3.5.4 Command Substitution
Command substitution allows the output of a command to replace
the command itself.
Command substitution occurs when a command is enclosed as follows:
Bash performs the expansion by executing command and
replacing the command substitution with the standard output of the
command, with any trailing newlines deleted.
Embedded newlines are not deleted, but they may be removed during
The command substitution
$(cat file) can be
replaced by the equivalent but faster
When the old-style backquote form of substitution is used,
backslash retains its literal meaning except when followed by
'$', '`', or '\'.
The first backquote not preceded by a backslash terminates the
When using the
$(command) form, all characters between
the parentheses make up the command; none are treated specially.
Command substitutions may be nested. To nest when using the backquoted
form, escape the inner backquotes with backslashes.
If the substitution appears within double quotes, word splitting and
filename expansion are not performed on the results.