If you use GitHub actions to build and deploy (CI/CD) a project, you can schedule those actions to automatically run on a specific time interval using cron. This is sweet if you're building a pseudo-static site with data that doesn't change all the time, but does change often enough to want to automate the build process. I initially implemented cron into my GitHub Actions to build my side project Live Music Forecast every 6 hours.
What is Cron?
Cron (aka Cron Jobs, Crontab) is a command line utility about as old as linux itself. Cron allows for the automatic running of code, scripts, & commands on a time based interval.
Defining the Cron Schedule
Cron takes in a string to determine which minutes, hours, days, & months to run your script on.
┌───────────── minute (0 - 59) │ ┌───────────── hour (0 - 23) │ │ ┌───────────── day of the month (1 - 31) │ │ │ ┌───────────── month (1 - 12 or JAN-DEC) │ │ │ │ ┌───────────── day of the week (0 - 6 or SUN-SAT) │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ * * * * *
*/5 * * * * will run your GitHub Action every 5 minutes of every day. This is most likely over kill and will probably run you into GitHub's Action time limits very quickly. Every 5 minutes is the fastest interval GitHub allows actions to be ran.
Some more example cron schedules:
59 23 * * 6will run at 11:59 PM on Saturday
0 12 1 * *will run at 12 noon, on the 1st of every month
1 */6 * * *runs at the first minute of every 6th hour. (12:01a, 6:01a, 12:01p, and 6:01p)
I highly recommend using a tool like crontab.guru to make sure your schedule is set up properly. Debugging cron schedules is basically just publishing it and waiting for the scheduled time to come and go, so it can be annoying if you mess it up.
Scheduling your GitHub Action
In your action's configuration file, your schedule is defined as one of the workflow triggers under the
name: GH Actions Cron Schedule on: schedule: # * is a special character in YAML so you have to quote this string - cron: '1 */6 * * *' # rest of your action here...
If you need to, you can configure your action to run on multiple different schedules:
name: GH Actions Multi-Cron on: schedule: - cron: '1 */6 * * 0-4' # Run every 6hrs Sun-Thur - cron: '1 */3 * * 5-6' # Run every 3hrs Fri-Sat # rest of your action here...
For the full list of things that can trigger your GitHub Workflows, check out the GitHub Docs.
Heads Up, The Timing Isn't Going To Be Exact
GitHub Actions run in UTC time, so you'll need to adjust your cron schedule based on your timezone if you want it to run at a specific time.
The other thing GitHub mentions is that your cron schedule is when the action will be queued to run. This doesn't mean your action will run at that exact time, depending on server availability. If that slight timing ambiguity is going to cause issues with your workflow, Contact Me for a free consultation call to look at other alternatives.