How to automate docker prune commands using crontab

Question

Can someone help me on how to clean up containers and images which are 3 months older automatically by using crontab scheduling? Which type of method need to use for automation? I have manually tried deleting by using below commands and it worked but I need to automate it, to perform action for every 3 months without manual action.

docker image prune　 --force --filter “until=2020-01-01T00:00:00”
docker container prune　 --force --filter “until=2020-01-01T00:00:00”

Answer 1

1. create a new file in the Cron file to house our command

    cd /etc/cron.daily
    sudo nano docker-prune-daily
   

2. Once the file opens add the prune command we created and inform the OS it's a bash file

   #!/bin/bash
   docker image prune　 --force --filter “until=2020-01-01T00:00:00”
   docker container prune　 --force --filter “until=2020-01-01T00:00:00”
   

3. Once you've added the command, hit Ctrl + O and then Ctrl + X to save and close the file

4. Now that we've created the file we need to tell the operating system that it needs to be executable with the following command:

   sudo chmod +x /etc/cron.daily/docker-prune-daily
   

** If you want to test out to see if it will execute correctly next time the cronjob runs, you can force the daily Cron register to run with the following command:

    run-parts /etc/cron.daily

Answer 2

You can add a new cron entry by prompting:

sudo crontab -e

You can then edit the file with your command and its frequency. Every three months is not a standard cron frequency (i.e. there's no built-in solution to transcribe this into cron) so you can use these frequencies:

30 03 01 Jan,Apr,Jul,Oct *

or else

30 03 01 */3 *

(Based on this answer by Richard Holloway on ServerFault)

So eventually your cron entry will look like:

30 03 01 Jan,Apr,Jul,Oct * docker image prune　 --force --filter “until=2020-01-01T00:00:00”

You can check out your new entry by prompting:

sudo crontab -l

Edit

chicks's comment pointed out that the goal wasn't to automate an action to be performed every three months but rather to automatically find which objects are older than months. So guessing that you would like to perform this action everyday, I would rather go for:

0 0 3 * * find /var/lib/docker -type f -ctime +90 -delete

This finds all Docker files older than three months and deletes them every day at 3AM.