107.2. Automate system administration tasks by scheduling jobs
Weight: 4
Description: Candidates should be able to use cron or anacron to run jobs at regular intervals and to use at to run jobs at a specific time.
Key Knowledge Areas:
Manage cron and at jobs
Configure user access to cron and at services
Configure anacron
Terms and Utilities:
/etc/cron.{d,daily,hourly,monthly,weekly}/
/etc/at.deny
/etc/at.allow
/etc/crontab
/etc/cron.allow
/etc/cron.deny
/var/spool/cron/
crontab
at
atq
atrm
anacron
/etc/anacrontab
Many system administration tasks must be done regularly, such as rotating log files, backing up files or databases, preparing reports, or installing system updates. In this lesson we will learn how to automate these kinds of jobs by setting up scheduling for them
By scheduling :
We can start a job for a time at which system usage is low
Errors will be reduced due to less manual interaction is needed.
We are sure that jobs always run the same way
System administrators can sleep more!
Run jobs at regular intervals
Linux systems have two facilities for scheduling jobs to run at regular intervals:
The original
cron
facility is best suited to servers and systems that are continuously powered on.The
anacron
(or anachronisticcron
) facility is suited to systems such as desktops or laptops that can be asleep or running on battery power.
cron | anacron |
Good for servers | Good for laptops and desktops |
Granularity from one minute to one year | Daily, weekly, and monthly granularity |
Job runs only if system is running at scheduled time | Job runs when system is next available |
Can be used by normal users | Needs root authority |
Schedule periodic jobs with cron
The cron
facility consists of the cron
daemon and a set of tables that describe what work is to be done and with what frequency. The cron
daemon is usually started by the init
, upstart
, or systemd
process at system startup.
crontab file syntax and Operators
Crontab (cron table) is a text file that specifies the schedule of cron jobs. The cron
daemon wakes up every minute and checks each crontab for jobs that need to run.
There are two types of crontab files. The individual user crontab files and system-wide crontab files.
crontab syntax and operators
Each line in the user crontab file contains six fields separated by a space followed by the command to be run.
*
-The asterisk operator means any value or always.,
-The comma operator allows you to specify a list of values for repetition.-
-The hyphen operator allows you to specify a range of values./
-The slash operator allows you to specify values that will be repeated over a certain interval between them.
Also if you have
@reboot
or@daily
instead of time fields, the command will be run once after the reboot or daily.
Lets see some examples:
user specific crons
/var/spool/cron/
Users crontab files are stored by the user’s name, and their location varies by operating systems. In Red Hat based system such as CentOS, crontab files are stored in the /var/spool/cron
directory while on Debian and Ubuntu files are stored in the /var/spool/cron/crontabs
directory.
Although you can edit the user crontab files manually, it is recommended to use the crontab
command.
crontab command
The crontab command allows you to install or open a crontab file for editing.
You can use the crontab command to view, add, remove, or modify cron jobs using the following options:
crontab -e
: Edit crontab file, or create one if it doesn’t already exist.crontab -l
: Display crontab file contents.crontab -r
: Remove your current crontab file.crontab -i
:Remove your current crontab file with a prompt before removal.crontab -u <username>
: Edit other user crontab file. Requires system administrator privileges.
The crontab -e command opens the crontab file using the editor specified by the VISUAL
or EDITOR
environment variables.
as an example add bellow line to above and it would send and email every 5 mintues:
crontab -e also check the syntax before exiting the file , which is really helpful.
crontab -l would show the above contents. Lets check if user crontab file has been created:
system wide cron
In addition to the user crontab files in /var/spool/cron, the cron
daemon also checks /etc/crontab and any crontabs in the /etc/cron.d directory.
/etc/crontab , /etc/cron.d
/etc/crontab
and the files inside the /etc/cron.d
directory are system-wide crontab files that can be edited only by the system administrators.
/etc/crontab is updated by direct editing. You cannot use the
crontab
command to update file files or files in the /etc/cron.d directory.
System-wide Crontab Files
The syntax of system-wide crontab files is slightly different than user crontabs. It contains an additional mandatory user field that specifies which user will run the cron job.
This file should be edited with an editor directly and we can mention which user runs command(s).
/etc/cron.{daily,hourly,monthly,weekly}/
In most Linux distributions you can also put scripts inside the /etc/cron.{hourly,daily,weekly,monthly}
directories and the scripts will be executed every hour/day/week/month
.
as an example lets take look at one of them:
anacron
The cron facility works well for systems that run continuously.If the system is down when the cron should run a task, that cron job wont run till the next occurrence! But anacron creates the timestamp each time a daily, weekly or monthly job runs.
Note: anacron checks the timestamps at BOOT TIME and does not handle jobs that must run hourly or every minute.
/etc/anacron
The table of jobs for anacron
is stored in /etc/anacrontab, which has a slightly different format from /etc/crontab.
just like/etc/crontab , /etc/anacrontab is updated by direct editing.
Anacrontab Format
period in days : specifies the frequency of execution of a job in N days.
delay in minutes: number of minutes anacron should wait before executing the job after reboot.
job-identifier :It is the name for the job’s timestamp file. It should be unique for each job. This will be available as a file under the /var/spool/anacron directory.
command: specifies the command to execute.
/var/spool/anacron
anacron
keeps a time stamp file in /var/spool/anacron for each job to record when the job runs. When anacron
runs, it checks to see if the required number of days has passed since a job last ran and runs the job if necessary.
This file will contain a single line that indicates the last time when this job was executed.
at
Sometimes you need to run a job at a future time just once, rather than regularly. For this purpose you use the at
command. (ubuntu: apt install at
)
A typical at command sequence looks like this
By running at command It then places you at a special prompt, where you can type in the command (or series of commands) to be run at the scheduled time. When you're done, press Control-D on a new line, and your command will be placed in the queue.
warning: commands will be executed using /bin/sh
Some other examples of at command:
at command has other members in its family:
atq
lists the pending jobs of users
atrm
delete jobs by their job number
atq command only shows the list of jobs but if you want to check what script/commands are scheduled with that task use at -c JobNum
command and see the last line.
both cron and are system services.
Configure user access to job scheduling
We can control access to the crontab command by using two files in the /etc/cron.d directory: cron.deny and cron.allow. These files permit only specified users to perform crontab command tasks such as creating, editing, displaying, or removing their own crontab files.
The cron.deny and cron.allow files consist of a list of user names, one user name per line.
/etc/cron.allow , /etc/cron.deny
These access control files work together as follows:
If cron.allow exists, only the users who are listed in this file can create, edit, display, or remove crontab files.
If cron.allow does not exist, all users can submit crontab files, except for users who are listed in cron.deny.
If neither cron.allow nor cron.deny exists, superuser privileges are required to run the crontab command.
Superuser privileges are required to edit or create the cron.deny and cron.allow files.
/etc/at.allow , /etc/at.deny
The corresponding /etc/at.allow and /etc/at.deny files have similar effects for the at
facility.
.
.
.
Crontab Variables
The cron daemon automatically sets several environment variables.
The default path is set to
PATH=/usr/bin:/bin
. If the command you are calling is present in the cron specified path, you can either use the absolute path to the command or change the cron$PATH
variable. You can’t implicitly append:$PATH
as you would do with a regular script.The default shell is set to
/bin/sh
. You can set a different shell by changing theSHELL
variable.Cron invokes the command from the user’s home directory. The
HOME
variable can be overridden by settings in the crontab.The email notification is sent to the owner of the crontab. To overwrite the default behavior, you can use the
MAILTO
environment variable with a list (comma separated) of all the email addresses you want to receive the email notifications. IfMAILTO
is defined but empty (MAILTO=""
), no mail is sent.
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.
https://developer.ibm.com/tutorials/l-lpic1-107-2/
https://linuxize.com/post/scheduling-cron-jobs-with-crontab/
https://www.thegeekdiary.com/centos-rhel-anacron-basics-what-is-anacron-and-how-to-configure-it/
https://www.thegeekstuff.com/2011/05/anacron-examples/
https://www.computerhope.com/unix/uat.htm
https://tecadmin.net/one-time-task-scheduling-using-at-commad-in-linux/
https://docs.oracle.com/cd/E19253-01/817-0403/sysrescron-23/index.html
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