A cron job is a scheduled task that runs automatically at specific times on Linux and Unix systems. You define when to run using a cron expression (5 fields for minute, hour, day, month, weekday) and what command to execute. The cron daemon checks every minute and runs matching tasks.

How do I write a cron expression?

A cron expression has 5 fields separated by spaces: minute (0-59), hour (0-23), day of month (1-31), month (1-12), and day of week (0-6, Sunday=0). Use * for every value, */n for every n units, comma for lists, and hyphen for ranges. Example: 0 9 * * 1-5 means 9 AM on weekdays.

How do I view and edit my cron jobs?

Use 'crontab -l' to list your cron jobs, 'crontab -e' to edit them in a text editor, and 'crontab -r' to remove all jobs. Each user has their own crontab. System-wide cron jobs are in /etc/crontab and /etc/cron.d/ directory.

What does */15 mean in a cron expression?

The /15 is a step value meaning 'every 15 units'. So */15 * * * * means every 15 minutes (at 0, 15, 30, 45 minutes past each hour). You can use steps with ranges too: 0-30/10 means every 10 minutes from 0 to 30 (0, 10, 20, 30).

Why is my cron job not running?

Common reasons: wrong timezone (cron uses system time), missing execute permissions on script, environment variables not set (cron has minimal PATH), wrong file paths (use absolute paths), or syntax errors. Check /var/log/syslog or /var/log/cron for error messages.

@Ajit5ingh

Cron Jobs Explained

Schedule tasks to run automatically on Linux

What is a Cron Job?

A cron job is a scheduled task that runs automatically at specific times on Linux and Unix systems. Want to back up your database every night at 2 AM? Or clear temp files every Sunday? That's what cron does. You write the schedule, cron handles the rest.

Fun fact: "Cron" comes from the Greek word "chronos" meaning time. It's been around since the 1970s.

How Cron Works


flowchart LR
    A[Crontab File] --> B[Cron Daemon]
    B --> C{Time Match?}
    C -->|Yes| D[Run Command]
    C -->|No| E[Wait]
    E --> C
    D --> F[Log Output]

The cron daemon runs in the background, checking every minute if any scheduled tasks need to run.

Cron Expression Format

A cron expression has 5 fields separated by spaces. Each field tells cron when to run:

* Minute
0-59

* Hour
0-23

* Day
1-31

* Month
1-12

* Weekday
0-6

💡 Tip: An asterisk (*) means "every" - so * * * * * runs every minute.

Special Characters

`*` Asterisk

Matches every value. * * * * * = every minute of every hour of every day.

`,` Comma

List multiple values. 0 9,17 * * * = at 9 AM and 5 PM.

`-` Hyphen

Range of values. 0 9-17 * * * = every hour from 9 AM to 5 PM.

`/` Slash

Step values. */15 * * * * = every 15 minutes.

Build Your Cron Expression

Not sure if you got the syntax right? Use our free tool to parse or build cron expressions.

Try Cron Expression Tool →

Common Examples

0 0 * * *

Every day at midnight. Good for daily backups or cleanup tasks.

0 9 * * 1-5

Every weekday at 9 AM. Send daily reports, skip weekends.

*/10 * * * *

Every 10 minutes. Health checks or monitoring.

0 2 * * 0

Every Sunday at 2 AM. Weekly maintenance window.

0 0 1 * *

First day of every month at midnight. Monthly reports or billing.

Managing Cron Jobs

Use the crontab command to manage your scheduled tasks:

# View your cron jobs

crontab -l

# Edit your cron jobs

crontab -e

# Remove all your cron jobs (careful!)

crontab -r

⚠️ Watch out: Cron uses the system timezone. If your server is in UTC but you want 9 AM New York time, you'll need to convert.

Crontab File Format


flowchart TB
    subgraph crontab[Crontab Entry]
        A["* * * * *"] --> B["/path/to/script.sh"]
    end
    
    A --> C[Schedule]
    B --> D[Command to Run]
    
    C --> E["When to run:
 minute, hour, day, month, weekday"]
    D --> F["What to run:
 script, command, or program"]