this will work incorrectly if there are any filenames containing newlines or spaces

find /tmp -name core -type f -print0 | xargs -0 /bin/rm -f

Find files named core in or below the directory /tmp and delete them, processing filenames in such a way that file or directory names containing spaces or newlines are correctly handled

find /tmp -depth -name core -type f -delete

Find files named core in or below the directory /tmp and delete them, but more efficiently than in the previous example (because we avoid the need to use fork(2) and exec(2) to launch rm and we don't need the extra xargs process)

cut -d: -f1 < /etc/passwd | sort | xargs echo

Generates a compact listing of all the users on the system

EXIT STATUS

xargs exits with the following status

0 if it succeeds
123 if any invocation of the command exited with status 1–125
124 if the command exited with status 255
125 if the command is killed by a signal
126 if the command cannot be run
127 if the command is not found
1 if some other error occurred

Exit codes greater than 128 are used by the shell to indicate that a program died due to a fatal signal

STANDARDS CONFORMANCE

As of GNU xargs version 4.2.9, the default behaviour of xargs is not to have a logical end-of-file marker

POSIX (IEEE Std 1003.1, 2004 Edition) allows this

The -l and -i options appear in the 1997 version of the POSIX standard, but do not appear in the 2004 version of the standard

Therefore you should use -L and -I instead, respectively

The -o option is an extension to the POSIX standard for better compatibility with BSD

The POSIX standard allows implementations to have a limit on the size of arguments to the exec functions

This limit could be as low as 4096 bytes including the size of the environment
For scripts to be portable, they must not rely on a larger value
However, I know of no implementation whose actual limit is that small
The --show-limits option can be used to discover the actual limits in force on the current system

In versions of xargs up to and including version 4.9.0, SIGUSR1 and SIGUSR2 would not cause xargs to terminate even if the -P option was not used

BUGS

It is not possible for xargs to be used securely, since there will always be a time gap between the production of the list of input files and their use in the commands that xargs issues

If other users have access to the system, they can manipulate the filesystem during this time window to force the action of the commands xargs runs to apply to files that you didn't intend
For a more detailed discussion of this and related problems, please refer to the ``Security Considerations chapter in the findutils Texinfo documentation
The -execdir option of find can often be used as a more secure alternative

When you use the -I option, each line read from the input is buffered internally

This means that there is an upper limit on the length of input line that xargs will accept when used with the -I option
To work around this limitation, you can use the -s option to increase the amount of buffer space that xargs uses, and you can also use an extra invocation of xargs to ensure that very long lines do not occur
For example

somecommand | xargs -s 50000 echo | xargs -I '{}' -s 100000 rm '{}'

Here, the first invocation of xargs has no input line length limit because it doesn't use the -i option

The second invocation of xargs does have such a limit, but we have ensured that it never encounters a line which is longer than it can handle
This is not an ideal solution
Instead, the -i option should not impose a line length limit, which is why this discussion appears in the BUGS section
The problem doesn't occur with the output of find(1) because it emits just one filename per line

In versions of xargs up to and including version 4.9.0, xargs -P would exit while some of its children were still running, if one of them exited with status 255

REPORTING BUGS

GNU findutils online help: <https://www.gnu.org/software/findutils/#get-help> Report any translation bugs to <https://translationproject.org/team/>

Report any other issue via the form at the GNU Savannah bug tracker <https://savannah.gnu.org/bugs/?group=findutils> General topics about the GNU findutils package are discussed at the bug-findutils mailing list <https://lists.gnu.org/mailman/listinfo/bug-findutils>