Replace command in Bash script from another script

I'm trying to setup a script that will fetch changes from github where a script has a command that's not executable in Mac OS X. What I'm aiming for is to use sed etc to replace the command within that script. These are the commands: Original command (to be replaced in script): DIR=$(dirname "$(readlink -f $0)") New command (I want to replace with): DIR="$(cd "$(dirname "$0")" && pwd -P)" What I've tried so far: StartStopScript="/path/to/script.sh" DIRnew=""$(cd "$(dirname "$0")" && pwd -P)"" DIRold=""$(dirname "$(readlink -f $0)")"" echo "$StartStopScript" | sed -e 's/"$DIRold"/"$DIRnew"/g' This fails and the commands seems to be executed instead of interpreted as a string. Figure it comes down to quoting in sed. Grateful for any help I can get with this.