Getting a Process ID & Assigning to a Variable in a Bash Script - bash

Question

I have an interesting situation. All code here is a functional pseudo-code example of the exact issue I am facing so no jokes about assign the output of date. I actually want to capture the output of a slower more resource dependent function, but date works well to show the functional obstacle I have run into.
I am writing bash script where I want to assign the output of a process to a variable like so:
RESPONSE=$(nice -n 19 date);
Now that gives me the RESPONSE in a nice variable, right? Okay, what if I want to get the process ID of the function called within $()? How would I do that? I assumed this would work:
RESPONSE=$(nice -n 19 date & PID=(`jobs -l | awk '{print $2}'`));
Which does give me the process ID in the variable PID, but then I no longer get the output sent to RESPONSE.
The code I am using as a functional example is this. This example works, but no PID; yes I am not assigning a PID but again this is an example:
RESPONSE=$(nice -n 19 date);
wait ${PID};
echo "${RESPONSE}";
echo "${PID}";
This example gives me a PID but no RESPONSE:
RESPONSE=$(nice -n 19 date & PID=(`jobs -l | awk '{print $2}'`));
wait ${PID};
echo "${RESPONSE}";
echo "${PID}";
Anyone know how I can get the value of RESPONSE with the PID as well?

Answer 1

Depending on exactly how you set it up, using RESPONSE=$(backgroundcommand) will either wait for the command to complete (in which case it's too late to get its PID), or won't wait for the command to complete (in which case its output won't exist yet, and this can't be assigned to RESPONSE). You're going to have to store the command's output someplace else (like a temporary file), and then collect it when the process finishes:
responsefile=$(mktemp -t response)
nice -n 19 date >$responsefile &
pid=$!
wait $pid
response=$(<$responsefile)
rm $responsefile
(Note that the $(<$responsefile) construct is only available in bash, not in plain posix shells. If you don't start the script with #!/bin/bash, use $(cat $responsefile) instead.)
This still may not do quite what you're looking for, because (at least as far as I can see) what you're looking for doesn't really make sense. The command's PID and output never exist at the same time, so getting both isn't really meaningful. The script I gave above technically gives you both, but the pid is irrelevant (the process has existed) by the time response gets assigned, so while you have the pid at the end, it's meaningless.