git_checkout.sh

#!/bin/bash
# Demo using a hook to pull a file from master branch
# !Run in a temp dir!

set -ue

main () {
echo \> Setup
    mkdir work
    cd work
    git init
    echo "Initial state" > file
    git add file
    echo "Initial config" > master.cfg
    git add master.cfg
    git commit -m "Initial commit"
    cd -

    cd work
echo \> Write ref update hook
    cat <<-EOF > .git/hooks/post-checkout
	#!/bin/bash
	set -ue

	old_head=\$1
	new_head=\$2
	branch_checkout=\$3

	if [ \$branch_checkout == 1 ]; then
	    echo "Checkout from branch: \$old_head → \$new_head"
	    git checkout master master.cfg
	else
	    echo "Checkout file from: \$old_head → \$new_head"
	fi
	EOF
    chmod +x .git/hooks/post-checkout
    cd -
echo
echo \> Make a change on a new branch
    cd work
    git checkout -b dev
    echo "Modified state" > file
    git commit -m "Modification" file
    cd -
echo
echo \> Checkout branches
    cd work
    git checkout master
    grep --color=always . ./*
    git checkout dev
    grep --color=always . ./*
    cd -
echo
echo \> Make a change to master config
    cd work
    git checkout master
    echo "Modified config" > master.cfg
    git commit -m "Config change" master.cfg
    cd -
echo
echo \> Checkout branches
    cd work
    git checkout master
    grep --color=always . ./*
    git checkout dev
    grep --color=always . ./*
    cd -
}

if [ "${1:-}" = "clean" ]; then
    rm -rf work
else
    main
fi