Cron – Monitoring file for changes and immediately take action on change

background-processcronmonitoringscheduled-tasktask-scheduler

How can I constantly monitor (small) file for changes?

Eg. when file is updated (action from web application), a script is executed (if not already running).

Right now, I do it every minute using cron, but this has delay up to one minute. I'd like to take the action immediately after the file is changed.

Maybe I need to write some low level process running in the background, once the server is started?

The reason I want to do it is to separate web application from root actions (performed on demand, once when file is updated).

Best Answer

incrond (inotify cron daemon) behaves like cron but uses inotify as the triggering mechanism. This guide has some pretty comprehensive examples.

An example incron entry for the case you described might be:

/path/to/file IN_CLOSE_WRITE /path/to/your_script

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Cron Job – How to Run a Cron Job Manually and Immediately

Here's what I did, and it seems to work in this situation. At least, it shows me an error, whereas running from the command line as the user doesn't show the error.

Step 1: I put this line temporarily in the user's crontab:

* * * * *   /usr/bin/env > /home/username/tmp/cron-env

then took it out once the file was written.

Step 2: Made myself a little run-as-cron bash script containing:

#!/bin/bash
/usr/bin/env -i $(cat /home/username/tmp/cron-env) "$@"

So then, as the user in question, I was able to

run-as-cron /the/problematic/script --with arguments --and parameters

This solution could obviously be expanded to make use of sudo or such for more flexibility.

Hope this helps others.

Linux – How to kill a runaway cronjob

To show daemons in ps you need to include the -e option (System V/POSIX) or a (BSDish). Try ps aux or ps -ef.

Best Answer

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Cron Job – How to Run a Cron Job Manually and Immediately

Linux – How to kill a runaway cronjob

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