xargs.md

xargs

build and execute command lines from standard input

---

References

• man xargs

Synopsis

xargs [-0prtx] [-E eof-str]
    [-e[eof-str]] [--eof[=eof-str]] [--null]
    [-d delimiter] [--delimiter delimiter]
    [-I replace-str] [-i[replace-str]]
    [--replace[=replace-str]]
    [-l[max-lines]] [-L max-lines]
    [--max-lines[=max-lines]]
    [-n max-args] [--max-args=max-args]
    [-s max-chars] [--max-chars=max-chars]
    [-P max-procs] [--max-procs=max-procs]
    [--process-slot-var=name] [--interactive]
    [--verbose] [--exit] [--no-run-if-empty]
    [--arg-file=file] [--show-limits]
    [--version] [--help]
    [command [initial-arguments]]

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.

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 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-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.
  • Also, unquoted blanks do not terminate input items; instead the separator is the newline character.
  • Implies -x and -L 1.
• -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 specified, it defaults to one.
  • The -l option is deprecated since the POSIX standard specifies -L instead.
• -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 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.
• -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-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.
• --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.

Usage

Sample 1

$ cat sample1
1
2
3
4

Default

-a : read from file

$ cat sample1 | xargs
# same as
$ xargs -a sample1
1 2 3 4

Delimiter

-d

$ echo "1X2X3X4" | xargs -dX
1 2 3 4

Max Args

-n

$ cat sample1 | xargs -n 2
# same as
$ xargs -a sample1 -n 2
1 2
3 4

1 to Multiple Lines

$ echo "1X2X3X4" | xargs -d X -n 1
1
2
3
4

Replace Str

-I replace_str

$ cat sample1 | xargs -I {} echo prefix {} suffix
# same as
$ xargs -a sample1 -I {} echo prefix {} suffix
prefix 1 suffix
prefix 2 suffix
prefix 3 suffix
prefix 4 suffix

Run in multiple processes

See -P ( 暂略 )

Samples

Line-Count of Files

$ find . -type f -print0 | xargs -0 wc -l | grep './sample'
       4 ./sample4
       4 ./sample1
       4 ./sample2
       5 ./sample3