I have a process which can run one instance at a time. Currently we have multiple scripts trying to kickoff this process. I wanted to implement the semaphore mechanism to achieve this.
I was going through few examples. The below code seems to be reasonable solution.
I cannot see the problems / limitations that you quote. There are other lock mechanisms, but they are more sophisticated, so possibly less suitable for your team. What's the problem with the file name?
With any POSIX-conforming shell, you can also use:
Code:
set -C # Set noclobber mode. (Attempts to overwrite an existing file fail.)
if echo $$ > lockfile
then printf 'Process ID %d now has the lock.\n' $$
else read pid < lockfile
printf 'Process ID %d holds the lock; exiting.\n' "$pid"
exit 1
fi
set +C # Return to default (no noclobber) mode.
trap 'rm -rf lockfile' EXIT # Remove the lock when we exit.
This works to guarantee that only one process holds the lock (assuming that lockfile is an absolute pathname), but the read could fail if the lock was held at the time you requested the lock but the lock holder terminated before the read was processed. (So, you need to verify that the read worked rather than assume that it will always succeed.)
#!/bin/sh
trap "rmdir lockdir" EXIT
if mkdir lockdir
then
echo start process
fi
This is backwards. If you do it this way, you will destroy a lock set by another process if you don't get the lock... You only want to set that trap after you acquire the lock.
Code:
#!/bin/sh
if mkdir lockdir
then
trap "rmdir lockdir" EXIT
echo start process
fi
This User Gave Thanks to Don Cragun For This Post:
Hi i have a script that check pings and i use flock to so the script wont run
multipul times :
its not the whole script but this is the idea :
(
flock -x -w 3 200 || exit 1
/usr/sbin/fping -c$count -i$interval -a $hosts > $FILE1 2>&1
) 200>/var/lock/.myscript.exclusivelock
now i... (4 Replies)
I'm changing my mindset from a few big processes moving data from a few sources under an external, dependency-based scheduler to multiple processes moving data from many sources run by each client cron and possibly interfering with each other. It has the benefits of more granular code but I'm... (11 Replies)
Hello,
I have been working on using "flock"/file lock to prevent two instances of a bash script from being executed. Below is a simplified version of what I have to illustrate the flock part. It works as it is set up there below however the piece I am trying to figure out is how to get it to... (2 Replies)
Hi,
I am new to this forum, could any one please help me to understand the LOCKFILE command with an example and what exactly it is used for and how it is used.
Thanks
Reshu289 (4 Replies)
Hi all,
I have to test some user priviliges. The goal is to be sure that an unauthorized user can't restart some modules (ssh, mysql etc...).
I'm trying to automate it with a shell script but in same cases I got the syslog broadcast message.
Is there any way to simply get a return code... (3 Replies)
Just tried out the following code from "Using lockfiles in shell scripts":
#!/bin/bash
if ; then
echo "File exists"
else
echo "File does not exists. Create file now."
echo 1 > number.txt
fi
lockfile script.lock
a=$(/usr/bin/tail -n 1 number.txt)
if ; then... (1 Reply)
Hi,
I have written below check lockfile script but need some tweaking on it.
If there is a lockfile from present, I need the script to retry every 10 seconds to see if the lockfile is still there. After 120 seconds it should send an email.
In my current version, if the script encounters... (6 Replies)
05-30-2015
