BASH – customizing PS1 to run a command

I wanted to modify my PS1 to run some commands every time. Let’s say I want it so that if the last executed command was successful, it would add a green smile at the end of PS1, otherwise the smile should be red.
I extracted it to a function:

function exit_smile {

    EXITSTATUS="$?"
    RED="[e[1;31m]"
    GREEN="[e[32;1m]"

    if [ "${EXITSTATUS}" -eq 0 ]
    then
       SMILE="${GREEN}:)"
    else
       SMILE="${RED}:("
    fi

    echo -n "$SMILE"
}

and then tried both using `exit_smile` and $(exit_smile) when modifying the PS1 variable, but it executes it once when modifying PS1 or prints literal [e...] instead of a color.
For example

PROMPT="<a href="https://getridbug.com/cdn-cgi/l/email-protection" class="__cf_email__" data-cfemail="750035">[email protected]</a>h W"
PS1="${PROMPT} $ $(exit_smile) ${OFF}n"

Gives [email protected] ~ $ [e[32;1m]:)
What am I missing?

Answers:

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Method 1

I’m not sure if this has changed between versions^(*), but my man page for Bash says that

Bash allows these prompt strings to be customized by inserting a number of backslash-escaped special characters that are decoded as follows:

(list contains e, [, ] etc.)

After the string is decoded, it is expanded via parameter expansion, command substitution, …

Which means that the [..] can’t come from the command substitution, but must be there before that.

(It also means you could use u or w as arguments to a command substitution, and they’d get replaced before the command runs. And I have no idea what putting [..] inside a command substitution would do… This would make more sense the other way around.)

So, we’ll have to put the color codes in separate expansions and protect them with [..] by hand. I’ll use variables instead of command substitution, and also the $'...' expansion to get the ESC character:

prompt_smile() {
        if [ "$?" = 0 ] ; then
                smile=' :) '
                smilecolor=$'e[1;32m'
        else
                smile=' :( '
                smilecolor=$'e[1;31m'
        fi
        normalcolor=$'e[0m'
}

PROMPT_COMMAND=prompt_smile
PS1='<a href="https://getridbug.com/cdn-cgi/l/email-protection" class="__cf_email__" data-cfemail="354075">[email protected]</a>h W $ [$smilecolor]$smile[$normalcolor]n'

_{(* the reason I wonder about that, is that the answers to the older and similar but no so duplicate question seem to output the [..] from within an expansion)}

All methods was sourced from stackoverflow.com or stackexchange.com, is licensed under cc by-sa 2.5, cc by-sa 3.0 and cc by-sa 4.0