kill command

sang_froid · 02-09-2010, 11:23 AM

what does the following command do ?

kill -0 $PID 1> /dev/null 2> /dev/null

crabboy · 02-09-2010, 11:33 AM

0 is not a valid signal. I think it will check for the existence of process with $PID, routes errors and stdout to /dev/null and sets the return code to 1 if it was found and 0 if not.

echo $?

sang_froid · 02-09-2010, 11:37 AM

well man page of kill says that it will kill all processes belonging to that group...that is what made me confused..

If that be correct, then say if i run that code in proftpd, then it is going to kill all proftpd child processes so far as i believe.

Quote:

Originally Posted by crabboy

0 is not a valid signal. I think it will check for the existence of process with $PID, routes errors and stdout to /dev/null and sets the return code to 1 if it was found and 0 if not.

echo $?

crabboy · 02-09-2010, 11:44 AM

Maybe the machine I tested on has an older version of kill, but kill -0 fails to kill anything.

crabboy · 02-09-2010, 11:47 AM

This is from the man page:

Quote:

If sig is 0, then no signal is sent, but error checking is still per-
formed.

I think what you read is regarding the PID:

Quote:

OPTIONS
pid... Specify the list of processes that kill should signal. Each pid
can be one of five things:

n where n is larger than 0. The process with pid n will be
signaled.

0 All processes in the current process group are signaled.
...

amani · 02-09-2010, 11:50 AM

man kill says

0 0 n/a exit code indicates if a signal may be sent

(*buntu-9.10 )

sang_froid · 02-09-2010, 01:30 PM

I am just trying to find out what is this piece of code trying to do

Quote:

RUN=./file
OPT=
#OPT=-d

# Send output to...
OUTPUT=./time.cron
#OUTPUT=/dev/null

if [ -r mech.pid ]; then
PID=`cat mech.pid`
if [ -r /proc/$PID ]; then
exit 0
fi
if ( kill -0 $PID 1> /dev/null 2> /dev/null ); then
exit 0
fi
fi

# its not running...

echo >> $OUTPUT
echo "Mech restarted:" >> $OUTPUT
( date 2>&1 ) >> $OUTPUT
echo >> $OUTPUT
( $RUN $OPT 2>&1 ) >> $OUTPUT

crabboy · 02-09-2010, 02:46 PM

It is checking to see if the PID in the pid file is valid. kill -0 PID will return a 1 if the process with PID is alive and if so the script will exit. If not, it will restart the process.