Cron is a daemon that runs scheduled tasks on a Linux system. These tasks are called "cron jobs" and are defined in a configuration file called a crontab. Cron is often used to schedule tasks to run automatically at a specific time or at a specified interval.
Cron jobs are typically used for tasks that need to be run on a regular basis, such as system maintenance tasks, backups, and other automated processes. For example, you could use crontab to schedule a script to run every day at midnight to update a database, or to run a backup script every week.
Crontab is a powerful tool that allows you to automate a wide variety of tasks on your Linux system. It is commonly used in conjunction with shell scripts, but can be used to run any command or script that is executable on the system.
How to use crontab on a Linux system¶
Open a terminal and enter the command
crontab -e. This will open the crontab configuration file in a text editor. If this is the first time you are using crontab, you will be asked to choose a text editor.
To create a new cron job, add a new line to the crontab configuration file in the following format:
* * * * * command_to_execute
The asterisks represent the following: - The first asterisk represents the minute (0-59) - The second asterisk represents the hour (0-23) - The third asterisk represents the day of the month (1-31) - The fourth asterisk represents the month (1-12) - The fifth asterisk represents the day of the week (0-6, where 0 represents Sunday)
For example, to run a command at 10:15 every day, you would enter the following line in the crontab configuration file:
15 10 * * * command_to_execute
You can also use special characters to specify more complex scheduling patterns.
For example, to run a command every hour at 15 minutes, you can use the
15 in the minute field and character
* in the hour field:
15 * * * * command_to_execute
Or, to run a command every 5 minutes, you can use the character
the minute field:
*/5 * * * * command_to_execute
Save and close the crontab configuration file. The cron daemon will automatically pick up the changes and start running the new cron job.
To view your current cron jobs, enter the command
To remove a cron job, edit the crontab configuration file and delete the line that corresponds to the cron job you want to remove. Save and close the file, and the cron daemon will automatically pick up the changes.
Running cron task when the server starts¶
@reboot rule is a special keyword that can be used in place of the timing
fields (minute, hour, day of month, month, and day of week) to specify that a
cron job should be run when the system reboots. This can be useful for tasks
that need to be run every time the system starts up, such as starting a service
or running a script.
To use the
@reboot rule, simply add it as the first field in the crontab
configuration file, followed by the command to be executed:
For example, to run a script every time the system reboots, you could add the following line to the crontab configuration file:
Note that the
@reboot rule is only processed when the cron daemon is started,
which typically happens when the system boots. If the cron daemon is already
running when you add a new
@reboot rule, it will not be executed until the
next time the system reboots.
Cron service environment¶
Cron jobs are typically run in a limited environment, which means that certain environmental variables and paths may not be set up as they are in a normal interactive shell session. This can cause problems when running scripts or commands that rely on these variables or paths being set correctly.
One common issue that can arise is that the
PATH variable may not be set to
include all the directories that you need to access. The
PATH variable is
a list of directories that the shell searches for executables when you run a
command. If a command is not found in the directories listed in
shell will not be able to find and execute it.
To configure the
PATH variable correctly for your cron jobs, you can add the
following line to the top of your crontab configuration file:
This sets the
PATH variable to a default value that includes common
directories where executables are typically stored. You can also add additional
directories to the
PATH variable as needed, separated by colons.
For example, if you want to include the
/usr/local/bin directory in the
variable, you can add it as follows: