eval

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.) A Bourne shell example can be found under eval in Chapter 4 . Other uses of eval are shown next.

Examples

The following lines can be placed in the .login file to set up terminal characteristics:

set noglob
eval `tset -s xterm`
unset noglob

The following commands show the effect of eval :

% set b='$a'


% set a=hello



% echo $b

         Read the command line once

$a
% eval echo $b

    Read the command line twice

hello