Help to run exec only if `branch === main`

I didn't find a way to run some `exec` commands only in `main`.  
I have a release config that has to run in `main` and `dev`. `main` is the production branch and `dev` is the development branch, on both, the release is similar, except that in `main` I want to execute a script that I can't in `dev`. How would I achieve this?