# To delete local old/merged branches:
git fetch --prune
git branch -r | awk '{print $1}' | egrep -v -f /dev/fd/0 <(git branch -vv | grep origin) | awk '{print $1}' | xargs git branch -d
# This string gets the list of remote branches and passes it into egrep through the standard input. And filters the branches that have a remote tracking branch (using git branch -vv and filtering for those that have origin) then getting the first column of that output which will be the branch name.
# Finally passing all the branch names into the delete branch command.
# Since it is using the -d option, it will not delete branches that have not been merged into the branch that you are on when you run this command.
# Also remember that you'll need to run git fetch --prune first, otherwise git branch -r will still see the remote branches.