/usr/sbin/update-service is in runit 2.1.1-6.2ubuntu2.
This file is owned by root:root, with mode 0o755.
The actual contents of the file can be viewed below.
| 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 | #!/bin/sh
set -e
servicedir=$SVDIR
test -n "$servicedir" || servicedir=/etc/service
err() { >&2 printf '%s\n\n' "$*"; exit 1; }
fatal() { err "${0##*/}: fatal: $*"; }
warn() { err "${0##*/}: warning: $*"; }
usage() {
  err "Usage: ${0##*/} --add|--remove <service-directory> [<service-name>]
       ${0##*/} --list|--check [<service-name>]"
}
opt=$1
svdir=${2%/}
sv=$3
test -z "${opt##-*}" || usage
case "$opt" in
  -c|--check) exec >/dev/null; exec 2>/dev/null; opt=-l;;
esac
case "$opt" in
  -l|--list)
    test -n "$svdir" || exec ls -1 "$servicedir"
    test -h "$servicedir"/"$svdir" || err "Service $svdir not registered."
    printf '%s -> %s\n' "$svdir" "$(readlink "$servicedir"/"$svdir")"
    exit 0
  ;;
esac
test -n "$svdir" || usage
test -d "$svdir" ||
  fatal "$svdir does not exist, or is not a directory."
test -z "${svdir%%/*}" ||
  fatal "The <service-directory> must start with a slash."
test -n "$sv" || sv=${svdir##*/}
test -n "${sv##.*}" ||
  fatal "The <service-name> must not start with a dot."
test "$sv" = "${sv#*/}" ||
  fatal "The <service-name> must not contain a slash."
case "$opt" in
  -a|--add)
    test "$(id -u)" = 0 || fatal "${0##*/} -a must be run by root."
    if test -e "$servicedir"/"$sv"; then
      test -h "$servicedir"/"$sv" ||
        fatal "$servicedir/$sv exists, but is not a symbolic link."
      test "$(readlink "$servicedir"/"$sv")" = "$svdir" ||
        test "$(readlink "$servicedir"/"$sv")" = "$svdir"/ ||
          fatal "$servicedir/$sv exists, but doesn't point to $svdir."
      printf '%s\n' "Service $sv already added."
      exit 0
    fi
    ! sv stat "$svdir" >/dev/null 2>&1 ||
      fatal "$svdir is currently controlled by a runsv(8) process."
    if test "${svdir#/etc/}" != "$svdir"; then
      if test ! -h "$svdir"/supervise; then
        rm -rf "$svdir"/supervise
        ln -s /var/lib/supervise/"$sv" "$svdir"/supervise
      fi
      if test -d "$svdir"/log && test ! -h "$svdir"/log/supervise; then
        rm -rf "$svdir"/log/supervise
        ln -s /var/lib/supervise/"$sv".log "$svdir"/log/supervise
      fi
    fi
    ln -s "$svdir" "$servicedir"/"$sv"
    printf '%s\n' "Service $sv added."
    exit 0
  ;;
  -r|--remove)
    test "$(id -u)" = 0 || fatal "${0##*/} -r must be run by root."
    test -e "$servicedir"/"$sv" ||
      warn "$servicedir/$sv does not exist."
    test -h "$servicedir"/"$sv" ||
      fatal "$servicedir/$sv is not a symbolic link."
    test "$svdir" = "$(readlink "$servicedir"/"$sv")" ||
      test "$svdir"/ = "$(readlink "$servicedir"/"$sv")" ||
        fatal "$servicedir/$sv does not point to $svdir."
    rm -f "$servicedir"/"$sv"
    printf '%s %s\n' \
      "Service $sv removed," \
      "the service daemon received the TERM and CONT signals."
    exit 0
  ;;
  *) usage
  ;;
esac
 |