DEV Community

zhihu wu
zhihu wu

Posted on

Cron Jobs in 2026: Docker, CI/CD, and Why Your Old crontab Tricks Still Work

#cron #devops #docker #tutorial

Cron has been scheduling Unix jobs since the 1970s, and it is not going anywhere. But the way we run cron jobs has shifted: from bare-metal servers to Docker containers, from crontab files to CI/CD YAML, and from cron.d to Kubernetes CronJobs. Here is what changed — and what has not.

1. Cron Inside Docker

Docker containers prefer a single foreground process, so running a cron daemon inside a container feels unnatural. Three modern approaches:

  • Host cron + docker exec: Run cron on the host, and have each entry invoke docker exec container command. Zero container changes, works today.
  • supercronic: A drop-in crontab-compatible runner that logs to stdout/stderr. Perfect for containers whose log drivers expect stdout.
  • ofelia: A Docker-native scheduler that reads job config from container labels instead of a crontab file.

2. CI/CD Cron: GitHub Actions & Friends

GitHub Actions, GitLab CI, and Jenkins all support cron-triggered pipelines. The syntax is identical to standard 5-field cron:

# GitHub Actions — runs every weekday at 9 AM UTC
on:
  schedule:
    - cron: "0 9 * * 1-5"
Enter fullscreen mode Exit fullscreen mode

Key difference: CI/CD schedulers use UTC by default, not your local timezone. A 0 0 * * * job that reads "midnight" actually fires at midnight UTC — which may be 8 PM your time. Always verify the timezone offset.

3. The Classic Traps Have Not Changed

Some mistakes transcend the platform:

  • Weekday numbering: 0 is Sunday (POSIX), but Quartz Scheduler and some libraries start at 1 for Sunday. Check your platform convention.
  • OR logic: When both day-of-month AND day-of-week are set, the job fires when EITHER matches — not both. This catches everyone at least once.
  • Minimal environment: Cron runs without your shell PATH, aliases, or environment variables. Use absolute paths and set SHELL=/bin/bash.

Quick tip: I use a free visual cron generator to build expressions, see human-readable descriptions, and preview the next 5 execution times before pasting into production. Beats memorizing field order at 2 AM.

What is the worst cron bug you have deployed? Mine was a timezone mismatch that sent customer emails at 3 AM local time instead of 9 AM.

Top comments (0)