Bash: wait with timeout

Question

In a Bash script, I would like to do something like:

app1 &
pidApp1=$!
app2 &
pidApp2=$1

timeout 60 wait $pidApp1 $pidApp2
kill -9 $pidApp1 $pidApp2         


        

Answer 1

To put in my 2c, we can boild down Teixeira's solution to:

try_wait() {
    # Usage: [PID]...
    for ((i = 0; i < $#; i += 1)); do
        kill -0 $@ && sleep 0.001 || return 0
    done
    return 1 # timeout or no PIDs
} &>/dev/null

Bash's sleep accepts fractional seconds, and 0.001s = 1 ms = 1 KHz = plenty of time. However, UNIX has no loopholes when it comes to files and processes. try_wait accomplishes very little.

$ cat &
[1] 16574
$ try_wait %1 && echo 'exited' || echo 'timeout'
timeout
$ kill %1
$ try_wait %1 && echo 'exited' || echo 'timeout'
exited

We have to answer some hard questions to get further.

Why has wait no timeout parameter? Maybe because the timeout, kill -0, wait and wait -n commands can tell the machine more precisely what we want.

Why is wait builtin to Bash in the first place, so that timeout wait PID is not working? Maybe only so Bash can implement proper signal handling.

Consider:

$ timeout 30s cat &
[1] 6680
$ jobs
[1]+    Running   timeout 30s cat &
$ kill -0 %1 && echo 'running'
running
$ # now meditate a bit and then...
$ kill -0 %1 && echo 'running' || echo 'vanished'
bash: kill: (NNN) - No such process
vanished

Whether in the material world or in machines, as we require some ground on which to run, we require some ground on which to wait too.

• When kill fails you hardly know why. Unless you wrote the process, or its manual names the circumstances, there is no way to determine a reasonable timeout value.

• When you have written the process, you can implement a proper TERM handler or even respond to "Auf Wiedersehen!" send to it through a named pipe. Then you have some ground even for a spell like try_wait :-)