UNIX for Dummies Questions & Answers

Hi,
I have a ksh shell script that accesses databases to drop and create tables and the script also creates text files.
This shell script is accessed thru a java application that i would like to turn multi-user, but the only way that i can do that is if I can figure out a way to lock the shell script so that only 1 user can access it at a time.
If 2 or more users access it, then bad things happen when 2 users and trying to drop and create the same database at the same time, or trying to create the same text file at the same time.

I would also like for the user to wait till the shell script is unlocked, instead of just passing over it or ending the application.

please help!!

thanks!!

have the script issue a "lock file"...either touch or write to a file that it alone knows about (ie, formatted file name), and then the script would check for this file upon launch. If the lock file is present, then someone else is running...or was, if not set it, process and then remove it. Secondary scripts can use a while [ -f $touch_file ] ;then sleep x; fi...and then run once it clears out.

Be sure that your Admins can identify the lock file externally in case it might fail mid-process, and would need to be deleted. Also, it might not hurt to use a file name format that allows for a random or session-based component so that each script launch would have its own lock file to manage, but one that could still be seen by other runs of the script.

Maybe someone else will have a more elegant idea, but here's a way that comes to mind.

At the top of your script test for the presence of a file which indicates the script is running. If it is, loop, checking every 5 minutes or whatever until the file does not exist.

If it does not exist, then create it to indicate we are running and remove it at the end.

You could also write a row to a table in the database which logically does the same thing.

Partial code to show the logic:

Code:

#!/bin/ksh

#  Define constants
typeset -r LOCKFILE=/tmp/${0}.lock
typeset -ir LOCKWAITTIME=300   # seconds

# Remove the lock file if the script is interrupted before it ends normally.
trap 'rm $LOCKFILE' 0 2 3 9 15  # 0=normal exit

#  If lockfile exists, wait for it to go away.
if [[ -f $LOCKFILE ]]; then
     print "Waiting...\c"
     while [[ -f $LOCKFILE ]]
     do
       sleep $LOCKWAITTIME
       print ".\c"
     done
fi
# LOCKFILE does not exist so create it.
print "$0: locked by $(logname)" > $LOCKFILE

#script body here

exit 0

This example loops forever, you would most likely want to put a timeout in there I suppose that would exit after maybe 2 or 3 LOCKWAITTIME iterations. Also if more than one process is waiting there could be trouble if they both see the lock file go away at the same time.

Gary