The GNU/Linux system supports several utilities for scheduling tasks. The cron utility is the most widely supported. It allows you to schedule tasks to be run in the background at regular intervals. The cron utility uses a table (crontab) with a list of scripts or commands to be executed and the time when they are to be executed.
Normal users can use the crontab command to manage their jobs. This command can be called in four different ways:
|crontab -l||List the jobs for the current user|
|crontab -r||Remove all jobs for the current users.|
|crontab -e||Edit jobs for the current user.|
|crontab [filename]||Remove all jobs, and replace with the jobs read from [filename]. If no file is specified, stdin will be used.|
How to edit and view crontab
When editing jobs with the “crontab -e“, an editor will be started (vi by default. unless the EDITOR environment variable has been set to something different). The file being edited will have one job per line. Empty lines are allowed, and comments start their line with a hash symbol (#).
# crontab -e
When you execute the command “crontab -e” without any options, the crontab of the currently logged in user will be edited by default. To edit the crontab of another user, you can use the -u option with crontab command. For example, to edit the crontab of the user john.
# crontab -u john -e
Similarly, to view the crontab of the current user, use the “crontab -l” command.
# crontab -l
And to list the crontab of a specific user, use the below command.
# crontab -u john -l
Cron configuration files
The cron background process is mostly idle. It wakes up once every minute and checks /etc/crontab, /etc/cron.d, and the user cron table files and determines whether there are any jobs that need to be executed.
Table below summarizes the purpose of the various files and directories used by cron. Knowledge of these files and directories will help you troubleshoot any issues as well as understand cron in more detail.
|/etc/init.d/crond||Starts the cron daemon in system boot.|
|/var/log/cron||System messages related to the cron process. Useful for troubleshooting problems.|
||User crontab files are stored in the /var/spool/crondirectory.|
|/etc/cron.allow||Specifies users who can create a cron table.|
|/etc/cron.deny||Specifies users who are not allowed to create a cron table.|
|/etc/crontab||The system cron table that has commands to run scripts located in the following directories: /etc/cron.hourly, /etc/cron.daily, /etc/cron.weekly, and /etc/cron.monthly.|
|/etc/cron.d||A directory that contains cron tables for jobs that need to run on a schedule other than hourly, daily, weekly, or monthly.|
|/etc/cron.hourly||Directory that contains system scripts to run on an hourly basis.|
|/etc/cron.daily||Directory that contains system scripts to run on a daily basis.|
|/etc/cron.weekly||Directory that contains system scripts to run on a weekly basis.|
|/etc/cron.monthly||Directory that contains system scripts to run on a monthly basis.|
The jobs of individual users are stored in the directory /var/spool/cron in files matching the usernames. These files always belong to the user root. The files in /var/spool/cron are not edited directly. Instead, a program called crontab is used to manipulate them. The figure below shows the syntax of a cron job.
Each line in a file defines a job. There are 6 fields in a line. The first 5 fields define the time, the final field contains the command to run. This can be any type of command or shell script. The first 5 fields have the following format:
|Day of the Month||1-31|
Example cron jobs
1. Schedule a job at February 2nd, every year
You need to run a backup script once every year on a specific date in february month. The sytax is shown below.
0 9 2 2 * /usr/local/bin/yearly_backup
2. Schedule a job every hour at the fifth minute, every day
Use the following command to run a script every hour at the fifth minute, every day:
5 * * * * $HOME/bin/daily.job >> $HOME/tmp/out 2>&1
3. Schedule a job 5 minutes after midnight every day
Use the following command to run 5 minutes after midnight every day:
5 0 * * * $HOME/bin/daily.job >> $HOME/tmp/out 2>&1
4. Schedule a job at a specific time on on the first of every month
Use the following command to run at 2.15 P.M. on the first of every month:
15 14 1 * * * $HOME/bin/monthly
5. Schedule a job at a specific time only on weekdays (exclude Saturday and Sunday)
Use the following command to run at 10 P.M. on weekdays:
0 22 * * 1-5 $HOME/bin/weekday.job >> $HOME/tmp/out 2>&1
6. Scheduling multiple jobs in a single cron job
We can also schedule multiple jobs in a single cron job using a semicolon(;) separated jobs as showb below:
0 12 * * * /var/tmp/script01.sh; /var/tmp/script02.sh
7. Using ranges to specify starting times
Ranges can be specified through the use of a hyphen. A value of 1-5 indicates that this field is valid for numbers 1 through 5. If you use a name instead of a number, you cannot specify a range. Example of specifying ranges is shown below. The Job is scheduled to run every hour from 3 P.M. To 10 P.M.
* 3-10 * * * /var/tmp/script.sh
8. Using step values to specify starting times
Step values can be used in conjunction with ranges. To specify a step value, follow the range with a forward slash (https://cdn.thegeekdiary.com/) and a number. The number specified is the step value. For example, the following specifies that every third value (in this case, 2, 5, 8, and 11) should be matched:
* 0-12/3 * * * /var/tmp/script.sh
Step values can also be used with asterisks. The value */3 in the hour field would match every third hour (0, 3, 6, 9, 12, 15, 18, and 21).
9. Using lists to specify starting times
Lists are also acceptable; each item in a list is separated by a comma. It is common to use lists in conjunction with ranges, like so:
1-15,31-45 * * * * /var/tmp/script.sh
This example matches all numbers(minutes in our case) from 1 through 15 and from 31 through 45. If you use a name instead of a number, you cannot specify a list.
10. Schedule a job at 4:45 a.m., on the 1st, 10th and 22nd of each month
Shown below is an example of running a job at a specific time on few specific days of the month.
45 4 1,10,22 * * /apps/bin/backup.sh
We can add macros in the crontab file. For example, use the following to restart “my_script” after each reboot:
The following is a summary of a few more macros:
|@reboot||Run once at start-up||None|
|@weekly||Run once a week||0 0 * * 0|
|@daily||Run once a day||0 0 * * *|
|@midnight||(same as @daily)||0 0 * * *|
|@hourly||Run once an hour||0 * * * *|
Dealing with Output
A script often prints something to the screen, either for debugging, status updates, or to log an error. Anything that a job prints to the screen is sent in an email to the current user, which can be overridden with the MAILTO variable inside the crontab.
# crontab -l MAIL=john 0 2 * * * /apps/bin/backup.sh
Defining System Jobs Using Cron under Linux
UNIX / Linux : How crontab validates the access based on the cron.allow and cron.deny files
CentOS / RHEL : anacron basics (What is anacron and how to configure it)
CentOS / RHEL : Begginners guide to cron