'crontab' in Linux with Examples

If you do manually backups , update logs, or restart services on your Linux machine? Imagine that running repetitive tasks overnight so your machine works for you while you rest. Here crontab, the native job scheduler in Linux, which enables users to easily automate commands, scripts, and system tasks.

In this guide, you will learn how to use cron (the tool behind crontab) to schedule tasks by the minute, hour, or day. We’ll explain crontab’s syntax in plain language, walk you through practical examples (like automating backups or log updates), and share easy tips to avoid mistakes.

What is Linux Crontab?

Linux Crontab is a powerful utility used for Task Scheduling and Task Automation in Unix-like operating systems. It facilitates the users to run the Linux Commands or scripts at specified time and intervals. It is ideal for recurring tasks such as system maintenance, backup, and updating.

Every user can have his/her own Crontab file, whereby he/she can define the scheduling commands to be executed. It is used to simplify automating the task, enhancing system efficiency and reliability. Crontab is a shortened name of "cron table," as it uses the cron job scheduler to execute tasks; the "cron" was named from the Greek god "Chronos," which refers to time.

What is the Syntax of Linux Crontab?

The Linux Crontab Format is represented by the following syntax:

MIN HOUR DOM MON DOW CMD

Let's break down each field and discuss as follows:

How Linux Crontab Works?

Crontab functions by enabling the users to program their tasks or commands that assist in running automatically at given times and intervals. The following are the points that describe how the crontab will function:

• Crontab Configuration: Any user on a Linux server can have their crontab file that assists in listing the commands or scripts to be run as their schedule.
• Time Specification: It has operands that specify the time when the tasks need to be executed. Its time operands are the fields that represent the minute, hour, day of the month, month, and day of the week.
• Scheduled Execution: The cron daemon (cron) will repeatedly scan crontab files to see if there are any scheduled jobs matching the current time. When it finds a match, it runs the given command or script.
• Logging and Notifications: The output of cron jobs is usually emailed to the user's email address by default. Users can send output to files or turn off email notices whenever necessary.
• System-wide and User-specific: System administrators configure system-wide cron jobs in /etc/crontab and /etc/cron.d/, while individual users control their tasks using the crontab -e command to modify their crontab files.

What are Some Usage and Examples of Linux Crontab Jobs?

The following are some examples of cron jobs:

1. Scheduling a Job For a Specific Time:

• The basic usage of cron is to execute a job in a specific time as shown below. This will execute the Full backup shell script (full-backup) on 10th June at 08:30 AM.
• The time field uses 24 hours format. So, for 8 AM use 8, and 8 PM use 20.

30 08 10 06 * /home/maverick/full-backup

• 30 – 30th Minute 08 – 08 AM 10 – 10th Day 06 – 6th Month (June) * – Every day of the week

2. To View the Crontab entries:

• View Current Logged-In User’s Crontab entries: To view your crontab entries type crontab -l from your Unix account.
• View Root Crontab entries: Login as root user (su – root) and do crontab -l.
• To view crontab entries of other Linux users: Login to root and use -u {username} -l.

3. To Edit Crontab Entries Edit the Current Logged-In User’s Crontab Entries:

• To edit a crontab entries, use crontab -e. By default, this will edit the currently logged-in user crontab.

4. To Schedule a Job for Every Minute using Cron:

• Ideally, you may not have a requirement to schedule a job every minute. But understanding this example will help you understand the other examples.

* * * * * CMD

• The * means all the possible units — i.e. every minute of every hour throughout the year. More than using this * directly, you will find it very useful in the following cases.
• When you specify */5 in the minute field means every 5 minutes. When you specify 0-10/2 in the minute field means every 2 minutes in the first 10 minutes. Thus the above convention can be used for all the other 4 fields.

5. To Schedule a Job For More Than One Time (e.g. Twice a Day):

• The following script takes an incremental backup twice a day every day. This example executes the specified incremental backup shell script (incremental-backup) at 11:00 and 16:00 every day.
• The comma-separated value in a field specifies that the command needs to be executed at all the mentioned times.

00 11, 16 * * * /home/maverick/bin/incremental-backup

• 00 – 0th Minute (Top of the hour) 11, 16 – 11 AM and 4 PM * – Every day * – Every month * – Every day of the week

6. To Schedule a Job for a Within Certain Range of Time (e.g. Only on Weekdays):

If you wanted a job to be scheduled for every hour within a specific range of time then use the following.

• Cron Job every day during working hours: This example checks the status of the database every day (including weekends) during the working hours 9 a.m – 6 p.m

00 09-18 * * * /home/maverick/bin/check-db-status

• 00 – 0th Minute (Top of the hour) 09-18 – 9 am, 10 am, 11 am, 12 am, 1 pm, 2 pm, 3 pm, 4 pm, 5 pm, 6 pm * – Every day * – Every month * – Every day of the week
• Cron Job every weekday during working hours: This example checks the status of the database every weekday (i.e. excluding Sat and Sun) during the working hours 9 a.m – 6 p.m.

00 09-18 * * 1-5 /home/maverick/bin/check-db-status

• 00 – 0th Minute (Top of the hour) 09-18 – 9 am, 10 am, 11 am, 12 am, 1 pm, 2 pm, 3 pm, 4 pm, 5 pm, 6 pm * – Every day * – Every month 1-5 -Mon, Tue, Wed, Thu and Fri (Every Weekday)

7. To Schedule a Background Cron Job for Every 10 Minutes:

• Use the following, if you want to check the disk space every 10 minutes.

*/10 * * * * /home/maverick/check-disk-space

• It executes the specified command check-disk-space every 10 minutes throughout the year. But you may have a requirement of executing the command only during certain hours or vice versa.
• The above examples show how to do those things. Instead of specifying values in the 5 fields, we can specify them using a single keyword as mentioned below.
• There are special cases in which instead of the above 5 fields you can use @ followed by a keyword — such as reboot, midnight, yearly, or hourly.

Keyword    Equivalent
@yearly    0 0 1 1 *
@daily     0 0 * * *
@hourly    0 * * * *
@reboot    Run at startup.

8. To Schedule a Job for the First Minute of Every Year Using @yearly:

• If you want a job to be executed on the first minute of every year, then you can use the @yearly cron keyword as shown below.
• This will execute the system annual maintenance using the annual-maintenance shell script at 00:00 on Jan 1st for every year.

@yearly /home/maverick/bin/annual-maintenance

9. To Schedule a Cron Job Beginning of Every Month Using @monthly:

• It is similar to the @yearly as above. But executes the command monthly once using the @monthly cron keyword. This will execute the shell script tape-backup at 00:00 on the 1st of every month.

@monthly /home/maverick/bin/tape-backup

10. To Schedule a Background Job Every Day Using @daily:

• Using the @daily cron keyword, this will do a daily log file cleanup using the cleanup-logs shell script at 00:00 every day.

@daily /home/maverick/bin/cleanup-logs "day started"

11. To Execute a Command after Every Reboot Using @reboot:

• Using the @reboot cron keyword, this will execute the specified command once after the machine gets booted every time.

@reboot CMD

What are Some Features of Linux Crontab?

The following are the features of crontab:

• Flexible Scheduling: The Crontab allows users to schedule tasks with a high degree of flexibility, specifying exact times, intervals, days of the week, and months for execution.
• Automated Task Execution: It facilitates with automatic execution of commands or scripts without user intervention, making it ideal for repetitive tasks like backups, updates, and system maintenance.
• User-specific Configuration: Through each user it can maintain their crontab file, allowing personalized scheduling of tasks tailored to individual needs and preferences.
• Special Time Indicators: Offers convenience with predefined time indicators (@reboot, @daily, @weekly, etc.) for commonly used schedules, simplifying the setup of recurring tasks.

Conclusion

From the above extensive discussion, we can conclude that the 'Crontab' Command on Linux Distributions is one of the powerful tools that can be used to Automate Tasks on Linux. If you want to perform System Maintenance, Backups, and Updates then the Linux 'Crontab' Command should be used as we have discussed.