To delete local branches which have alread been merged into master:

$ git branch --merged master | grep -v "\* master" | xargs -n 1 git branch -d

Omit the master branch argument to remove local branches which have already been merged into the current HEAD:

$ git branch --merged | grep -v "\*" | xargs -n 1 git branch -d

To delete all local branches except for master:

$ git branch | grep -v "master" | xargs git branch -D