Unix in a Nutshell, 4th Edition

Name

eval

Synopsis

    eval args

Typically, eval is used in shell scripts, and args is a line of code that contains shell variables. eval forces variable expansion to happen first and then runs the resulting command. This “double-scanning” is useful any time shell variables contain input/output redirection symbols, aliases, or other shell variables. (For example, redirection normally happens before variable expansion, so a variable containing redirection symbols must be expanded first using eval; otherwise, the redirection symbols remain uninterpreted.) See the C shell eval entry (Chapter 5 ) for another example.

Example

This fragment of a shell script shows how eval constructs a command that is interpreted in the right order:

    for option
    do
       case "$option" in   Define where output goes
          save) out=' > $newfile' ;;
          show) out=' | more' ;;
       esac
    done

    eval sort $file $out