Bash prompt with the last exit code

I've been trying to customize my Bash prompt so that it will look like

[feralin@localhost ~]$ _

with colors. I managed to get constant colors (the same colors every time I see the prompt), but I want the username ('feralin') to appear red, instead of green, if the last command had a nonzero exit status. I came up with:

\e[1;33m[$(if [[ $? == 0  ]]; then echo "\e[0;31m"; else echo "\e[0;32m"; fi)\u\e[m@\e[1;34m\h \e[0;35m\W\e[1;33m]$ \e[m

However, from my observations, the $(if ...; fi) seems to be evaluated once, when the .bashrc is run, and the result is substituted forever after. This makes the name always green, even if the last exit code is nonzero (as in, echo $?). Is this what is happening? Or is it simply something else wrong with my prompt? Long question short, how do I get my prompt to use the last exit code?

As you are starting to border on a complex PS1, you might consider using PROMPT_COMMAND. With this, you set it to a function, and it will be run after each command to generate the prompt.

You could try the following in your ~/.bashrc file:

PROMPT_COMMAND=__prompt_command    # Function to generate PS1 after CMDs

__prompt_command() {
    local EXIT="$?"                # This needs to be first
    PS1=""

    local RCol='\[\e[0m\]'

    local Red='\[\e[0;31m\]'
    local Gre='\[\e[0;32m\]'
    local BYel='\[\e[1;33m\]'
    local BBlu='\[\e[1;34m\]'
    local Pur='\[\e[0;35m\]'

    if [ $EXIT != 0 ]; then
        PS1+="${Red}\u${RCol}"        # Add red if exit code non 0
    else
        PS1+="${Gre}\u${RCol}"
    fi

    PS1+="${RCol}@${BBlu}\h ${Pur}\W${BYel}$ ${RCol}"
}

This should do what it sounds like you want. Take a look a my bashrc's sub file if you want to see all the things I do with my __prompt_command function.

If you don't want to use the prompt command there are two things you need to take into account:

1. getting the value of $? before anything else. Otherwise it'll be overridden.
2. escaping all the $'s in the PS1 (so it's not evaluated when you assign it)

Working example using a variable

PS1="\$(VALU="\$?" ; echo \$VALU ; date ; if [ \$VALU == 0 ]; then echo zero; else echo nonzero; fi) "

Working example without a variable

Here the if needs to be the first thing, before any command that would override the $?.

PS1="\$(if [ \$? == 0 ]; then echo zero; else echo nonzero; fi) "

Notice how the \$() is escaped so it's not executed right away, but each time PS1 is used. Also all the uses of \$?.

I wanted to keep default Debian colors, print the exact code, and only print it on failure:

# Show exit status on failure.
PROMPT_COMMAND=__prompt_command

__prompt_command() {
    local curr_exit="$?"

    local BRed='\[\e[0;91m\]'
    local RCol='\[\e[0m\]'

    PS1='${debian_chroot:+($debian_chroot)}\[\033[01;32m\]\u@\h\[\033[00m\]:\[\033[01;34m\]\w\[\033[00m\]\$ '

    if [ "$curr_exit" != 0 ]; then
        PS1="[${BRed}$curr_exit${RCol}]$PS1"
    fi
}