Replace environment variables in configuration files with optional default using perl regular expressions.

cpconfig.sh

#!/bin/bash
#
# Replaces ${var:def} expressions in text files with environment variables with
# an optional default.
#
# Assuming VAR1 contains the value "foo" nad VAR2 is undefined, the following
# expressions will evaluate as given on the right side:
#
# ${VAR1}     ~> foo
# ${VAR2:bar} ~> bar
# ${VAR1:bar} ~> foo
# ${VAR2}     ~>     (empty string)
#
perl -p -e 's/\$\{([^}:]+)(:([^}:]+))?\}/defined $ENV{$1} ? $ENV{$1} : $3/eg' $1

I found this which does exactly what I want

perl -wpne 's#${?(\w+)}?# $ENV{$1} // $& #ge;' inputfile.txt