Xargs

xargs(pronounced zargs) is a command of Unix and most Unix-like operating systems. It is useful when one wants to pass a large number of arguments to a command. Arbitrarily long lists of parameters can't be passed to a command, [ [http://www.gnu.org/software/coreutils/faq/coreutils-faq.html#Argument-list-too-long GNU Core Utilities FAQ] ] so xargs will break the list of arguments into sublists small enough to be acceptable.

For example, commands like:

rm /path/* rm `find /path -type f`

will fail with an error message of "Argument list too long" if there are too many files in "/path". However this version (functionally equivalent with "rm `find /path -type f`") will not fail:

find /path -type f -print0 | xargs -0 rm

In this example, "find" feeds the input of "xargs" with a long list of file names. "xargs" then splits this list into sublists and calls "rm" once for every sublist. This is more efficient than this functionally equivalent version: find /path -type f -exec rm '{}' ;

which calls "rm" once for every single file. Note however that with modern versions of "find", the following variant does the same thing as the "xargs" version:

find /path -type f -exec rm '{}' +

xargs often covers the same functionality as the backquote feature of many shells, but is more flexible and often also safer, especially if there are blanks or special characters in the input. It is a perfect companion for commands that output long lists of files like find, locate and grep.

Examples

find . -name "*.foo" | xargs grep bar

does the same as

grep bar `find . -name "*.foo"`

Note that the preceding command uses backticks (`), not single quotes ('). It searches in all files in the current directory and its subdirectories which end in .foo for occurrences of the string bar. These commands will not work as expected if there are whitespace characters, including newlines, in the filenames. In order to avoid this limitation one may use:

find . -name "*.foo" -print0 | xargs -0 grep bar

which uses GNU specific extensions to find and xargs to separate filenames using the null character;

find . -name "*.foo" -print0 | xargs -0 -t -r vi

is similar to above, but launches the vi editor for each of the files. The -t prints the command to stderr before issuing it. The -r is a GNU extension that tells xargs not to run the command if no input was received.

find . -name "*.foo" -print0 | xargs -0 -I mv {} /tmp/trash

uses -I to tell xargs to replace {} with the argument list. Note that not all versions of xargs supports the {} syntax. In those cases you may specify a string after -I that will be replaced, e.g.

find . -name "*.foo" -print0 | xargs -0 -I xxx mv xxx /tmp/trash

uses string xxx instead of {} as the argument list marker.

find . -maxdepth 1 -type f -name "*.mp3" -print0 | xargs -0 -r cp -v -p --target-directory=/home/media

does the same as:

cp -v -p *.mp3 /home/media

however, the former command which uses find/xargs/cp is more resource efficient and will not halt with an error if the number of files is too large for the cp command to handle.

References

External links

* [http://unixhelp.ed.ac.uk/CGI/man-cgi?xargs GNU man page on xargs]