Xargs (Bash)

xargs is an explicit way of reusing the output of a command. I often use it, when more implicit ways of pipelining like through > or | doesn't work, like with WP-CLI:

$ man xargs

XARGS(1)                                      General Commands Manual                                      XARGS(1)

NAME
       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  /bin/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  op‐
       tions are used).  The specified command will be invoked as many times as necessary to use up the list of in‐
       put 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 paral‐
       lel too; see the -P option.

       Because Unix filenames can contain blanks and newlines, this default behaviour is often  problematic;  file‐
       names  containing blanks and/or newlines are incorrectly processed by xargs.  In these situations it is bet‐
       ter 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 pro‐
       gram 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
       -0, --null
              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 file, --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 char‐
              acter, 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-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.

       -I replace-str
              Replace occurrences of replace-str in the initial-arguments with names read from standard input.  Al‐
              so,  unquoted  blanks  do  not terminate input items; instead the separator is the newline character.
              Implies -x and -L 1.

       -i[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.

       -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[=max-lines]
              Synonym for the -L option.  Unlike -L, the max-lines argument is optional.  If max-lines is not spec‐
              ified,  it  defaults  to  one.  The -l option is deprecated since the POSIX standard specifies -L in‐
              stead.

       -n max-args, --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=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.

              Please  note  that  it is up to the called processes to properly manage parallel access to shared re‐
              sources.  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=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-de‐
              pendent,  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.

       --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.

       --help Print a summary of the options to xargs and exit.

       --version
              Print the version number of xargs and exit.

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  pre‐
       vious 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 ver‐
       sion 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.

SEE ALSO
       find(1), locate(1), locatedb(5), updatedb(1), fork(2), execvp(3), kill(1), signal(7),

       The   full  documentation  for  xargs is maintained as a Texinfo manual.  If the info and xargs programs are
       properly installed at your site, the command info xargs should give you access to the complete manual.

COPYRIGHT
       Copyright © 1990-2019  Free  Software  Foundation,  Inc.   License  GPLv3+:  GNU  GPL  version  3  or  later
       <https://gnu.org/licenses/gpl.html>.
       This is free software: you are free to change and redistribute it.  There is NO WARRANTY, to the extent per‐
       mitted by law.

BUGS
       The -L option is incompatible with the -I option, but perhaps should not be.

       It is not possible for xargs to be used securely, since there will always be a time gap between the  produc‐
       tion 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  com‐
       mands  xargs  runs to apply to files that you didn't intend.  For a more detailed discussion of this and re‐
       lated problems, please refer to the ``Security Considerations'' chapter in the findutils Texinfo  documenta‐
       tion.  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 the 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.

       The best way to report a bug is to use the form at https://savannah.gnu.org/bugs/?group=findutils.  The rea‐
       son  for  this is that you will then be able to track progress in fixing the problem.   Other comments about
       xargs(1) and about the findutils package in general can be sent to the bug-findutils mailing list.  To  join
       the list, send email to bug-findutils-request@gnu.org.

Examples WP-CLI

# Remove orders
########################################
#
wp wc shop_order list --user=4 --field=id --per_page=100 --offset=$(($i*100))  | xargs -n1 wp wc shop_order delete --user=4 --force=1


# Remove tax rules
########################################
#
wp --user=4 wc tax list --field=id | xargs -n1 wp --user=4 wc tax delete --force=true


# Process a sequence of numbers
########################################
#
seq $post_start $post_end | xargs wp media regenerate --only-missing --yes

See also

• -exec
• Pipelining & redirection