support coreutils chroot's --userspec

arch-chroot.in

@@ -8,7 +8,8 @@ usage() {
   cat <<EOF
 usage: ${0##*/} chroot-dir [command]
 
-    -h             Print this help message
+    -h                  Print this help message
+    -u <user>[:group]   Specify non-root user and optional group to use
 
 If 'command' is unspecified, ${0##*/} will launch /bin/sh.
 
@@ -41,10 +42,24 @@ chroot_add_resolv_conf() {
   chroot_add_mount /etc/resolv.conf "$resolv_conf" --bind
 }
 
-if [[ -z $1 || $1 = @(-h|--help) ]]; then
-  usage
-  exit $(( $# ? 0 : 1 ))
-fi
+while getopts ':hu:' flag; do
+  case $flag in
+    h)
+      usage
+      exit 0
+      ;;
+    u)
+      userspec=$OPTARG
+      ;;
+    :)
+      die '%s: option requires an argument -- '\''%s'\' "${0##*/}" "$OPTARG"
+      ;;
+    ?)
+      die '%s: invalid option -- '\''%s'\' "${0##*/}" "$OPTARG"
+      ;;
+  esac
+done
+shift $(( OPTIND - 1 ))
 
 (( EUID == 0 )) || die 'This script must be run with root privileges'
 chrootdir=$1
@@ -55,4 +70,8 @@ shift
 chroot_setup "$chrootdir" || die "failed to setup chroot %s" "$chrootdir"
 chroot_add_resolv_conf "$chrootdir" || die "failed to setup resolv.conf"
 
-SHELL=/bin/bash unshare --fork --pid chroot "$chrootdir" "$@"
+chroot_args=()
+[[ $userspec ]] && chroot_args+=(--userspec "$userspec")
+chroot_args+=("$chrootdir" "$@")
+
+SHELL=/bin/bash unshare --fork --pid chroot "${chroot_args[@]}"