/usr/sbin/update-tirfs is in tiny-initramfs 0.1-4~deb9u1.
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  | #!/bin/sh
set -e
umask 0022
usage() {
  echo "Usage: $0 [-k version]"
  echo ""
  echo "Updates system tiny-initramfs images"
  echo ""
  echo "Options:"
  echo "    -k, --kernel-version=VERSION  The kernel version for which the initramfs"
  echo "                                  image is to be updated. (default: all)"
}
if ! OPTIONS=$(getopt -o k:h -l kernel-version:,help -n "$0" -- "$@") ; then
  usage >&2
  exit 1
fi
eval set -- "$OPTIONS"
VERSION="all"
while true; do
  case "$1" in
    -h|--help)
      usage
      exit 0
      ;;
    -k|--kernel-version)
      VERSION="$2"
      shift 2
      ;;
    --)
      shift
      break
      ;;
    *)
      echo "$0: internal error" >&2
      exit 1
      ;;
  esac
done
if [ $# -gt 0 ] ; then
  echo "$0: too many options" >&2
  usage >&2
  exit 1
fi
# Called from a maintainer script -> resort to triggers instead
if [ -n "$DPKG_MAINTSCRIPT_PACKAGE" ]; then
  echo "$0: WARNING: called from dpkg maintscript, please use triggers instead." >&2
  if dpkg-trigger --no-await update-initramfs; then
    echo "update-tirfs: deferring update (trigger activated)"
    exit 0
  fi
fi
if [ x"$VERSION" = x"all" ] ; then
  for V in /boot/vmlinu[xz]-* ; do
    if ! [ -r "${V}" ] ; then continue ; fi
    V=${V#/boot/vmlinu?-}
    mktirfs -o /boot/initrd.img-"${V}" "${V}"
  done
else
  if ! ls /boot/vmlinu[xz]-"${VERSION}" >/dev/null 2>&1 ; then
    echo "$0: kernel version ${VERSION} does not appear to be installed." >&2
    exit 1
  fi
  mktirfs -o /boot/initrd.img-"${VERSION}" "${VERSION}"
fi
 |