Show exit code of last command in Bash prompt if it returned error

Question

I've added the return value of the last command to PS1 (aka "the prompt") in my .bashrc.

Now I'd like to have it shown only if the value is nonzero.

Android's shell has it:

${| local e=$? (( e )) && REPLY+="$e|" return $e }

Question: how to convert it to bash?

Answer 1

PS1='${?#0}$ '

It uses a special form of parameter expansion, ${?#0}, which means: "Remove the character zero if it is the first character of ${?}, the exit code of the previous command."

You can also change the color of the prompt if the last exit code were not zero:

PS1='\[\e[0;$(($?==0?0:91))m\]$ \[\e[0m\]'

That uses a if-else ternary expression $(($?==0?0:91)) that makes the color code 0;91m (red, see color codes) if the last command exits with non-zero, or 0;0m (your default color) otherwise.

Answer 2

This is what I use in my .bashrc:

PS1_PROMPT() {
  local e=$?
  (( e )) && printf "$e|"
  return $e
}
PS1='$(PS1_PROMPT)'"$PS1"

ibug@example:~ $ false
1|ibug@example:~ $ ^C
130|ibug@exame:~ $ true
ibug@example:~ $

This has the advantage over JoL's answer that the value of $? is preserved after displaying (via return $e). It's also basically a rework of Android's default /system/etc/mkshrc that does the same job.

Note that $(PS1_PROMPT) is enclosed in single quotes because $PS1 is evaluated every time it's printed, in addition to when set in a variable assignment expression, so single quotes prevents it from being evaluted right now and instead defers that to when printing.

Alternatively, as suggested by ilkkachu, you can make use of Bash PROMPT_COMMAND special variable:

PS1_PROMPT() {
  local e=$?
  PROMPT_ECODE=
  (( e )) && PROMPT_ECODE="$e|"
  return $e
}
PROMPT_COMMAND=PS1_PROMPT
PS1='$PROMPT_ECODE'"$PS1"

This has one advantage that no subshell is spawned every time PS1 is printed.

Answer 3

The entire Android PS1 is:

${|
    local e=$?

    (( e )) && REPLY+="$e|"

    return $e
}$HOSTNAME:${PWD:-?} $ 

I don't recognize ${|, but this bash is equivalent:

PS1='$(e=$?; (( e )) && echo "$e|")$HOSTNAME:${PWD:-?} $ '

local is not needed because $() is a subshell.

Answer 4

Thank you, everyone, that's what I settled for:

PS1_PROMPT() {
  local e=$?
  #(( e )) && printf "$e|" # BW
  (( e )) && printf "\033[01;31m$e\033[00m|"  # color
  return $e
}
PS1='$(PS1_PROMPT)'"$PS1"

This displays errno, a.k.a. $?, in red if it's different than 0.

Note that the condition found in .bashrc, "$color_prompt" = yes could not be used above, because the function PS1_PROMPT() is not evaluated in .bashrc, but each time in the prompt. I did not want to export $color_prompt systemwide.