builder/mount-chroot.sh

#!/usr/bin/env bash

err() { echo "$1" >&2; }

usage() {
cat <<- EOF
	Usage: "${0##*/}" [options] <chroot>
	Options:
		-s	chroot base dir
		-n	mount name
		-u	unmount chroot
EOF
}

chroot.mount() {
	for i in "$target_dir" "$chroot_base"; do
		[[ "$i" ]] || {
			usage
			return 1
		}
	done

	[[ -d "$chroot_base" ]] || {
		err "Chroot base ($chroot_base) does not exist."
		return 1
	}

	mount_name="${mount_name:-chroot}"

	for i in "$target_dir" "${target_dir}.work"; do
		[[ -d "$i" ]] || {
			mkdir -p "$i" || {
				return 3
			}
		}
	done

	mount -t overlay -o lowerdir="$chroot_base",upperdir="$target_dir",workdir="${target_dir}.work" "$mount_name" "$target_dir"
	mount -t proc "${mount_name}_proc" "${target_dir}/proc"
	mount --rbind /dev "${target_dir}/dev"
	mount --rbind /sys "${target_dir}/sys"
}

chroot.umount() {
	umount --recursive "$target_dir"
}

main() {
	while [[ "$1" ]]; do
		case "$1" in
			-s) chroot_base="$2"; shift;;
			-n) mount_name="$2"; shift;;
			-u) action='umount';;

			--) shift; break;;
			-*)
				err "Unknown key: $1"
				usage
				return 1
			;;
			*) break;;
		esac
		shift
	done

	target_dir="$1"
	action="${action:-mount}"
	mount_name="${mount_name:-chroot}"

	case "$action" in
		mount) chroot.mount;;
		umount) chroot.umount;;
	esac
}

main "$@"