If you do daily management on Unix/Linux systems, then checking the return code of a command is something you'll do often. If you do SELinux development, you might not even notice that a command has failed without checking its return code, as policies might prevent the application from showing any output.

To make sure I don't miss out on application failures, I wanted to add the return code of the last executed command to my PS1 (i.e. the prompt displayed on my terminal).
I wasn't able to add it to the prompt easily - in fact, I had to use a bash feature called the prompt command.

When the PROMPT_COMMMAND variable is defined, then bash will execute its content (which I declare as a function) to generate the prompt. Inside the function, I obtain the return code of the last command ($?) and then add it to the PS1 variable. This results in the following code snippet inside my ~/.bashrc:

export PROMPT_COMMAND=__gen_ps1

function __gen_ps1() {
  local EXITCODE="$?";
  # Enable colors for ls, etc.  Prefer ~/.dir_colors #64489
  if type -P dircolors >/dev/null ; then
    if [[ -f ~/.dir_colors ]] ; then
      eval $(dircolors -b ~/.dir_colors)
    elif [[ -f /etc/DIR_COLORS ]] ; then
      eval $(dircolors -b /etc/DIR_COLORS)
    fi
  fi

  if [[ ${EUID} == 0 ]] ; then
    PS1="RC=${EXITCODE} \[\033[01;31m\]\h\[\033[01;34m\] \W \$\[\033[00m\] "
  else
    PS1="RC=${EXITCODE} \[\033[01;32m\]\u@\h\[\033[01;34m\] \w \$\[\033[00m\] "
  fi
}

With it, my prompt now nicely shows the return code of the last executed command. Neat.

Edit: Sean Patrick Santos showed me my utter failure in that this can be accomplished with the PS1 variable immediately, without using the overhead of the PROMPT_COMMAND. Just make sure to properly escape the $ sign which I of course forgot in my late-night experiments :-(.