Execute loop while commands is running

Question

I want to create a loop (blinking LED) while a command (in this case ping) is running. I am using the Raspberry Pi (Raspbian)

while [ `nmap -p 80 example.com` ] # something like this
    do 
    echo "1">/sys/class/gpio/...
    sleep 0.2
    echo "0">/sys/class/gpio/...
    sleep 0.2
done

Perhaps you could run ping as a child process, and have the parent process do the light-blinking whilst it waits for the child to finish what it's doing. — GoBusto
– GoBusto, Commented Nov 10, 2014 at 16:54

Gilles Quénot · Accepted Answer · 2014-11-10 18:16:19Z

7

What I would do :

any_command & _pid=$!
while kill &>/dev/null -0 $_pid; do
    echo "1">/sys/class/gpio/...
    sleep 0.2
    echo "0">/sys/class/gpio/...
    sleep 0.2
done

kill -0 just test if the pid exists =)
the command any_command is launched in the background
& put the command in the background
$! is the integer of the pid of the latest backgrounded job

edited Nov 10, 2014 at 18:16

answered Nov 10, 2014 at 16:54

Gilles Quénot

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4 Comments

Olivier Dulac Over a year ago

+1, nice. But why do you put the "&>/dev/null" in such a weird place?

Olivier Dulac Over a year ago

And maybe change the test by some use of jobs | grep any_command as the pid could be reused

Gilles Quénot Over a year ago

@OlivierDulac I like to place &>/dev/null here, that demonstrate this possibility. And your command will have a bigger footprint than mine.

Gilles Quénot Over a year ago

Yes, no subshell needed, indeed

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