How do I make sure my bash script isn't already running?

Question

I have a bash script I want to run every 5 minutes from cron... but there's a chance the previous run of the script isn't done yet... in this case, i want the new run to just exit. I don't want to rely on just a lock file in /tmp.. I want to make sure sure the process is actually running before i honor the lock file (or whatever)...

Here is what I have stolen from the internet so far... how do i smarten it up a bit? or is there a completely different way that's better?

if [ -f /tmp/mylockFile ] ; then
  echo 'Script is still running'
else
  echo 1 > /tmp/mylockFile
  /* Do some stuff */
  rm -f /tmp/mylockFile
fi

Answer 1

If you want to check the process's existence, just look at the output of

ps aux | grep your_script_name

If it's there, it's not dead...

As pointed out in the comments and other answers, using the PID stored in the lockfile is much safer and is the standard approach most apps take. I just do this because it's convenient and I almost never see the corner cases (e.g. editing the file when the cron executes) in practice.

Answer 2

Store your pid in mylockFile. When you need to check, look up ps for the process with the pid you read from file. If it exists, your script is running.

Answer 3

You can always just:

 if ps -e -o cmd | grep scriptname > /dev/null; then 
     exit
 fi

But I like the lockfile myself, so I wouldn't do this without the lock file as well.

Answer 4

lf=/tmp/pidLockFile
cat /dev/null >> $lf
read lastPID < $lf
[ ! -z "$lastPID" -a -d /proc/$lastPID ] && exit
echo not running
echo $$ > $lf
sleep 5

Answer 5

You might want to have a look at the man page for the flock command, if you're lucky enough to get it on your distribution.

NAME
       flock - Manage locks from shell scripts
SYNOPSIS
       flock [-sxon] [-w timeout] lockfile [-c] command...

Answer 6

In some cases, you might want to be able to distinguish between who is running the script and allow some concurrency but not all. In that case, you can use per-user, per-tty or cron-specific locks.

You can use environment variables such as $USER or the output of a program such as tty to create the filename. For cron, you can set a variable in the crontab file and test for it in your script.

Answer 7

Never use a lock file always use a lock directory. In your specific case, it's not so important because the start of the script is scheduled in 5min intervals. But if you ever reuse this code for a webserver cgi-script you are toast.

if mkdir /tmp/my_lock_dir 2>/dev/null
then
   echo "running now the script"
   sleep 10
   rmdir /tmp/my_lock_dir
fi

This has a problem if you have a stale lock, means the lock is there but no associated process. Your cron will never run.

Why use a directory? Because mkdir is an atomic operation. Only one process at a time can create a directory, all other processes get an error. This even works across shared filesystems and probably even between different OS types.