overlay/overlay

#!/bin/sh

set -eu

fstab_fs_mntopt() {
    awk "\"overlay\" == \$1 && \"$1\" == \$2 { printf(\"%s\n\", \$4) }"
}

fstab_option_iter() {
    tr ',' '\n'
}

fstab_option_value() {
    cut -d= -f2
}

overlay_add() {
    o_lowerdir="$1"
    o_upperdir=$(mktemp -d)
    o_workdir=$(mktemp -d)
    chmod $(stat -c '%a' $o_lowerdir) $o_upperdir $o_workdir
    mount -t overlay -o lowerdir="${o_lowerdir}",upperdir=${o_upperdir},workdir=${o_workdir} overlay "${o_lowerdir}"
}

overlay_remove() {
    o_lowerdir="$1"
    o_options="$(fstab_fs_mntopt $o_lowerdir < /proc/mounts)"
    umount $o_lowerdir
    if [ -z "$o_options" ]
    then
        echo >&2 $o_lowerdir is not an overlay
        exit 1
    else
        echo "${o_options}" |
            fstab_option_iter |
            while read FS_MNTOPT
            do
                case $FS_MNTOPT in
                    upperdir=*|workdir=*)
                        rm -rf "$(echo $FS_MNTOPT | fstab_option_value)"
                        ;;
                esac
            done
    fi
}

if [ $# -lt 2 ]
then
    cat $0
    exit 1
fi

subcmd=$1
shift

case $subcmd in
    add)
        for target in "$@"
        do
            overlay_add "$target"
        done
        ;;
    remove)
        for target in "$@"
        do
            overlay_remove "$target"
        done
        ;;
    -h|--help|*)
        cat $0
        ;;
esac