Cron Jobs

Also Known As

TL;DR

Explanation

Cron syntax: five fields (minute hour day month weekday) followed by the command. Special strings (@hourly, @daily, @weekly, @monthly, @reboot) simplify common schedules. Cron runs in a minimal environment — PATH is different from interactive shells, so use absolute paths. Output goes to the user's email unless redirected. PHP cron jobs should use the CLI SAPI, not the web server. Consider systemd timers or a task scheduler like Laravel Scheduler for more complex needs.

Diagram

flowchart LR
    CRON[cron daemon] -->|reads| CRONTAB[crontab -e]
    CRONTAB --> SYNTAX[minute hour day month weekday command]
    subgraph Examples
        EV_MIN[every minute: * * * * *]
        EV_HOUR[every hour: 0 * * * *]
        EV_DAY[daily 2am: 0 2 * * *]
        EV_WEEK[weekly Mon: 0 0 * * 1]
    end
    CRON --> EXEC[Execute command]
    EXEC -->|stdout| MAIL[Email to user<br/>or redirect to log]
    subgraph Issues
        OVERLAP[Job overlaps<br/>previous still running]
        ENV2[Wrong environment<br/>PATH not set]
        OVERLAP & ENV2 -->|fix with| LOCKFILE[flock or systemd timers]
    end
style EXEC fill:#238636,color:#fff
style OVERLAP fill:#f85149,color:#fff
style LOCKFILE fill:#1f6feb,color:#fff

Common Misconception

Why It Matters

Common Mistakes

• Using relative paths — cron's PATH does not include /usr/local/bin; use /usr/bin/php explicitly.
• Not redirecting output — all stdout/stderr goes to cron email which is often unmonitored.
• Overlapping executions — a slow job runs again before the previous one completes; use file locking.
• Running cron jobs as root when a less privileged user is sufficient.

Code Examples

# Broken cron — relative path, no output handling:
*/5 * * * * php /var/www/app/artisan schedule:run
# Fails silently: 'php' not found in cron PATH
# No output captured — errors invisible

# Correct cron entry:
*/5 * * * * /usr/bin/php /var/www/app/artisan schedule:run \
    >> /var/log/app/schedule.log 2>&1

# With flock to prevent overlap:
*/5 * * * * flock -n /tmp/schedule.lock \
    /usr/bin/php /var/www/app/artisan schedule:run >> /var/log/app/schedule.log 2>&1

References

• ↗ https://man7.org/linux/man-pages/man5/crontab.5.html

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