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$ cat input
ALT=someone@acme.com
TO=whoever@lalalulu.com
$
$ flag="ALT"
$
$ grep "$flag" input | cut -d= -f2
someone@acme.com
$

How about using awk:

var1=2
var2=4
var3=$(awk "BEGIN { print $var1 / $var2 }" /dev/null)
echo $var3