Korn Shell: Unix and Linux Programming Manual, Third Edition, The

Command Substitution

The standard output from a command enclosed in parentheses preceded by a dollar sign ($()) or a pair of grave accents (` `) may be used as part or all of a word; trailing new-lines are removed. In the second (obsolete) form, the string between the quotes is processed for special quoting characters before the command is executed (see Quoting below). The command substitution $(cat file) can be replaced by the equivalent but faster $(<file).