cron command in Linux with Examples

22 Jan, 2021
The cron is a software utility, offered by a Linux-like operating system that automates the scheduled task at a predetermined time. It is a daemon process, which runs as a background process and performs the specified operations at the predefined time when a certain event or condition is triggered without the intervention of a user. Dealing with a repeated task frequently is an intimidating task for the system administrator and thus he can schedule such processes to run automatically in the background at regular intervals of time by creating a list of those commands using cron. It enables the users to execute the scheduled task on a regular basis unobtrusively like doing the backup every day at midnight, scheduling updates on a weekly basis, synchronizing the files at some regular interval. Cron checks for the scheduled job recurrently and when the scheduled time fields match the current time fields, the scheduled commands are executed. It is started automatically from /etc/init.d on entering multi-user run levels. Syntax: 
cron [-f] [-l] [-L loglevel]
Options:  
-f : Used to stay in foreground mode, and don’t daemonize.
-l : This will enable the LSB compliant names for /etc/cron.d files.
-n : Used to add the FQDN in the subject when sending mails.
-L loglevel : This option will tell the cron what to log about the jobs with the following values: 1 : It will log the start of all cron jobs.2 : It will log the end of all cron jobs.4 : It will log all the failed jobs. Here the exit status will not equal to zero.8 : It will log the process number of all the cron jobs.
1 : It will log the start of all cron jobs.
2 : It will log the end of all cron jobs.
4 : It will log all the failed jobs. Here the exit status will not equal to zero.
8 : It will log the process number of all the cron jobs.
The crontab (abbreviation for “cron table”) is list of commands to execute the scheduled tasks at specific time. It allows the user to add, remove or modify the scheduled tasks. The crontab command syntax has six fields separated by space where the first five represent the time to run the task and the last one is for the command. 
Minute (holds a value between 0-59)
Hour (holds value between 0-23)
Day of Month (holds value between 1-31)
Month of the year (holds a value between 1-12 or Jan-Dec, the first three letters of the month’s name shall be used)
Day of the week (holds a value between 0-6 or Sun-Sat, here also first three letters of the day shall be used)
Command
The rules which govern the format of date and time field as follows: 
When any of the first five fields are set to an asterisk(*), it stands for all the values of the field. For instance, to execute a command daily, we can put an asterisk(*) in the week’s field.
One can also use a range of numbers, separated with a hyphen(-) in the time and date field to include more than one contiguous value but not all the values of the field. For example, we can use the 7-10 to run a command from July to October.
The comma (, ) operator is used to include a list of numbers which may or may not be consecutive. For example, “1, 3, 5” in the weeks’ field signifies execution of a command every Monday, Wednesday, and Friday.
A slash character(/) is included to skip given number of values. For instance, “*/4” in the hour’s field specifies ‘every 4 hours’ which is equivalent to 0, 4, 8, 12, 16, 20.
Permitting users to run cron jobs: 
The user must be listed in this file to be able to run cron jobs if the file exists.
/etc/cron.allow
If the cron.allow file doesn’t exist but the cron.deny file exists, then a user must not be listed in this file to be able to run the cron job.
/etc/cron.deny
Note: If neither of these files exists then only the superuser(system administrator) will be allowed to use a given command. Sample commands: 
Run /home/folder/gfg-code.sh every hour, from 9:00 AM to 6:00 PM, everyday.
00 09-18 * * * /home/folder/gfg-code.sh
Run /usr/local/bin/backup at 11:30 PM, every weekday.
30 23 * * Mon, Tue, Wed, Thu, Fri /usr/local/bin/backup
Run sample-command.sh at 07:30, 09:30, 13:30 and 15:30.
30 07, 09, 13, 15 * * * sample-command.sh
The following points should be remembered while working with cron: 
Have a source version control to track and maintain the changes to the cron expressions.
Organize the scheduled jobs based on their importance or the frequency and group them by their action or the time range.
Test the scheduled job by having a high frequency initially.
Do not write complex code or several pipings and redirection in the cron expression directly. Instead, write them to a script and schedule the script to the cron tab.
Use aliases when the same set of commands are frequently repeated.
Avoid running commands or scripts through cron as a root user.
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