How can I delete all Git branches which have been merged?

Question

I have many Git branches. How do I delete branches which have already been merged? Is there an easy way to delete them all instead of deleting them one by one?

Answer 1

Alias version of Adam's updated answer:

[alias]
    branch-cleanup = "!git branch --merged | egrep -v \"(^\\*|master|dev)\" | xargs git branch -d #"

Also, see this answer for handy tips on escaping complex aliases.