A simple git hook that let's you know whenever you need to update your CocoaPods after merging or pulling from a remote.

post-merge

#!/bin/bash
#
# Let's you know when your pods should be updated right after a git pull or git merge
# Also works for fast-forward pulls.
#
# To enable this hook for your project, save it as .git/hooks/post-merge

diff "Podfile.lock" "Pods/Manifest.lock" > /dev/null

if [ $? != 0 ] ; then
    echo
    echo "[COCOAPODS] The sandbox is not in sync with the Podfile.lock."
    echo "            Run 'pod install' or update your CocoaPods installation."
    echo
    #pod install # uncomment this line to update the pods whenever necessary
fi

Why /bin/bash? /bin/sh should do fine

You could also use this in post-checkout