bash Quick Reference

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.)

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