[SOLVED] Stop the running Command if a String Is found on stdout

blueray · 06-08-2020, 07:56 AM

How Can I close ping if a string is found in the output:

Code:

$ ping google.com
PING google.com (74.125.130.138) 56(84) bytes of data.
64 bytes from sb-in-f138.1e100.net (74.125.130.138): icmp_seq=1 ttl=44 time=50.3 ms
64 bytes from sb-in-f138.1e100.net (74.125.130.138): icmp_seq=2 ttl=44 time=49.5 ms
64 bytes from sb-in-f138.1e100.net (74.125.130.138): icmp_seq=3 ttl=44 time=49.5 ms
64 bytes from sb-in-f138.1e100.net (74.125.130.138): icmp_seq=4 ttl=44 time=49.5 ms

Now I want something like:

Code:

if ping google.com | grep -q 'time=49.5'; then
  //terminate ping
fi

So that, if time=49.5 is there it will terminate ping command.

pan64 · 06-08-2020, 08:14 AM

I would say this is much better to use:

Code:

while <command1>
do
   <command2>
done

berndbausch · 06-08-2020, 09:27 AM

Code:

if ping google.com | grep -q 'time=49.5'; then
  pkill ping
fi

Problem: You risk mass-murder instead of a surgical kill (i.e. it may not only kill ping).

EDIT: The above grep will never stop, thus you will never enter the then branch.

Perhaps better:

Code:

ping google.com > /tmp/pingoogle &
PING=$!
while ! grep -q time=49.5 /tmp/pingoogle
do sleep 1
done
kill $PING

Or something like that.

crts · 06-08-2020, 10:49 PM

Maybe I am missing something but I ran the following examples:

Code:

if ping -c3 google.com | grep -q 'time=14.. ms';then
    echo true
else
    echo false
fi
pgrep ping

if ping google.com | grep -q 'time=14.. ms';then
    echo true
else
    echo false
fi
pgrep ping

Both times there was no ping command running when pgrep ran. In the first example ping terminates because it has reached is max count (-c3) and false is echo'ed in the console. In the second example grep
terminates and true is echo'ed. The ping command terminates due to a broken pipe condition. So in your example you should not need the if condition at all.

PS:
Your grep is very specific. If your ping is not terminating it is likely because the grep does not find the string. I have a feeling that we are facing an XY problem here. What are you actually trying to do?

berndbausch · 06-09-2020, 12:07 AM

Quote:

Originally Posted by crts

In the second example grep terminates and true is echo'ed.

This is strange. Why should grep terminate? It should read until its input reaches end-of-file - which never happens, because ping keeps on generating output.

However I do agree with your assessment that the OP probably asks the wrong question.

blueray · 06-09-2020, 12:33 AM

@berndbausch

There is a new question based on top of this question.

https://www.linuxquestions.org/quest....php?p=6132308

Can you please help me with that?

pan64 · 06-09-2020, 01:56 AM

Quote:

Originally Posted by berndbausch

This is strange. Why should grep terminate? It should read until its input reaches end-of-file - which never happens, because ping keeps on generating output.

However I do agree with your assessment that the OP probably asks the wrong question.

I assume that would be something like that:

Code:

while ping -c 1 host | grep whatever
do
  sleep well
done

and that loop will be stopped based on the result of grep.

pan64 · 06-09-2020, 01:58 AM

Quote:

Originally Posted by blueray

@berndbausch

There is a new question based on top of this question.

https://www.linuxquestions.org/quest....php?p=6132308

Can you please help me with that?

you ought set this thread solved if it was solved and also would be nice to say thanks (if you think so) - by clicking on yes.

crts · 06-09-2020, 02:11 AM

Quote:

Originally Posted by berndbausch

This is strange. Why should grep terminate? It should read until its input reaches end-of-file - which never happens, because ping keeps on generating output.

However I do agree with your assessment that the OP probably asks the wrong question.

From the manpage (emphasis by me):

Code:

       -q, --quiet, --silent
              Quiet;  do  not  write  anything  to  standard   output.    Exit
              immediately  with  zero status if any match is found, even if an
              error was detected.  Also see the -s or --no-messages option.

It does not make much sense to keep matching results if they are not going to be displayed or written anywhere. But I agree, it is not obvious that grep would behave this way.

berndbausch · 06-09-2020, 02:23 AM

Quote:

Originally Posted by crts

it is not obvious that grep would behave this way.

Not obvious, but useful. Thanks for teaching me something new.

blueray · 06-09-2020, 02:25 AM

@pan64 I actually marked it solved and hit the "yes" button prior to your suggestion. Earlier, I used to thank via a reply as well. However, I stopped doing it because I thought it is not the norm here.

Please be sure of the fact that I can not thank you (plural) enough to spend your precious time helping me solve my problem. All of you who help me owe me big time.