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UNIX in a Nutshell: System V Edition

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The Bourne Shell and Korn Shell
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eval

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 (Section 5) for another example.

Example

This fragment of a Bourne 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


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