$ for p in A B C; do (cd ~/git-repositories/$p; git checkout -b new-branch); done
Branch new-branch set up to track local branch master.
Switched to a new branch 'new-branch'
Branch new-branch set up to track local branch master by rebasing.
Switched to a new branch 'new-branch'
Branch new-branch set up to track local branch master.
Switched to a new branch 'new-branch'