Git pre-push hook to prevent force pushing master branch

pre-push.sh

#!/bin/sh

# Called by "git push" after it has checked the remote status,
# but before anything has been pushed.
#
# If this script exits with a non-zero status nothing will be pushed.
#
# Steps to install, from the root directory of your repo...
# 1. Copy the file into your repo at `.git/hooks/pre-push`
# 2. Set executable permissions, run `chmod +x .git/hooks/pre-push`
# 3. Or, use `rake hooks:pre_push` to install
#
# Try a force push to master, you should get a message `*** [Policy] never force push...`
#
# The commands below will not be allowed...
# `git push --force origin master`
# `git push --delete origin master`
# `git push origin :master`
#
# Nor will a force push while on the master branch be allowed...
# `git co master`
# `git push --force origin`
# 
# Requires git 1.8.2 or newer
#
# Git 1.8.2 release notes cover the new pre-push hook:
# <https://github.com/git/git/blob/master/Documentation/RelNotes/1.8.2.txt>
#
# See Sample pre-push script:
# <https://github.com/git/git/blob/87c86dd14abe8db7d00b0df5661ef8cf147a72a3/templates/hooks--pre-push.sample>


protected_branch='master'

policy='[Policy] Never force push or delete the '$protected_branch' branch! (Prevented with pre-push hook.)'

current_branch=$(git symbolic-ref HEAD | sed -e 's,.*/\(.*\),\1,')

push_command=$(ps -ocommand= -p $PPID)

is_destructive='force|delete|\-f'

will_remove_protected_branch=':'$protected_branch

do_exit(){
  echo $policy
  exit 1
}

if [[ $push_command =~ $is_destructive ]] && [ $current_branch = $protected_branch ]; then
  do_exit
fi

if [[ $push_command =~ $is_destructive ]] && [[ $push_command =~ $protected_branch ]]; then
  do_exit
fi

if [[ $push_command =~ $will_remove_protected_branch ]]; then
  do_exit
fi

unset do_exit

exit 0