delete all remote branches that have already been merged into master

gistfile1.txt

$ git branch -r --merged | 
  awk -F'/' '/^ *origin/{if(!match($0, /(>|master)/)){print $2}}' | 
  xargs git push origin --delete

@talsafran Yep, -r is not an option on MacOS xargs (see: man xargs)
-r is a GNU extension

You can install GNU xargs through Homebrew and the GNU findutils package.
brew install findutils

This will give you GNU xargs as gxargs, and you can use the -r option

The same goes for other basic commands found in the findutils package such as gfind or glocate or gupdatedb, which have different BSD counterparts on OS X.

After installing findutils, the command on MacOS will be:

git branch -r --merged origin/master \
    | awk -F/ '/^\s*origin/ {if (!match($0, /origin\/master/)) {sub("^\\s*origin/", ""); print}}' \
    | gxargs -rpn1 git push origin --delete