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Introduction
Cron is a special feature of the Linux Operating System for automating scheduled tasks. Users can plan actions or scripts to run at predetermined intervals, such as daily backups, log rotation, system maintenance, database optimization, and much more, with the help of a cron job.
In this blog, we will discuss the “Linux Cron Cheat Sheet” for quickly and easily automating repetitive processes in your Linux environment, saving time, and boosting productivity.
Crontab in Linux
Crontab manages and schedules cron jobs by configuration files in Linux and other Unix-like operating systems. They are also known as Cron Table. It enables users to specify commands or scripts to be automatically run at predefined times. Each user on the system can have their Crontab, which contains the jobs they want to perform at specific times. Crontab functions as a time-based job scheduler.
Creating a New Crontab
In this section of "Linux Cron Cheat Sheet," we will see how to create a new Crontab, following the given below steps:
Step 1: Start a shell session or a terminal
Step 2: To open or create a new Cron table for the current user, type the following command:
crontab -e
Step 3: Select an editor to edit the Cron table if required
Commands in Crontab
In this "Linux Cron Cheat Sheet" section, we will see some commands in Crontab.
crontab -e: If a crontab file is not already there, edit it or create one
crontab -l: It is used to view the crontab file
crontab -r: It is used to delete the crontab file
crontab -v: It is used to show the last time your crontab file was modified
Syntax of Cron Job in Cron Table
There are five fields in the syntax of a cron job entry that let users specify when and how often a command or script should be run. The fields represent the following, with spaces separating each field.
Each field with an asterisk (*) denotes one that defines the Cron Job's timing. The fields, which are separated by spaces, specify the time when the command will be executed.
Cron Job Field Format
The fields in Cron Job Format are:-
Minute (0–59): It indicates the minute the cron job is supposed to run. If you set it to 12, the job will start running at 12 minutes of the hour
Hour (0–23): It indicates when the cron job should be executed. If you set it to 2, for example, the work will start at 2 AM
Month day (1–31): It indicates the day the cron job should be executed. If you set it to 8, for example, the job will run on the eighth day of each month
Month (1–12): It indicates the month the cron job shall be executed. By setting it to 6, for example, the task will be executed in June
Weekday (0–7): It denotes the day of the week the cron job shall execute. Sunday is represented by 0 and 7, Monday by 1, and so on. For example, if you set it to 4, the job will run on Thursdays
Cron Job Expression Examples
In this section of "Linux Cron Cheat Sheet," we will see various examples to understand the working of Cron Job.
Example 1: To run a script daily at 6 AM, run the following command.
0 6 * * * Command_to_executed
Example 2: To run a script daily at 4:40 PM, run the following command.
40 16 * * * Command_to_executed
Example 3: To run a script every Tuesday at 6:00 AM, run the following command.
0 6 * * 2 Command_to_executed
Example 4: To run a script every 20 minutes, run the following command.
* /20 * * * * Command_to_executed
Example 5: To run a script every hour, run the following command.
0 * * * * Command_to_executed
Example 6: To run a script every 4 hours, run the following command.
0 */4 * * * Command_to_executed
Example 7: To run a script every 1st of the month, run the following command.
0 0 1 * * Command_to_executed
Uses of Special Strings
Special strings offer an efficient way to establish the schedule without declaring each field individually. Some special strings are:-
@reboot
This unique string causes the supplied command or script to run once the system boots up. It is beneficial for tasks that must be carried out as the system boots up.
Example
@reboot /home/user/file.sh
@yearly
This unique string stands for "0 0 1 1 *", indicating that the cron job will execute just once annually, on January 1.
Example
@yearly /home/user/file.sh
@monthly
This unique string stands for "0 0 1 * *", indicating that the cron job will execute once every month, on the first of the month.
Example
@monthly /home/user/file.sh
@weekly
The cron job will execute once a week, on Sunday, according to the special string "0 0 * * 0," which it symbolizes.
Example
@weekly /home/user/file.sh
@daily
This unique string stands for "0 0 * * *," indicating that the cron job will be executed once a day at midnight.
Example
@daily /home/user/file.sh
@hourly
This unique string stands for "0 * * * *," indicating that the cron job will be executed once every hour at the start of the hour.
Example
@hourly /home/user/file.sh
Special Characters in Cron Table
Special Characters are used to express different scheduling choices and specify timing for running cron jobs. Some special characters used in Cron Table are:-
Asterisk: The asterisk is a wildcard that accepts any value. This term denotes "every possible value" for a field in a cron. The symbol * * * * *, for example, denotes "every minute, every hour, every day, every month, and every day of the week"
Comma: It denotes a list of values in a cron field. You can use it to run the cron job several times at predetermined times. For Example, “5,15,25 * * *” means "at the 5th, 15th, and 25th minute of every hour"
Hyphen: It is used to designate a range of values for a cron field. It enables you to run the cron job across the entire range that has been defined. As an illustration, 1-5 * * * * stands for "every minute from 1 to 5 past the hour"
Slash: In a cron field, the slash is used to specify step values. It enables you to run the cron job regularly. As an illustration, the notation */10 * * * * denotes "every 10 minutes"
Hash: In a cron table, comments are added using the hash character
Question Marks: Day of Month and Day of Week can be used instead of the symbol "* ."In a cron expression, only Day of Week or Day of Month may be used
Best Practice in Linux Cron
In this section of "Linux Cron Cheat Sheet," we will see some best practices while working with Cron.
Generating Log Files
The closing bracket (>) must be used once more if the cron output is to be saved to a file:
Cron Jobs are executed based on the system's selected time zone. However, you can specify the required time zone within the Cron Job entry if you need to perform a Cron Job based on a different time zone. For Example:
System Maintenance: Cron Jobs can carry out standard system maintenance procedures such as disc cleanup activities, updating software packages, and deleting temporary files.
Website Maintenance: Cron Jobs can run scripts or instructions to carry out operations connected to websites, such as database optimization, content updates, and scheduled publication or republication of articles.
Scheduled tasks for applications: Many services and applications use cron jobs to do certain operations at predetermined intervals. Cron jobs, for instance, may be used by email servers to send out recurring reports or content management systems to publish scheduled posts.
Data backups: Automating data backup processes with Cron Jobs ensures that important files and databases are regularly and consistently backed up.
Frequently Asked Question
What is a cron job?
The Cron job is a Linux task scheduled to be executed at predetermined intervals. It is controlled by the cron daemon, which runs scripts or instructions by the timetable specified in the cron table.
What fields are there in the cron schedule?
The minute, hour, day of the month, month, and day of the week are the five fields that make up a cron schedule. The following is the syntax: minute hour day month weekday.
How can a user momentarily disable a particular cron job?
Users can temporarily disable a particular cron task by commenting out the relevant line in the user's cron table. To deactivate a line, start it with a # by editing the cron table using the Crontab -e command.
What are the cron default time variables?
Cron offers several helpful predefined time macros, @reboot executes the command when the system boots, @daily execute it once per day, and @weekly executes it once per week. Additionally, macros are available for annual, monthly, and hourly chores.
Conclusion
Crontab, an important tool for job automation on Linux systems, makes it simple for users to plan out repetitive activities, system maintenance, and application-specific processes. Therefore, this cheat sheet will help you to do hands-on Linux Cron.
We hope this blog has helped you enhance your knowledge of Linux Cron. Do not stop learning! We recommend you read some of our articles related to the Linux Cron Cheat Sheet:
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