Cron Syntax Reference Guide

Cron's PATH is not your PATH

Cron runs with a stripped-down environment. php, node, python - none of them are on cron's PATH by default. Your script works fine when you run it manually and fails silently every night. Use full paths:

Run which php to find the full path on your server.

Silent failures are the default

By default, cron discards all output. If your script crashes, you will never know. Always redirect both stdout and stderr to a log file:

The 2>&1 part redirects errors too. Without it you only see half the picture.

Timezone mismatch

Cron uses the server's system timezone - not UTC, not your local time, not the app's timezone. Your backup runs at "2 AM" but which 2 AM depends entirely on what timedatectl says. AWS EventBridge is the only platform that always uses UTC, which is actually a feature. Check your timezone before assuming anything: cat /etc/timezone

Day-of-month AND day-of-week is OR, not AND

In Linux cron, 0 9 1 * 1 does not mean "the 1st of the month if it falls on Monday". It means "the 1st of the month, OR any Monday". Most people expect AND logic here and get surprised. If you need AND logic (run only when both conditions are true), you need to add a date check inside the script itself.

GitHub Actions

Standard 5-field cron in UTC. Minimum interval is every 5 minutes.

on:
  schedule:
    - cron: '0 2 * * *'

GitHub may delay scheduled runs during peak load.

GitLab CI

Configured in the GitLab UI under CI/CD › Schedules, not in the YAML. Uses standard 5-field syntax.

0 4 * * 1-5

Timezone is configurable per schedule in GitLab.

Jenkins

Jenkins uses standard cron syntax in the triggers block. Also supports H (hash) to spread load across jobs.

triggers {
  cron('0 2 * * *')
}

H * * * * means "once per hour at an arbitrary minute" - useful to avoid all jobs firing at :00.

CircleCI

Defined in .circleci/config.yml under triggers. Standard 5-field syntax in UTC.

triggers:
  - schedule:
      cron: "0 0 * * *"
      filters:
        branches:
          only: main

CircleCI requires a filters.branches block - easy to forget.