Another shell feature that is useful for programmers is command substitution. When encountering a construct of the form:

 `command `

in an input command line, the shell runs the given command. It then puts the output of the command, after converting newlines into spaces, back into the command line, replacing command, and runs the new command line. This is called command substitution.

As an example of how a programmer could use command substitution, consider a file called srclist, containing the following list of source code file names: alpha.c, beta.c, and gamma.c. If you enter the command:

grep printf `cat srclist`

the shell runs cat against the contents of srclist, and rewrites the original command line, so that this line appears as:

grep printf alpha.c beta.c gamma.c

This line is then run, with grep searching through the given files, displaying lines that contain the string printf. This type of construct quickly locates all references to a particular variable or function in the source code for a program.