Advanced Bash−Scripting Guide

Example 11−11. A version of "rot13"
Advanced Bash−Scripting Guide
#!/bin/bash
# A version of "rot13" using 'eval'.
# Compare to "rot13.sh" example.
setvar_rot_13()
# "rot13" scrambling
{
local varname=$1 varvalue=$2
eval $varname='$(echo "$varvalue" | tr a−z n−za−m)'
}
setvar_rot_13 var "foobar"
echo $var
echo $var | tr a−z n−za−m
# Run "foobar" through rot13.
# sbbone
# foobar
# Back to original variable.
# This example by Stephane Chazelas.
exit 0
Rory Winston contributed the following instance of how useful eval can be.
Example 11−12. Using eval to force variable substitution in a Perl script
In the Perl script "test.pl":
...
my $WEBROOT = ;
...
To force variable substitution try:
$export WEBROOT_PATH=/usr/local/webroot
$sed 's//$WEBROOT_PATH/' < test.pl > out
But this just gives:
my $WEBROOT = $WEBROOT_PATH;
However:
$export WEBROOT_PATH=/usr/local/webroot
$eval sed 's//$WEBROOT_PATH/' < test.pl > out
# ====
That works fine, and gives the expected substitution:
my $WEBROOT = /usr/local/webroot
set
The eval command can be risky, and normally should be avoided when there exists a
reasonable alternative. An eval $COMMANDS executes the contents of COMMANDS,
which may contain such unpleasant surprises as rm −rf *. Running an eval on
unfamiliar code written by persons unknown is living dangerously.
The set command changes the value of internal script variables. One use for this is to toggle option
flags which help determine the behavior of the script. Another application for it is to reset the
positional parameters that a script sees as the result of a command (set `command`). The script
can then parse the fields of the command output.
Chapter 11. Internal Commands and Builtins 134