Classic Shell Scripting

Running Commands: xargs

When find produces a list of files, it is often useful to be able to supply that list as arguments to another command. Normally, this is done with the shell's command substitution feature, as in this example of searching for the symbol POSIX_OPEN_MAX in system header files:

$ grep POSIX_OPEN_MAX /dev/null $(find /usr/include -type f | sort)
/usr/include/limits.h:#define   _POSIX_OPEN_MAX            16

Whenever you write a program or a command that deals with a list of objects, you should make sure that it behaves properly if the list is empty. Because grep reads standard input when it is given no file arguments, we supplied an argument of /dev/null to ensure that it does not hang waiting for terminal input if find produces no output: that will not happen here, but it is good to develop defensive programming habits.

The output from the substituted command can sometimes be lengthy, with the result that a nasty kernel limit on the combined length of a command line and its environment variables is exceeded. When that happens, you'll see this instead:

$ grep POSIX_OPEN_MAX /dev/null $(find /usr/include -type f | sort)
/usr/local/bin/grep: Argument list too long.

That limit can be found with getconf:

$ getconf ARG_MAX                        
            Get system configuration value of ARG_MAX
131072

On the systems that we tested, the reported values ranged from a low of 24,576 (IBM AIX) to a high of 1,048,320 (Sun Solaris).

The solution to the ARG_MAX problem is provided by xargs: it takes a list of arguments on ...