#!/bin/sh
set -e

backup() {
	if exists "$1"; then
		cp -dp "$1" "$2" || return $?
	fi
	return 0
}

exists() {
	[ -e "$1" ] || return 1
}

force_symlink() {
	if [ -e "$3" ] && ! unlink "$3"; then
		echo "cannot create symlink $2 -> $1 (unlink)" >&2
		exit 1
	fi

	if ! symlink "$1" "$3"; then
		echo "cannot create symlink $2 -> $1 (symlink)" >&2
		exit 1
	fi

	if ! rename "$3" "$2"; then
		echo "cannot create symlink $2 -> $1 (rename)" >&2
		exit 1
	fi

	return 0
}

symlink() {
	ln -s "$1" "$2" || return $?
}

rename() {
	mv -fT "$1" "$2" || return $?
}

reset_diversion() {
	dpkg-divert --package bash --remove "$2" || return $?
	dpkg-divert --package "$1" --divert "$3" --add "$2" || return $?
}

has_binsh_line() {
	while read -r item; do
		if [ "$item" = '/bin/sh' ]; then
			return 0
		fi
	done
	return 1
}

binsh_in_filelist() {
	dpkg-query -L "$1" 2> /dev/null | has_binsh_line || return $?
}

undiverted() {
	in="`dpkg-divert --listpackage "$1"`" || return $?
	[ -z "$in" -o "$in" = 'bash' ] || return 1
}

access() {
	# This isn't perfectly accurate, but it's sufficient for our purposes.  The syscall is a little different.
	[ -e "$1" ] || return 1
}

main() {
	if access /bin/sh; then
		backup /bin/sh /bin/sh.distrib || return $?
		backup /usr/share/man/man1/sh.1.gz /usr/share/man/man1/sh.distrib.1.gz || return $?

		force_symlink bash /bin/sh /bin/sh.temp || return $?
		force_symlink bash.1.gz /usr/share/man/man1/sh.1.gz /usr/share/man/man1/sh.1.gz.temp || return $?

		if ! binsh_in_filelist bash; then
			return 0
		fi
	fi

	if undiverted /bin/sh; then
		reset_diversion dash /bin/sh /bin/sh.distrib || return $?
	fi
	if undiverted /usr/share/man/man1/sh.1.gz; then
		reset_diversion dash /usr/share/man/man1/sh.1.gz || return $?
	fi

	return 0
}

main || exit $?