Command substitution in the C shell

In command substitution, the shell executes a specified command and replaces that command with its output.

To perform command substitution in the C shell, enclose the command or command string in backquotes (` `). The shell normally breaks the output from the command into separate words at blanks, tabs, and newline characters. It then replaces the original command with this output.

In the following example, the backquotes (` `) around the date command indicate that the output of the command will be substituted:

echo The current date and time is: `date`

The output from this command might look like the following:

The current date and time is: Wed Apr 8 13:52:14 CDT 1992

The C shell performs command substitution selectively on the arguments of built-in shell commands. This means that it does not expand those parts of expressions that are not evaluated. For commands that are not built-in, the shell substitutes the command name separately from the argument list. The substitution occurs in a child of the main shell, but only after the shell performs input or output redirection.

If a command string is surrounded by " ", the shell treats only newline characters as word separators, thus preserving blanks and tabs within the word. In all cases, the single final newline character does not force a new word.