The Korn Shell, or POSIX Shell, enables you to do command substitution.
In command substitution, the shell executes a specified command in a subshell environment and replaces that command with its output. To execute command substitution in the Korn shell or POSIX shell, perform the following:
or, for the backquoted version, use:
Note: Although the backquote syntax is accepted by ksh, it is considered obsolete by the XPG4 and POSIX standards. These standards recommend that portable applications use the $(command) syntax.
The shell expands the command substitution by executing command in a subshell environment and replacing the command substitution (the text of command plus the enclosing $( ) or backquotes) with the standard output of the command, removing sequences of one or more new-line characters at the end of the substitution.
In the following example, the $( ) surrounding the command indicates that the output of the whoami command is substituted:
echo My name is: $(whoami)
You can perform the same command substitution with:
echo My name is: `whoami`
The output from both examples for user dee is:
My name is: dee
You can also substitute arithmetic expressions by enclosing them in ( ). For example, the command:
echo Each hour contains $((60 * 60)) seconds
produces the following result:
Each hour contains 3600 seconds
The Korn shell or POSIX shell removes all trailing new-line characters when performing command substitution. For example, if your current directory contains the file1, file2, and file3 files, the command:
removes the new-line characters and produces the following output:
file1 file2 file3
To preserve new-line characters, insert the substituted command in " ":