Xargs
xargs is a command on Unix and most Unix-like operating systems used to build and execute command lines from standard input. Commands such as grep and awk can accept the standard input as a parameter, or argument by using a pipe. However, others such as cp and echo disregard the standard input stream and rely solely on the arguments found after the command. Additionally, under the Linux kernel before version 2.6.23, and under many other Unix-like systems, arbitrarily long lists of parameters cannot be passed to a command, so xargs breaks the list of arguments into sublists small enough to be acceptable.
For example, shell commands such as:
or
may fail with an error message of "Argument list too long" (meaning that the exec system call's limit on the length of a command line was exceeded) if there are too many files in /path.
However, the version below (functionally equivalent to rm `find /path -type f`) will not fail:
In the above example, the find utility 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.
The previous example is more efficient than this functionally equivalent version 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:
xargs can also be used to parallelize operations with the -P maxprocs argument to specify how many parallel processes should be used to execute the commands over the input argument lists. However, the output streams may not be synchronized. This can be overcome by using an --output file argument where possible, and then combining the results after processing. The following example queues 24 processes and waits on each to finish before launching another.
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 good companion for commands that output long lists of files such as find, locate and grep, but only if you use -0, since xargs without -0 deals badly with file names containing ', " and space. GNU Parallel is a similar tool that offers better compatibility with find, locate and grep when file names may contain ', " and space (newline still requires -0).
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