Pure CI/CD: Trigger CronJobs from Containers

Crontab UI running inside a docker container activating CI/CD procedures on the host machine

Having all CronJobs (such as CI/CD procedures) in an isolated container is cool, isn’t it?

Defining a new CronJob on your host machine (in this case Ubuntu) is as simple as running the command crontab -e. This will verify and save a file inside /etc/cron.d/your_username.

So if we mount our cron.d directory to the guest container, it will allow us to edit it from the inside. But there is an issue which is a feature at the same time. We know that crontab will run CronJobs with the same username. Then we can treat this as a feature to only allow the host to run the CronJobs.

The main difference between crontab -e and directly editing /etc/crond.d/<your_username> in syntax is the necessity of adding a related username to the definition of the command. etc:

*/30 * * * * ubuntu ./cicd.sh

Definition of a CronJob using Crontab UI / ref / You have to put username at the end of the `week` field (trick!)

Crontab UI is a web-based graphical user interface that allows us to simply edit CronJobs without direct access to the shell, which is a nice project by the way.

It can simply be done by creating the files below and running docker compose up -d:

variables.env

BASIC_AUTH_USER=tayyebi
BASIC_AUTH_PWD=my_voice_is_my_pa55p0rt

docker-compose.yml

version: '3.7'

services:
  crontab-ui:
    container_name: crontab-ui
    image: alseambusher/crontab-ui
    network_mode: bridge
    ports:
      - 8000:8000
    env_file: variables.env
    volumes:
      - /etc/cron.d:/etc/crontabs
      - ./data/crontabs:/crontab-ui/crontabs

There is a simple way to test the installation, and that is to create a CronJob wall $(date) with the username taken from the host machine every minute (* * * *). The wall command is somehow the same to echo, but it will also let the user broadcast a message on all active terminals.

Broadcast message sent using `wall` command on Linux Ubuntu/Debian

Finally, it is possible to create a cicd.sh bash file and put all git pull and docker ... --build commands inside that. Please don’t forget to navigate to your working directory (dirname) before any commands in the bash script.