Bash PS1 string that evaluates certain code every time it is looked up

Question

This question is very similar to this one: Colored PS1 string

But I still don't understand completely.

So say I have this function

function getjobs {
    echo $(jobs | wc -l)
}

And my PS1 is this

PS1="\u@\h: [jobs "
PS1="$PS1\$(if [[ $(getjobs) == 0 ]] ; then
               echo $(color $green);
            else
               echo $(color $red);
            fi)"

Now this only gets evaluated when .bashrc is loaded for the first time but what I want to have happen is this if statement evaluated every time a new prompt appears.

How can I do this?

EDIT: the color function returns escape sequences for those colors.

Answer 1

I usually use something like this:

method_that_echos_values(){
    if [[ ]]; then
      echo -n "something"
    else
      echo -n "something else"
    fi
}

export PS1="\$(method_that_echoes_values)"

Note that getting it extracted to a method is required

Answer 2

I would use bash's PROMPT_COMMAND for this. See the bash man page and this answer for an example.

Answer 3

So I was doing more experimenting with different placement of different quotes and I found something that works, but if someone knows a cleaner, better solution, please let me know.

PS1="\u@\h: [jobs "
PS1="$PS1\$(if [[ "'$(getjobs)'" == 0 ]] ; then
           echo $(color $green);
        else
           echo $(color $red);
        fi)"

All I had to do was put the part that I want reevaluated in single quotes with everything else in double quotes. I didn't think that it'd work since I had a double quoted string next to a single quoted string but I guess that's an ok thing.