A Comprehensive Guide to Using Cronjobs

There are times when there’s a need for running a group of tasks automatically at certain times in the future. These tasks are usually administrative, but could be anything - from making database backups to downloading emails when everyone is asleep.

Cron is a time-based job scheduler in Unix-like operating systems, which triggers certain tasks at a point in the future. The name originates from the Greek word χρόνος (chronos), which means time.

The most commonly used version of Cron is known as Vixie Cron, originally developed by Paul Vixie in 1987.

This article is an in-depth walkthrough of this program, and a reboot of this ancient, but still surprisingly relevant post.

Terminology

• Job: a unit of work, a series of steps to do something. For example, sending an email to a group of users. In this article, we’ll use tasks, job, cron job or event interchangeably.

• Daemon: (/ˈdiːmən/ or /ˈdeɪmən/) is a computer program which runs in the background, serving different purposes. Daemons are often started at boot time. A web server is a daemon serving HTTP requests. Cron is a daemon for running scheduled tasks.

• Cron Job: a cron job is a scheduled job, being run by Cron when it’s due.

• Webcron: a time-based job scheduler which runs within the web server environment. It’s used as an alternative to the standard Cron, often on shared web hosts that do not provide shell access.

Getting Started

This tutorial assumes you’re running a Unix-based operating system like Ubuntu. If you aren’t, we recommend setting up Homestead Improved - it’s a 5 minute process which will save you years down the line.

If we take a look inside the /etc directory, we can see directories like cron.hourly, cron.daily, cron.weekly and cron.monthly, each corresponding to a certain frequency of execution. One way to schedule our tasks is to place our scripts in the proper directory. For example, to run db_backup.php on a daily basis, we put it inside cron.daily. If the folder for a given frequency is missing, we would need to create it first.

Note: This approach uses the run-parts script, a command which runs every executable it finds within the specified directory.

This is the simplest way to schedule a task. However, if we need more flexibility, we should use Crontab.

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by Reza Lavaryan via SitePoint