Cron Jobs

By the end of this lesson you'll be able to read and write cron expressions, schedule a PHP CLI script to run automatically, log its output, and stop two runs from colliding — the toolkit behind every nightly backup and email digest.

Part of the free Php course at LearnCodingFast — hands-on lessons with examples you run in your browser, plus practice exercises and a quick quiz.

What You'll Learn in This Lesson

1️⃣ Cron Syntax: The Five Fields

A cron job is a command your server runs automatically on a schedule. That schedule is written as five space-separated fields followed by the command: minute hour day-of-month month day-of-week command . An asterisk ( * ) means "every value", */5 means "every 5th", a dash like 1-5 is a range , and a comma like 1,15 is a list . Learn to read these six lines and you can express almost any schedule.

Cron itself prints nothing — it just launches the command at the right moment. The fields only decide when . A common gotcha: the day-of-month and day-of-week fields are OR-ed together, so 0 0 13 * 5 runs on the 13th and on every Friday, not only Friday the 13th.

2️⃣ Installing a Job with crontab -e

Each user has their own crontab (cron table). You edit it with crontab -e , which opens an editor; the moment you save, the schedule is live. The command in a cron line is run by a bare shell, so you give the full path to PHP and the full path to your script : /usr/bin/php /var/www/app/backup.php . Cron throws away your script's output unless you capture it — so always redirect it to a log file with > > file.log 2 > &1 (append stdout, and send errors to the same place).

3️⃣ The PHP CLI Script Cron Runs

The script cron launches is ordinary PHP, but run from the command line rather than a browser. That mode is called the CLI SAPI (Server API), and php_sapi_name() returns "cli" there. There is no web page, so everything you echo goes to stdout — which the cron line redirects into your log file. Finish with a clear exit code : exit(0) for success, anything else for failure.

Because each line is timestamped with date('Y-m-d H:i:s') , the log tells you not just that the job ran but when — the first thing you'll want when something breaks at 2 AM.

4️⃣ Preventing Overlap with flock

What happens if a job scheduled */5 * * * * sometimes takes seven minutes? Cron starts the next copy on top of the first, and now two runs fight over the same data. A file lock stops this: the first run grabs an exclusive lock; any later run that can't grab it simply exits. You can do this in the cron line with the flock command (shown in section 2) or inside PHP with the flock() function below.

5️⃣ Your Turn: Write a Cron Line

Now you write the schedule. The line below is almost complete — fill in each ___ using the field order and the 👉 hints, then check it against the Output panel.

One more — this time the PHP script. Cron will redirect its output to a log, so give every line a timestamp . Fill in the blanks and run it.

Common Errors (and the fix)

• "php: command not found" (or the script just never runs) — cron uses a bare PATH and a different working directory than your login shell. Use absolute paths for both PHP and the script: /usr/bin/php /var/www/app/job.php (find PHP's path with which php ), and don't rely on relative paths or shell aliases.
• The job runs but reads the wrong files / env vars are missing — cron doesn't load your ~/.bashrc or shell profile, so variables you set there are absent. Set them at the top of the crontab ( PATH=... , APP_ENV=... ) or load them inside the script, and cd to the project directory in the command.
• Two copies clobber each other — a long run overlapped the next scheduled start. Wrap the command in flock -n /tmp/job.lock in the crontab, or call flock($handle, LOCK_EX | LOCK_NB) in PHP so the second run exits instead of piling up.
• It fails silently and you can't tell why — you didn't capture output. Add > > /var/log/app/job.log 2 > &1 to the cron line so both normal output and errors land in a file, then read it with tail -f .
• "Call to undefined function" for a web-only feature — you assumed the web SAPI. From cron PHP runs as cli : there's no $_SESSION , no header() that a browser will see, and a different php.ini may apply. Write CLI scripts to read arguments and print to stdout, not to expect a request.

Pro Tips

• 💡 Test the command by hand first. Run the exact line ( /usr/bin/php /var/www/app/job.php ) in a terminal before trusting cron with it — most "cron bugs" are path or permission bugs that show up immediately when run directly.
• 💡 Log a heartbeat, then alert on its absence. Have the job write a timestamp on success; if the log goes quiet, you know it stopped — far better than waiting for a user to notice.
• 💡 Prefer the Laravel scheduler or systemd timers for anything non-trivial. They keep timing logic in version control and give you proper logs ( journalctl ) instead of a crontab nobody remembers editing.

6️⃣ Alternatives: systemd Timers & the Laravel Scheduler

Plain crontab is everywhere, but two alternatives solve its weak spots. systemd timers pair a .timer unit (when) with a .service unit (what), giving you centralised logging via journalctl -u myjob , dependencies, and easy enable / disable . The Laravel scheduler flips the model: you define all schedules in PHP and add a single cron line that runs php artisan schedule:run every minute.

📋 Quick Reference — Cron Fields & Commands

Field / Token

Range / Example

Meaning

1st field

minute (0-59)

Minute of the hour

2nd field

hour (0-23)

Hour, 24-hour clock

3rd field

day of month (1-31)

Day number in the month

4th field

month (1-12)

Month of the year

5th field

day of week (0-6)

0 = Sunday … 6 = Saturday

*

* * * * *

Every value (here: every minute)

*/n a-b a,b

*/5 1-5 1,15

Every nth · range · list

crontab -e / -l

crontab -e

Edit / list your cron jobs

> > log 2 > &1

> > job.log 2 > &1

Append output + errors to a log

flock

flock -n /tmp/j.lock cmd

Prevent overlapping runs

Frequently Asked Questions

Mini-Challenge: A Self-Protecting Heartbeat

No code is filled in this time — just a brief and an outline. Write it yourself, run it on onecompiler.com/php or your own machine, then check your result against the expected output in the comments. This combines the lock and the timestamped log into one real cron-ready script.

🎉 Lesson Complete!

• ✅ A cron schedule is five fields — minute, hour, day-of-month, month, day-of-week — then the command
• ✅ crontab -e installs jobs; crontab -l lists them
• ✅ Run PHP from cron with absolute paths : /usr/bin/php /path/script.php
• ✅ Capture output and errors with > > file.log 2 > &1 — silent jobs are undebuggable
• ✅ Stop overlaps with flock (in the cron line or via PHP's flock() )
• ✅ Cron's bare environment is the #1 gotcha — set PATH /env explicitly, and remember PHP runs as cli , not the web SAPI
• ✅ For more, reach for systemd timers or the Laravel scheduler
• ✅ Next lesson: Environment Management — store config and secrets securely with .env files

Practice quiz

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