xargs

Reference

build and execute command lines from standard input

Category

Linux Command

Usage

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

Manual

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.

Because Unix filenames can contain blanks and newlines, this default behaviour is often problematic; filenames containing blanks and/or new-lines 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

  • -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.
  • -i [replace-str], --replace[=replace-str]: This option is a synonym for -I replace-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 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 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.
  • -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-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 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 an unrecognized option.
  • --help: Print a summary of the options to xargs and exit.
  • --version: Print the version number of xargs and exit.

Examples

Running a Custom Script with Multiple Arguments

The -I option in the xargs command is used to specify a placeholder that represents the input items when constructing and executing commands. It allows you to control how the input items are passed as arguments to the command. Suppose you have a list of URLs and you want to download them using a custom script called download.sh that accepts both the URL and an output file name:

cat urls.txt | xargs -I {} ./download.sh {} {}.html

In this example, -I {} is used to replace {} with each URL from urls.txt, and {} is also used to specify the output file name with .html appended. This way, the download.sh script is executed with two arguments: the URL and the output file name.

 

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