Cron

Definition: Cron is a time-based job scheduler that automatically runs commands or tasks at specified times or recurring intervals, without a person having to trigger them manually. (man7.org)
In a backend, you can use it for background jobs such as cleanup, backups, report generation, or periodically checking something. (Linuxize)

Simple example

Suppose you want to delete expired sessions every night:

       Cron Scheduler
             │
       Every night
          at 02:00
             ↓
    cleanupExpiredSessions()

You don’t need a user to click anything. Cron automatically starts the task at 02:00.

3 examples

1. Database backup

Every day at 03:00
        ↓
Backup PostgreSQL database

2. Cleanup

Every hour
    ↓
Delete expired sessions
    ↓
Delete temporary files

3. Generate reports

Every Monday at 08:00
          ↓
Generate weekly sales report
          ↓
Send it to the manager

What does a cron expression mean?

A standard Unix cron schedule has five time fields:

* * * * *
│ │ │ │ │
│ │ │ │ └── Day of week
│ │ │ └──── Month
│ │ └────── Day of month
│ └──────── Hour
└────────── Minute

For example:

0 2 * * *

means:

Run at 02:00 every day. (man7.org)

Cron vs. normal code

The important distinction is:

Normal function:
User/API request
      ↓
   Function()

Cron job:
      Clock
        ↓
   Scheduled time
        ↓
   Function()

So when someone says:

“Use a background job scheduler / cron.”

they basically mean:

“Run this task automatically in the background at a particular time or interval, rather than waiting for a user request.”

One small distinction: classic Unix cron is primarily for recurring schedules. If you need something like “run this once 20 minutes from now,” a job queue/scheduler or a one-time scheduler may be more appropriate. (Debian)

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