check_dependencies() {
  # just execute some dummy and/or version commands to see if required tools exist and are actually usable
  openssl version > /dev/null 2>&1 || _exiterr "This script requires an openssl binary."
  _sed "" < /dev/null > /dev/null 2>&1 || _exiterr "This script requires sed with support for extended (modern) regular expressions."
  command -v grep > /dev/null 2>&1 || _exiterr "This script requires grep."
  _mktemp -u > /dev/null 2>&1 || _exiterr "This script requires mktemp."
  diff -u /dev/null /dev/null || _exiterr "This script requires diff."

  # curl returns with an error code in some ancient versions so we have to catch that
  set +e
  curl -V > /dev/null 2>&1
  retcode="$?"
  set -e
  if [[ ! "${retcode}" = "0" ]] && [[ ! "${retcode}" = "2" ]]; then
    _exiterr "This script requires curl."
  fi
}

это все, если не нравится - напиши сам