Bash Scripting Tips

I went looking around for bash scripting tips, especially secure coding of bash. Can't find much information, so decided to consolidate whatever I found here.

• Salt string comparisons of variables to increase security
  

  if [[ "a$?" == "a4" ]]; then
  

• Use the full paths to any binaries, either by hardcoding them into the script or use variable substitution. This prevents the script from executing incorrect/rogue binaries in the path.
  

  /bin/grep "hardcoding the full path" *
  
  echo=/bin/echo
  ${echo} "From bash manpage under EXPANSION:
  The order of expansions is: brace expansion, tilde expansion,  parameter,
  variable  and  arithmetic  expansion  and command substitution (done in a
  left-to-right fashion), word splitting, and pathname expansion."
  

• Change the environment path at the start of the script to ensure no rouge directories are in the PATH
  

  #!/bin/bash
  # comments
  PATH=/bin:/usr/bin
  

• Write a function to explain the usage of the script
  

  function print_usage () {
      ${echo} "
  $0
  Usage: $0 [-a opts] [arguments]
   or    $0 -h
  Description: Something fishy
  Options:
    -a opts    (Optional) Options
    -h         (Optional) Help
    arguments  Smelly smelly fish
  "
  }
  

• Here's a sample code snippet to process script options
  

  if [ $# -lt 2 ]; then
      print_usage
      exit 1
  else
      while getopts ha:b: options; do
          case "${options}" in
              h)  print_usage
                  exit 1
                  ;;
              a)  flag=${options}
                  ;;
              b)  flag=${options}
                  ;;
              *)  echo "default case, everything else fits here"
                  ;;
          esac
      done
      shift $((${OPTIND} - 1))
  

• Variables should be enclosed in parenthesis when used, to indicate exactly which variable you are using. Of course, this can prevent an exploit involving longer variable names.
  

  a=erie
  ab=were
  if [[ "${a}b" == "erieb" ]]; then