Prevent directory traversal vulnerability in bash script

How can I prevent directory traversal attacks in a bash script, where arguments contain directory names?


$  STAGE=$  1 $  APP=$  2 dist/ /opt/apps/"$  STAGE"/"$  APP" 

The $ STAGE and $ APP variables are set from outside. An attacker could change this to an arbitrary path with "..".

I know the usual solution is to compare the directory string with the result of a function that returns the absolute path. But I couldn’t find a ready solution and don’t want to come up with my own.