xargs - build and execute command lines from standard input
xargs
[options] [command
[initial-arguments]]
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.
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.
file,
--arg-file=fileRead 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.
delim, -d
delimInput 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.
eof-strSet 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.
eof-str],
--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.
replace-strReplace 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.
replace-str],
--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{}.
This option is deprecated; use -I instead.
max-linesUse 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.
max-lines],
--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.
=max-argsUse 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.
=max-procsRun 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.
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).
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.
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.
nameSet 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.
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.
=max-charsUse 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 128KiB, 128Kib is used as the default value; otherwise, the default
value is the maximum. 1KiB is 1024 bytes. xargs
automatically adapts to tighter constraints.
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.
Print the command line on the standard error output before executing it.
Exit if the size (see the -s option) is exceeded.
Print a summary of the options to xargs and exit.
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.
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.
xargs exits with the following status:
if it succeeds
if any invocation of the command exited with status 1-125
if the command exited with status 255
if the command is killed by a signal
if the command cannot be run
if the command is not found
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.
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.
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.
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