CRONTAB Basic

Basics of Cron Tab (Crontab)

Crontab (short for "cron table") is a configuration file in Unix-like operating systems used to schedule jobs (commands or scripts) to run at specific times or intervals. The cron daemon runs in the background and checks the crontab file for jobs to execute.

1. Understanding Crontab Syntax

Each line in a crontab file represents a job and follows this format:

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* * * * * /path/to/command

- - - - -

| | | | |

| | | | | 

| | | | └─── Day of the week (0-6) (Sunday=0 or 7)

| | | └───── Month (1-12)

| | └─────── Day of the month (1-31)

| └───────── Hour (0-23)

└─────────── Minute (0-59)

• Minute: Specifies the minute (0-59) when the job should run.

• Hour: Specifies the hour (0-23) when the job should run.

• Day of the Month: Specifies the day of the month (1-31) when the job should run.

• Month: Specifies the month (1-12) when the job should run.

• Day of the Week: Specifies the day of the week (0-6, where 0 = Sunday) when the job should run.

You can use the following symbols to control the schedule:

• *: Any value (i.e., all possible values for that field).

• ,: Separate multiple values (e.g., 1,15 means run on the 1st and 15th).

• -: Specify a range (e.g., 1-5 means run from the 1st to the 5th).

• /n: Specify steps (e.g., */5 means every 5 minutes).

2. Common Examples of Crontab Scheduling

Here are some common crontab expressions:

a. Run a Job Every Minute

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* * * * * /path/to/command

b. Run a Job at 2:30 AM Every Day

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30 2 * * * /path/to/command

c. Run a Job Every Hour

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0 * * * * /path/to/command

d. Run a Job Every Day at Midnight

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0 0 * * * /path/to/command

e. Run a Job Every Sunday at 5 PM

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0 17 * * 0 /path/to/command

f. Run a Job on the 1st of Every Month at 12:00 PM

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0 12 1 * * /path/to/command

g. Run a Job Every 10 Minutes

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*/10 * * * * /path/to/command

h. Run a Job from Monday to Friday at 3:00 AM

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0 3 * * 1-5 /path/to/command

3. Viewing and Editing Crontab

To interact with your crontab file, you use the following commands:

a. Edit Crontab

To edit the crontab for the current user:

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$ crontab -e

This opens the crontab file in the default text editor (usually vi or nano), where you can add, edit, or remove jobs.

b. List Crontab Entries

To view the existing crontab jobs for the current user:

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$ crontab -l

c. Remove All Crontab Entries

To remove all crontab jobs for the current user:

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$ crontab -r

d. Edit Crontab for Another User

To edit the crontab of a different user (requires root privileges):

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$ sudo crontab -u username -e

4. Redirecting Output

By default, cron sends the output (both stdout and stderr) to the user's email. If you want to redirect output to a file, you can do it like this:

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* * * * * /path/to/command >> /path/to/logfile.log 2>&1

• >> /path/to/logfile.log: Redirect standard output (stdout) to a log file.

• 2>&1: Redirect standard error (stderr) to the same log file.

5. Cron Special Keywords

Instead of specifying the exact time for common schedules, you can use cron's special keywords:

Keyword Equivalent to Example

@reboot

Run once at boot

@reboot /path/to/command

@yearly

0 0 1 1 *

Run once a year at midnight on January 1st

@annually

Same as @yearly

@monthly

0 0 1 * *

Run once a month at midnight on the 1st

@weekly

0 0 * * 0

Run once a week at midnight on Sunday

@daily

0 0 * * *

Run once a day at midnight

@midnight

Same as @daily

@hourly

0 * * * *

Run once an hour at the beginning of the hour

6. Crontab Example: Run a Script Daily at 1 AM

To run a backup script daily at 1 AM, follow these steps:

Open the crontab editor:
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$ crontab -e

Add the following line:
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0 1 * * * /path/to/backup_script.sh >> /path/to/backup_log.txt 2>&1

2. Save and exit the crontab.

This will execute the backup_script.sh at 1:00 AM every day, and the output will be logged in backup_log.txt.

7. Useful Cron Commands

Check cron status (on most Linux systems):
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$ systemctl status cron

Restart cron:
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$ sudo systemctl restart cron

8. Crontab User Permissions

Only users listed in /etc/cron.allow are allowed to use cron, and users listed in /etc/cron.deny are denied access. If neither file exists, then cron is generally available to all users.