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Crontab Linux

Introduction to Crontab Command in Linux

The Crontab command lists, edits, removes, or submits cron jobs. The cron jobs are commands executed by the cron daemon at scheduled intervals regularly. We need to specify the command, i.e., crontab, with the flag, i.e., -e, to summit any cron job. The crontab command conjures an editing session that permits us to establish a crontab file. We create entries for all cron jobs in this file. All entries must be in a format decent to the cron daemon.

The crontab command copies entries into the directory, i.e., /var/spool/cron/crontabs, and places it inside a file name for our current username when we finish establishing entries and exit the file. We can alternatively establish a crontab file by describing the file parameter. The file must be in the form the cron daemon expects if it exists. The crontab command conjures the editor if it doesn't exist. The command conjures the editor it describes if the EDITOR environmental variable is available. Otherwise, the crontab command utilizes the vi editor.

  • We need to describe the crontab command along with the -l flag for listing the contents of our crontab file.
  • We can use the -r flag to delete an existing file.
  • The Username parameter (optional) can be applied by the crontab file owner or the root user to verify, remove, list, or edit the cron job status for the defined user.
  • An error message is produced, and the program exits if the username is not valid.
  • If the Username parameter (optional) is not specified, the crontab flags are present for the current user and the root user.
  • The crontab is a set of commands that we wish to execute on a regular schedule.
  • The command's name is also used to organize that list.

The full form of crontab is "cron table" because it utilizes the job scheduler cron to run tasks; cron itself is called after "chronos". Cron is the process of the system which will perform tasks automatically for us according to a fixed schedule. This schedule is known as the crontab, which is the program name used to alter that schedule.

Why do we use Cron Jobs?

The following are some reasons to use cron jobs in Linux:

  • Cron jobs are used for automating system maintenance.
  • Crontab is a perfect option for automating Unix jobs.
  • Cached data clean up regularly.
  • Sent out notification emails like Password expiration emails, Newsletters, etc.
  • Purge and archive database tables
  • Remove old log files
  • Helps the operating system grab a scheduled backup of the database or log files.

Cron configuration file types

The following are the types of cron configuration files:

  • The User Crontab
    User can install their cron jobs with the help of the crontab command. All users can have their crontab file, and hence these are files in crontabs /var/spool/cron/; they do not tend to be directly edited. We can use the crontab command to edit a set up our cron jobs.
  • The Linux/Unix System Crontab
    It requires sudo privileges because it's used for critical jobs and system services. It provides the ability to execute commands as a user to system crontab.

Crontab Unique String

We can use eight unique strings given by crontab. It will not just save our time but also increase readability.

Unique String Explanation
@reboot After startup, execute once.
@yearly Execute once a year, i.e., 0 0 1 1 *
@annually It is the same as @yearly.
@monthly Execute once a month, i.e., 0 0 1 * *
@weekly Execute once a week, i.e., 0 0 * * 0
@daily Execute once a day, i.e., 0 0 * * *
@midnight It is the same as @daily.
@hourly Execute once an hour, i.e., 0 * * * *

Examples of Crontab Unique String

  • To schedule operations on a yearly basis

We can run the above one to send a new year greeting. Also, it will run at the first minute of each year.

  • To schedule operations to run on a monthly basis

We can run the above one to implement a few cleanup activities at the start of each month. We can similarly use other crontab unique strings for hourly, daily, and weekly goals.

Cron Directory

A system administrator can utilize a system-wide cron schedule, which falls upon the predefined cron directory as displayed below:

  • /etc/cron.weekly
  • /etc/cron.monthly
  • /etc/cron.hourly
  • /etc/cron.daily
  • /etc/cron.d

Usage of Cron under Linux

The system pack of Linux contains a helpful task scheduler called crontab. Crontab is famous as it can be scheduled to execute an automated development as a root. So, having an automated development active as root makes system modifications easier. We just need to modify the task and wait until it is re-initiated.

Format of Linux Crontab

Linux crontab contains six different fields. The initial five fields specify the date and time of execution, and the last one is utilized for command execution.

Crontab syntax

How to Modify/Add Crontab

  • User can alter their crontab jobs using the below command:

This command opens our computer system's personal crontab configuration, which can be altered with our default text editor.

  • We don't need to reboot our crontab because it will automatically pick up our modifications when we use the below command:
  • We can use the below crontab command to remove our crontab tasks:
  • We can use the below crontab command to update or add jobs in crontab:
  • We can use the below crontab command to edit the crontab of other users:

List Crontab

  • We can use the below crontab command to see the crontab entries of the user:
  • We can use the below crontab command to see the crontab entries of a specific user:

Important Examples of Crontab

We will discuss some important crontab commands and their description as follows:

  • 0 7, 17 * * * /scripts/
    It is a cron command to implement several scheduling jobs. This command will daily run at 7 AM and 5 PM.
  • * / 5 * * * * * /scripts/
    The above command will run a cron after 5 minutes.
  • 0 5 * * mon /scripts/
    The command is a cron scheduler that will help us to run the task each Monday at 5 AM. It is useful to do weekly tasks, such as system cleanup.
  • * / 3 * * * * /scripts/
    The above command will execute our script at 3 minutes intervals.
  • * * * feb, jun, sep * /script/
    The above command can be used to schedule any cron to which run for a particular month. The command is used to execute tasks in the September, June, and February months. We sometimes require to schedule any task to run a monthly task.
  • 0 17 * * mon, wed /script/
    This command is used to run on some specific days. It will execute every Monday and Wednesday at 5 PM.
  • 0 2 * * sat [ $(date +%d) -le 06 ] && /script/
    The above command permits cron to run on the first Saturday of each month.
  • 0 * / 6 * * * /scripts/
    This command is used to execute a script for an interval of 6 hours so it can be set up like above.
  • 0 4, 17 * * mon, tue /scripts/
    The above command schedules a task to run twice on Tuesday and Monday. We can use the above settings to do it.
  • * * * * * /scripts/ * * * * * sleep 15; /scripts/
    The above command schedules a cron to run after every 15 seconds.
  • @yearly /scripts/
    The above command schedules a task yearly. @yearly timestamp is= to "0 0 5 1*". It will run the task in the 5th minute of each year. We can use it to send new year greetings.
  • @monthly /scripts/
    The above command schedules a task to run monthly. @monthly timestamp is the same as "0 0 1 * *". The command expression permits the task execution in 5 minutes per month.
  • * * * * * /scripts/; /scripts/
    The above command is used to run multiple tasks with one cron.
  • @weekly /bin/
    This command schedules tasks to run weekly. @weekly timestamp is the same as "0 0 4 * sun". It implements the weekly tasks, such as system clean-up.
  • @daily /scripts/
    This command is used to schedule tasks to run daily. @daily timestamp is the same as "0 2 * * *". It runs the task in the 2nd minute of each day.
  • @hourly /scripts/
    It permits tasks to run hourly. @hourly timestamp is the same as "0 * * * *". It runs the task for 5 minutes each hour.
  • @reboot /scripts/
    It permits tasks to run on system reboot. @reboot expression is helpful for tasks that the computer system wants to execute on our system startup. It is useful to automatically start tasks background.

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