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Cron Jobs – A Guide for Beginners

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Cron is one of the frequently used tools for Unix systems. It is used to schedule the execution of commands for a specific time. These “pending” teams or tasks are called “ Cron Jobs ”. Such a tool is great for regular backups, monitoring disk space, deleting files (for example, logs) and much more. This article will talk about working with Cron on Linux.

Introduction

The cron job template looks like this:

Minutes (0-59) Hours (0-24) Day (1-31) Month (1-12) Day of the week (0-6)

Here is an illustration of the same template that you can save yourself:

Cron Format
Cron Format

Asterisks indicate specific blocks of time.

To display the contents of the current user’s crontab file, use the command:

$ crontab -l

To edit a user’s tasks there is a command:

$ crontab -e

If this command is executed for the first time, you will be prompted to select an editor for Cron:


Select an editor. To change later, run 'select-editor'.
 1. /bin/nano <---- the simplest
 2. /usr/bin/vim.basic
 3. /usr/bin/vim.tiny
 4. /bin/ed

Choose 1-4 [1]:

Choose at your discretion. This is how the crontab file looks like:

In this file, you just need to list all the commands one by one.

To change another user’s crontab file (for example, ostechnix ):

$ crontab -u ostechnix -e

Below are some examples of cron jobs:

  1. To execute a command every minute, the task must be:* * * * * <command executable>
  2. A similar task, only the command will be called every five minutes :*/5 * * * * <command executable>
  3. Call the team 4 times per hour (every 15 minutes):*/15 * * * * <command executable>
  4. To execute a command every hour in 30 minutes, we write:30 * * * * <command executable> That is, the command will be executed not every 30 minutes, but when the value of minutes is 30 (for example, 10:30, 11:30, 12:30, etc.).
  5. Time values ​​can be combined by listing them separated by commas. The following code will execute the command three times per hour: at 0, 5, and 10 minutes.0,5,10 * * * * <command executable>
  6. The following task will be executed every hour :0 * * * * <command executable>
  7. Running a command every two hours :0 */2 * * * <command executable>
  8. To execute a command every day (at 00:00):0 0 * * * <command executable>
  9. Running the team every day at 03:00 :0 3 * * * <command executable>
  10. Running a command every Sunday (Sunday):0 0 * * SUN <command executable>
  11. Another variant of the task that will execute the command every Sunday (of course, also at 00:00):0 0 * * 0 <command executable>
  12. Running a command every day from Monday to Friday :0 0 * * 1-5 <command executable>
  13. The following task will execute the command every month, on the 1st day at 00:00 :0 0 1 * * <command executable>
  14. This task will be executed at 4:15 pm on each first day of the month :15 16 1 * * <command executable>
  15. Running a command every three months :0 0 1 */3 * <command executable>
  16. Running a command at a specific time and month:5 0 * 4 * <command executable>
  17. The task will call the team at the beginning of each semester (at 00:00 of the 1st day) :0 0 1 */6 * <command executable>
  18. Execution of the team every year on January 1st at 00:00 :0 0 1 1 *
    <command executable>

There are still ready tasks:

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  • @reboot – single command execution at boot;
  • @yearly – once a year;
  • @annually – also once a year;
  • @monthly – once a month;
  • @weekly – once a week;
  • @daily – once a day;
  • @midnight – also once a day;
  • @hourly – once an hour.

To execute a command every time after restarting the server, use this task:

@reboot <command executable>

The command to clear all tasks of the current user:

$ crontab -r

To learn about the details, there is a command:

$ man crontab

The above should already be enough for basic work with Cron and compiling assignments.

Crontab generator syntax

The process of writing assignments greatly simplify the web tools. They do not require knowledge of the Cron syntax, because they have a graphical interface, and the tasks are generated in accordance with the input data. The site generates a task that can be simply copied and pasted into a crontab file.

Crontab.guru

crontab.guru is a great site to explore various examples of cron jobs. Just enter the data and the site will generate the final task.

Cron Guru

The site has sections dedicated to examples and advice .

Crontag generator

crontab-generator.org is another site that helps to quickly generate crontab expressions. The principle is the same: you need to enter all the necessary data in the forms and click the Generate Crontab Line button at the bottom of the page.

Crontab Generator website
Crontab Generator website

Here is the final expression you see on the site:

In addition, there is a web tool ” Crontab UI “, which provides not only the ease of creating crontab-tasks but also security.

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