To delete all branches that are already merged into the currently checked out branch:
$ git branch --merged | grep -v "\*" | grep -v master | grep -v dev | xargs -n 1 git branch -d
To delete all branches that are already merged into the currently checked out branch:
$ git branch --merged | grep -v "\*" | grep -v master | grep -v dev | xargs -n 1 git branch -d