'crontab' is a command in Unix used to schedule jobs to run periodically at fixed times, dates, or intervals. Here's a basic overview of how to use 'crontab':
Viewing the Crontab
To view the current user's crontab, you can use the following command:
crontab -l
Editing the Crontab
To edit the crontab, use the following command:
crontab -e
This opens the crontab file in the default text editor specified in your environment (often "vi" or "nano").
Crontab Syntax
The crontab file has the following syntax:
* * * * * command_to_be_executed
- The five asterisks represent the time specification for the job.
- The five fields, from left to right, represent minute (0-59), hour (0-23), day of the month (1-31), month (1-12), and day of the week (0-6, where Sunday is 0 or 7).
Examples
- To run a job every day at midnight:
0 0 * * * command_to_be_executed
- To run a job every hour:
0 * * * * command_to_be_executed
- To run a job every Sunday at 3:30 PM:
30 15 * * 0 command_to_be_executed
Special Characters
- An asterisk (*) means "every" for that time unit.
- A comma (,) can be used to specify a list of values. For example, '1,15' in the day field means the 1st and 15th day of the month.
- A hyphen (-) can be used to specify a range. For example, '1-5' in the day field means the 1st through 5th day of the month.
- Forward slash (/) can be used to specify intervals. For example, '*/15' in the minutes field means every 15 minutes.
Removing the Crontab
To remove the current user's crontab, use:
crontab -r
These are the basics of using "crontab" for scheduling tasks on Unix-like systems. Adjust the time specifications and command as needed for your specific use case.