[SOLVED] Bash - get return code of background process only if it died

Issue

I have a simple server client test. I start the server as a background process, wait for 5 seconds and then start the client. Like so

server > /tmp/serverlog 2>&1 &

sleep 5

client > /tmp/clientlog 2>&1

ret=$?

cat /tmp/serverlog
cat /tmp/clientlog

exit ret

Sometimes, the server will fail to start. In that case, I want to exit with the exit code from the server and not start the client.

I can check if the server is alive with kill -0. But I can't get the exit code. I can use wait to wait for the server to finish but if the server doesn't fail, then wait will never return.

How can I check if the background server process died and get the return code only if it died?

Solution

Use kill -0 to see if the server died, and if it did, then use wait to get its exit status.

server > /tmp/serverlog 2>&1 & server_pid=$!

sleep 5

if ! kill -0 $server_pid; then
    wait $server_pid
    exit  # uses exit status of previous command, which is exit status of server
fi

client > /tmp/clientlog 2>&1

ret=$?

cat /tmp/serverlog
cat /tmp/clientlog

exit $ret

It shouldn't be a real issue, but be aware of the race condition of the server's process ID being available for reuse after it exists, but before you use it with kill and wait.

Answered By - chepner

Answer Checked By - Candace Johnson (WPSolving Volunteer)