Condition Inside String in Bashscript

Question

I want to print exit code of previous command in shell if it failed. So I changed PS1 variable in /etc/bash.bashrc like this:

PS1='${debian_chroot:+($debian_chroot)}\u@\h:\w [$?] \$ '
#_______________________________________________^^^^_____

but it always print the exit code of the previous command. Is there any way to add condition in the template so if $? is equal to 0, it does not print $?

Answer 1

You can create conditions and expressions inside $() like this:

echo "$( var=2; echo $var)"

in your example you can change PS1 variable like this:

PS1='${debian_chroot:+($debian_chroot)}\u@\h:\w $(
    status=$?; (( status != 0 )) && echo "[$status] "
)\$ '

looks like:

jackman@b7q9bw1:~ $ echo hello
hello
jackman@b7q9bw1:~ $ sh -c 'exit 42'
jackman@b7q9bw1:~ [42] $ 

Answer 2

Bash executes the code in the variable PROMPT_COMMAND before printing the prompt. One of the things you can do that is to calculate components of the prompt which are then assembled via the PS1 setting.

PROMPT_COMMAND_set_status () {
  if ((last_status == 0)); then
    PS1_status=
  else
    PS1_status="[$last_status] "
  fi
}
PS1=PS1='${debian_chroot:+($debian_chroot)}\u@\h:\w $PS1_status\$ '
PROMPT_COMMAND='last_status=$?; PROMPT_COMMAND_set_status'