Linux – using flock with cron

cronlinux

I need to prevent a script from being executed multiple times simultaneously and so flock seems right up my street.

What I can't figure out is if I need to manually delete the lock file after the completion of the cron job?

I'm using it like this:

# m h dom mon dow user  command
*/20 *  *  *  *  root /usr/bin/flock -w 0 /var/cron.lock /usr/bin/myscript

This script is also executed on machine startup by upstart (this is where the race condition is).

The file is created ok, but I can't really see how the command knows if the lock is being held by another process. It's empty and monitoring it for changes when the cron job is kicked off produces nothing (using fuser).

If someone can explain how it works I'd be very appreciative!

Best Answer

The lock file you specify as the option to /usr/bin/flock; /var/cron.lock remains locked with the flock(2) system call for the duration of your script /usr/bin/myscript. Once your script completes that lock is again released by /usr/bin/flock.

When the /usr/bin/flock command can't achieve a lock, , i.e. because /var/cron.lock is already locked because your script is still running (or any other error condition), /usr/bin/flock won't execute your script. Your script doesn't need to be modified and does not need to do any checking of the lock, that's all done by /usr/bin/flock.

The lock file itself will be created as an empty file by the /usr/bin/flock command if it does not yet exist, but after it has been created the lock file doesn't change, only an flock(2) will be applied and removed. The file won't be removed after your script completes.

Related Solutions

Cron – Ubuntu 9.10 cron script ‘ec2-consistent-snapshot’ unable to execute and access files

sudo crontab -e edits root's crontab right? When you say you have $HOME/.awssecret, what is $HOME? root's home or yours?

You might want to think about using /etc/cron.d, you can additionally add the name of the user to execute the script as in these files (e.g. caution: slightly different syntax for the scripts)

Edit (answering your question in the comment):

Create a file /etc/cron.d/myEc2Crontab

make it contain:

SHELL=/bin/sh
PATH=whatever you need as your path
12 18 4 2 *  root ec2-consistent-snapshot --mysql --xfs-filesystem /vol vol-xxxxxxx >>/mnt/backup.log 2>&1

note the added 'root' just after the time specification, prior to your command. This specifies the user the command runs as.

Cron – Redmine: fetch changesets from cron job

It's possible that ruby is in the Redmine user's $PATH but not in cron's. Try specifying the full path in the crontab entry.

*/5 * * * * /path/to/ruby /var/www/redmine/script/runner Repository.fetch_changesets -e production

Also, it would be helpful if you said what error messages or log entries you were getting instead of just saying "doesn't work". The additional information will help with diagnosing the problem.

Best Answer

Related Solutions

Cron – Ubuntu 9.10 cron script ‘ec2-consistent-snapshot’ unable to execute and access files

Cron – Redmine: fetch changesets from cron job

Related Topic