Q: I have a simple one line script that uploads a photo taken by my webcam to a remote server (/sbin/runuser -l myuser -c ‘scp /var/ftp/pub/snap/20131209-232415.jpg myuser@mydomain.com:www/cam/home.jpg).  How can I run this as a cron job every 30 seconds?

A:  This can be done, but not with cron alone.  There is no mechanism to run a cron job any more frequently than one minute.  This means you are limited to running your cron job once per minute.

One way you accomplish this is with a sleep in your script.  For example, something like this:

# Run the command
/sbin/runuser -l myuser -c 'scp /var/ftp/pub/snap/20131209-232415.jpg myuser@mydomain.com:www/cam/home.jpg
# Wait 30 seconds
sleep 30
#Run the command again
/sbin/runuser -l myuser -c 'scp /var/ftp/pub/snap/20131209-232415.jpg myuser@mydomain.com:www/cam/home.jpg

The problem you can run into is timing issues since the command will take time to complete.  So let’s say it takes 2 seconds for your upload to finish.  This means that the second command in actually running 32 seconds after the cron job launched (SLEEP + TIME TO EXECUTE). This puts you in a possible situation of launching a cron job, on the next minute, before the previous iteration has completed.

You can use flock to get around this by creating a lock file.  Below is a link to a good explanation of the basics of flock.

Bash Script Locking with flock

Resources for Scheduling Jobs in Linux