Question

Suppose I have test.sh as below. The intent is to run some background task(s) by this script, that continuously updates some file. If the background task is terminated for some reason, it should be started again.

#!/bin/sh

if [ -f pidfile ] && kill -0 $(cat pidfile); then
    cat somewhere
    exit
fi

while true; do
    echo "something" >> somewhere
    sleep 1
done &
echo $! > pidfile

and want to call it like ./test.sh | otherprogram, e. g. ./test.sh | cat.

The pipe is not being closed as the background process still exists and might produce some output. How can I tell the pipe to close at the end of test.sh? Is there a better way than checking for existence of pidfile before calling the pipe command?

As a variant I tried using #!/bin/bash and disown at the end of test.sh, but it is still waiting for the pipe to be closed.

---

What I actually try to achieve: I have a “status” script which collects the output of various scripts (uptime, free, date, get-xy-from-dbus, etc.), similar to this test.sh here. The output of the script is passed to my window manager, which displays it. It’s also used in my GNU screen bottom line.

Since some of the scripts that are used might take some time to create output, I want to detach them from output collection. So I put them in a while true; do script; sleep 1; done loop, which is started if it is not running yet.

The problem here is now that I don’t know how to tell the calling script to “really” detach the daemon process.

Answer 1

See if this serves your purpose:
(I am assuming that you are not interested in any stderr of commands in while loop. You would adjust the code, if you are. 🙂 )

#!/bin/bash

if [ -f pidfile ] && kill -0 $(cat pidfile); then
    cat somewhere
    exit
fi

while true; do
    echo "something" >> somewhere
    sleep 1
done >/dev/null 2>&1 &
echo $! > pidfile