one-way git repo synchronization with shallow clone

gistfile1.txt

export DESTINATION=[git URL]
export SOURCE=[git URL]
export BRANCH=main
# shallow clone of the destination - this should be at or behind the source
git clone --no-tags --progress --no-recurse-submodules --depth=1 $DESTINATION
git checkout $BRANCH
# add the source
git remote add source $SOURCE
# fetch only the delta
git fetch source $BRANCH
git merge refs/remotes/source/$BRANCH --ff-only
git push HEAD:$BRANCH --force