How do I make sure my script isn't already running before it attempts to run again?

my script it is very great

Well, I think this post is well-intentioned, but raises more issues than it solves.

1) Blindly believing a lock-file without checking if it belongs to a live process means that a single failure can permanently block your script from running.

2) Using /tmp for lockfiles is incredibly insecure, since an adversary can create the file there, and either block the execution of your script, or worse, if you read from the file, take advantage of any insecurity in your program to execute arbitrary code.

For (1), the solution is to write your process-id (pid) to the lockfile, and then check for the existence of that process when starting a new script:

LOCKDIR=/var/log/subsys LOCK=$LOCKDIR/$SCRIPTNAME echo $$ > $LOCK

and then to check if the previous script is still running:

function running() {

• #echo Checking pidfile $pidfile

• if ; then

• pid=cat $LOCK

• procfile=/proc/$pid

• if ; then

• true

• return

• fi

• fi

• false

As for (2), linux distros now generally default to using /var/lock/subsys for root processes. Check your platform for the acceptable lock directory. In general, picking a lock directory writable only by the user executing the script is key.

Thanks,

• --kirby