These are (currently) the only functions I recommend porting around when writing bash scripts

robust-bash.sh

#!/bin/bash -eu
if [ -z "${LIB_ROBUST_BASH_SH:-}" ]; then
  LIB_ROBUST_BASH_SH=included

  function error {
    echo "Error:  ${1:-}"
  }

  # Check to see that we have a required binary on the path
  function require_binary {
    if [ -z "${1:-}" ]; then
      error "${FUNCNAME[0]} requires an argument"
      exit 1
    fi

    if ! [ -x "$(command -v "$1")" ]; then
      error "The required executable '$1' is not on the path."
      exit 1
    fi
  }

  # Check to see that a required environment variable is set.
  # Use it without the $, as in:
  #   require_env_var VARIABLE_NAME
  # or 
  #   require_env_var VARIABLE_NAME "Some description of the variable printed when it is missing"
  function require_env_var {
    var_name="${1:-}"
    if [ -z "${!var_name:-}" ]; then
      error "The required environment variable ${var_name} is empty"
      if [ ! -z "${2:-}" ]; then
        echo "  - $2"
      fi
      exit 1
    fi
  }
fi