TIL: timeout in Bash scripts

The other day at work we had a Bash script that would set up a web server and wait for it to be up before proceeding to the next things. The script worked fine and we had no issues, until we had an infinite loop.

We were using the Bash built-in until to check if the web server was up:

until curl --silent --fail-with-body 10.0.0.1:8080/health; do
	sleep 1
done

This works fine. Unless our web server crashes during startup and we sleep 1 forever.

Here comes a handy utility: timeout. As the name suggests, this command adds a timeout to other commands. You specify the time limit you want to wait for a command and if that time passes, timeout sends a signal to terminate it and exits with non-zero. By default, timeout sends SIGTERM, but you can change it with the --signal flag, e.g. timeout --signal=SIGKILL 1s foo.

For example, timeout 1s sleep 5 will send the SIGTERM signal to sleep after 1 second:

$ time timeout 1s sleep 4

real    0m1,004s
user    0m0,000s
sys     0m0,005s

$ echo $?
124

The natural thing to do then is to combine timeout and until:

timeout 1m until curl --silent --fail-with-body 10.0.0.1:8080/health; do
	sleep 1
done

The only issue is that this doesn’t work. timeout expects a killable command and until is a shell keyword: you can’t SIGTERM until. We can’t use timeout with any shell built-in.

The way forward is to wrap that until in a Bash process:

timeout 1m bash -c "until curl --silent --fail-with-body 10.0.0.1:8080/health; do
	sleep 1
done"

Another approach is to move the until to a separate Bash script and timeout it:

timeout 1m ./until.sh

It’s a shame we can’t use timeout with until directly, that would be amazing. But wrapping it in a Bash process/script gets the job done.