website/content/blog/autopurgedocker.md

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---
title: "Auto Purge Old Docker Images"
date: 2020-09-28T23:30:22-04:00
draft: false
tags: ["Containers"]
medium_enabled: true
---
I use [Watchtower](https://github.com/containrrr/watchtower) to automatically update the docker images I use. After leaving it for several months, I've realized that I have been storing over 100GB of old docker images. I needed a way to automatically purge old images and [Systemd Timers](https://opensource.com/article/20/7/systemd-timers) is the solution.
First it's useful to know the docker command that purges unused images that are older than 24 hours old.
```bash
docker image prune -fa --filter "until=24h"
```
Then we can create a oneshot service file that will describe its dependencies and descriptions for Systemd to manage. This file is `/etc/systemd/system/docker-purge.service`.
```yml
[Unit]
Description=Purge Docker Images Older than 24 Hours
Requires=docker.service
Wants=docker-purge.timer
[Service]
Type=oneshot
ExecStart=/usr/bin/docker image prune -fa --filter "until=24h"
[Install]
WantedBy=multi-user.target
```
Now we can create the systemd timer that will hook to this service. This is `/etc/systemd/system/docker-purge.timer`.
```yaml
[Unit]
Description=Purge Docker Images Older than 24 Hours
Requires=docker-purge.service
[Timer]
Unit=docker-purge.service
OnCalendar=*-*-* 00:00:00
AccuracySec=24h
[Install]
WantedBy=timers.target
```
This tells Systemd to run the service every day if enabled/started, though not necessarily at midnight. Systemd will schedule a time to run the service within the `AccuracySec` parameter. That is, it will schedule a time to run sometime everyday.
Finally, let's enable and start the timer.
```bash
sudo systemctl enable docker-purge
sudo systemctl start docker-purge
```
We can check the time that docker-purge is scheduled to run next by asking Systemd to list its timers.
```bash
systemctl list-timers
```
If you want to play around with the `OnCalendar` parameter. A useful command is
```bash
systmed-analyze calendar --iterations=$N "$TIME"
```
Where you replace `$N` with an integer, and `$TIME` with your sample time string.