xargs

xargs is a command line utility that formats stdin correctly for another program's arguments and invokes that program

For example, by default the command ls outputs over a number of lines, but ls | xargs will push these onto one line (space-separated). This is very useful for substituting a command's results in another command. e.g. ls | xargs rm. N.B: this is a contrived example where rm -r * would be easier.

xargs is therefore very useful for scripting, but it also becomes essential if you would otherwise have a massive number of program arguments. For example if you were deleting thousands of files (by name (contrived example)) and were to put them all onto the command line with $(), some would be cut short since there is a maximum argument limit. xargs obeys this and will split the call into multiple calls if necessary to accommodate this. This can be manually achieved with the -n <n> argument which allows only n arguments per call. This also opens up much more scripting potential.