Shell Programming and Scripting

ps -ef | grep $0 | sed '/^$/ d' | sed '/grep/ d' | wc -l

#!/bin/ksh

# check for existence of a lock file
# if the file exists make sure the pid that 
#          created it is still running then exit.
if [ -e /tmp/myshellscript.lock ]; then 
         pid=`cat /tmp/myshellscript.lock`
         ps -p "$pid" >/dev/null 2&>1
# somebody else is running the script ?
         if [ $? -eq 0]; then
                echo "another processi is running - try again later"
                exit 1
         fi
# put the current process as owner of the lock
         echo "$$" > /tmp/myshellscript.lock

else
# no file found - make one, own the lock,too.

         echo "$$" > /tmp/myshellscript.lock
         chmod 777 /tmp/myshellscript.lock
fi
...

.....
# last lines of script - release lock file
rm -f /tmp/myshellscript.lock
exit 0