Overview of crontab
crontab is short for 'cron table', which is a file that contains the scheduled tasks of a user. Each line in this file defines a single job, and the cron daemon reads this file to execute the tasks at the specified times.
Structure of crontab Fields
Each line of the crontab file consists of 6 fields. The first 5 fields define the execution time, and the last field defines the command to be executed.
| Field | Description | Range |
|---|---|---|
| 1 | Minute | 0-59 |
| 2 | Hour | 0-23 |
| 3 | Day of month | 1-31 |
| 4 | Month | 1-12 |
| 5 | Day of week | 0-7 (0 and 7 are Sunday) |
| 6 | Command | The command or script to execute |
Special Characters
You can use special characters in the time fields to specify time more flexibly.
- `*`: Every time (e.g., every minute, every hour)
- `,`: Specify multiple values (e.g., `1,3,5` -> minutes 1, 3, 5)
- `-`: Specify a range (e.g., `10-15` -> from 10 minutes to 15 minutes)
- `/`: Specify an interval (e.g., `*/10` -> every 10 minutes)
Main Options
Take a look at the key features of the crontab command, including crontab file management options.
1. Managing crontab Files
2. User Management
Generated command:
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Description:
`crontab` Executes the command.
Combine the above options to virtually execute commands with AI.
Commonly Used Examples
Learn how to set up various repetitive tasks using crontab.
Run backup script at 2:30 AM every day
crontab -e
30 2 * * * /home/user/backup.shSets up to run the `/home/user/backup.sh` script at 2:30 AM every day.
System update at 8 AM every Monday
crontab -e
0 8 * * 1 sudo apt update && sudo apt upgrade -yUpdates system packages at 8 AM every Monday. (For Ubuntu/Debian)
Clean log file every 10 minutes
crontab -e
*/10 * * * * echo '' > /var/log/app.logDeletes the contents of `/var/log/app.log` every 10 minutes.
Restart web server at a specific time
crontab -e
0 4 1 * * sudo systemctl restart nginxRestarts the web server (nginx) at 4 AM on the 1st of every month.
Run task every hour from 6 AM to 6 PM
crontab -e
0 6-18 * * * /path/to/myscript.shRuns `myscript.sh` at the start of every hour from 6 AM to 6 PM.
Install crontab (if necessary)
Most Linux distributions have it installed by default, but in some minimal installations, you may need to install the cron daemon package.
Debian/Ubuntu
sudo apt update
sudo apt install cronInstalls the cron daemon on Ubuntu or Debian-based systems.
CentOS/RHEL/Fedora
sudo yum install cronie
sudo systemctl enable crond
sudo systemctl start crondInstalls the cron daemon on CentOS, RHEL, or Fedora-based systems.