TL;DR
To delete local branches which have already been merged into main:
$ git branch --merged main | grep -v "\* main" | xargs -n 1 git branch -d
You can omit the main 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
Breaking it down
We start by getting a list of local branches which have already been merged
into the current branch (i.e. HEAD)
$ git branch --merged
add_new_user_gravatar_links
assign_unique_key_to_uploads
* main
remember_the_last_activity_per_user
update_kaminari_to_thread_safe_version
We then pipe that to grep to match on the "\*" character, inverting that
match via -v to get all merged branches sans the current one.
$ git branch --merged | grep -v "\*"
add_new_user_gravatar_links
assign_unique_key_to_uploads
remember_the_last_activity_per_user
update_kaminari_to_thread_safe_version
Finally we pipe that list in to xargs so we can strip apart the input and
pass it on to a new command. We use -n 1 to ensure at most one argument is
taken from the input to be passed to the invocation of the new command. The
resulting commands that xargs will invoke are effectively
$ git branch -d add_new_user_gravatar_links
$ git branch -d assign_unique_key_to_uploads
$ git branch -d remember_the_last_activity_per_user
$ git branch -d update_kaminari_to_thread_safe_version
Pulling it all back together, we have
$ git branch --merged | grep -v "\*" | xargs -n 1 git branch -d
Deleted branch add_new_user_gravatar_links
Deleted branch assign_unique_key_to_uploads
Deleted branch remember_the_last_activity_per_user
Deleted branch update_kaminari_to_thread_safe_version
And there you have it. Go forth and clean up!