UNIX for Dummies Questions & Answers

I have a script which must not be run more than once at any given time. THis script will be scheduled to run every 20 mins as a cron job.

In my script can i have logic to say if this script is already running from the previous cron, then exit.

How do i go about doing that. If you describe the approach, can you please specify just the commands, which i can man to construct my approach.
Because I am not familiar with unix commands, if you tell me "find the file and do stuff..." i may not know the command to "find the file".

thanks!

Use ps to list running processes and use grep to filter the list...

Code:

ps -ef|grep '[m]yscript'

Ygor, this would list the processes whose effective userid is not that of the current user id,

instead,

Code:

ps -fu $LOGNAME | grep '[m]yscript'

You can also create a lock file with your current process PID. When starting your script, check for the lock file and, if found, retrieve the PID and check to see if the process is still running with ps -fp PID. I have encountered instances where ps -ef or -fu provided misleading results. The lock file method has never failed to work.

To add to the above stuff, if you checking with ps -ef for running process, you should also consider the other stuff like... if some opened the script with vi, or doing more with the executable or doing more with the log file name that contains executable name . log etc.... it will also come on ps -ef and your condition becomes true though your script is not running actually.

ps -fu $LOGNAME | grep '[m]yscript' | egrep -v "more | cat | vi | pg | ls | ......."

so better having lock file if possible, otherwise, use the above and do egrep -v with all possible combinations.

write a tmp file to the /tmp directory.
have your script check to see if the file exists, if it does then don't run.


#!/bin/sh

# our tmpfile
tmpfile="/tmp/mytmpfile"

# check to see if it exists.
# if it does then exit script
if [[ -f ${tmpfile} ]]; then
echo script already running.
exit
fi

# it doesn't exist at this point so lets make one
touch ${tmpfile}

# do whatever now.

# end of script
rm ${tmpfile}

I guess we can't simply touch a file, it should hold the pid of the process and check for that pid and whether it is same process or not.

#!/usr/bin/ksh

lockfile="/application/data/${pname}.lck"

pname="scripname"

if [ -f $lockfile ]
then
ps_cnt = $( ps -ef | grep $pname | fgrep -cf $lockfile )
fi

if [ ! -f $lockfile ] || (( $ps_cnt < 1 ))
then
echo "process not running, restarting it...."
nohup $lockfile &
sleep 5;
ptree $$ | grep $pname | sed 's/^[ ]*\([0-9]*\) .*$/\1/g' > /application/data/${pname}.lck
fi
fi

not tested...