6 Deploy, Configure, and Maintain Systems

6.1 : Schedule tasks to run at a set date and time

cron - 
anacron - smallest units for repetition is the day, can recover a job that was not run due to system off
at - tasks that should only run once


$ cat /etc/crontab
To view system wide cron definition

35 6 * * * root /bin/somme_command --some_options

35 - minutes(0 - 59)
6  - hour (0 - 23)
*  - day of the month (1 - 31)
*  - month (1-12)
*  - day week (0 - 6) (Sunday=0 or 7)

Special caracters

* = match all possible values (i.e, every hours)
, = match multiple values (i.e, 15,45)
- = range of values (i.e., 2 - 4)
/ = specifies steps (i.e., */4)

cron job will always like to use the full path to the command, use which command to view path to a command

Cronjobs can be set using special directories, what ever cron finds in these directories is going to be run

daily = /etc/cron.daily/
hourly = /etc/cron.hourly/
monthly = /etc/cron.monthly/
weekly = /etc/cron.weekly

NB: Shell scripts should have no extension when required to be run by cron

$ touch shellscript

$ sudo cp shellscript /etc/cron.hourly/

$ sudo chmod +rx /etc/cron.hourly/shellscript

To remove the job

$ sudo rm /etc/cron.hourly/shellscript


ANACON

No care about the time about when the job is going to be run. DAILY, MONTHLY

To schedule jobs with anacron
$ sudo vim /etc/anacrontab

#period in days		delai in minutes	job-identifier	command

3  	10		test_job		/usr/bin/touch 	/root/anacron_created_this

3 - job is run every 3 days
10 - 10 minutes dalay
test_job - job identifier necessary for loggin
/usr/bin/touch /root/anacron_created_this - command

To verify if our anacron syntax is correct
$ anacron -T



AT

Specific job launch
$ at 15:00
at> /usr/bin/touch file_created_by_at

Ctrl + d - to save the job

Run at specific date
$ at 'August 20 2022'
at>

Run at specific data and time
$ at '2:30 August 20 2022'
at>

Use of relative dates and time
$ at 'now + 30 minutes'

$ at 'now + 3 days'

$ at 'now + 3 weeks'

$ at 'now + 3 months'

To see schedules jobs, will output job identifier
$ atq

To find what the job identifier does
$ at -c <id>

To remove a scheduled job
$ atrm <id>


6.2 : Verify Completion of Scheduled Jobs

cron, anacon and at events are all logged

/var/log/cron

The MAILTO directive found in /etc/crontab can be edited
$ vim /etc/anacrontab

To force anacron to run scheduled job at a moment
$ sudo anacron -n 
$ sudo anacron -n -f



6.3 : Manage Startup Process and Services

You can view a service unit:
$ systemctl cat sshd.service

To edit service file
$ sudo systemctl edit --full sshd.service

To revert to factory default settings
$ sudo systemctl revert sshd.service

To see the status of a service
$ sudo systemctl status sshd.service


To restart a service
$ sudo systemctl restart sshd.service

To reload a service configurations
$ sudo systemctl reload sshd.service

To gracefully restart a service
$ sudo systemctl reload-or-restart sshd.service

To disabled sshd.service
$ sudo systemctl disable sshd.service

$ sudo systemctl is-enabled sshd.service

To make it automatically start at boot
$ sudo systemctl enable sshd.service

NB: Make sure you enable the sshd.service to be able to login at next reboot

To do both enable and start a service
$ sudo systemctl enable --now sshd.service

The disable command also supports teh --now option
$ sudo systemctl disable --now sshd.service


Some services can automatically startup other services
You can mask a service is you do not want it to be started by other services
$ sudo systemctl mask atd.service

NB: Masked services can not be enabled or started

To unmask a service
$ sudo systemctl unmask atd.service

To list all service units
$ sudo systemctl list-units --type service --all





6.3 : Install and update software packages from RHN or local file system

yum and dnf are interchangeable

Subscription manager
$ sudo subscription-manager register --username your-redhat-username --password your-redhat-password

To attach machine to Subscription
$ sudo subscription-manager attach --auto



+ List default repositories
$ sudo yum repolist

To get the url of the repositories
$ sudo yum repolist -v

To view all(optional) repositories
$ sudo yum repolist --all

To enable one of the optional repositories
$ sudo subscription-manager repos --enable <repo-id>

$ sudo subscription-manager repos --disable <repo-id>

This can be done with the yum config manager
$ sudo yum-config-manager --enable <repo-id>

$ sudo yum-config-manager --disable <repo-id>

To use repositories on local network
Install yum utils
$ sudo yum install yum-utils

$ sudo yum-config-manager --add-repo https://<repo-url>.repo

$ sudo yum-config-manager --add-repo 192.168.1.220/BaseOS.repo

To search
$ sudo yum search 'web server'

More information about a package
$ sudo yum info nginx

To install
$ sudo yum install nginx

$ sudo yum reinstall nginx

$ sudo yum remove nginx

To view groups of packages 
$ sudo yum group list

$ sudo yum group list --hidden

To install a group
$ sudo yum group install 'Server with GUI'

To remove a group of packages
$ sudo yum group remove 'Server with GUI'

You can use yum to install downloaded rpm
$ sudo yum install ./file_1.1_x68_64.rpm

To remove package+dependencies
$ sudo yum autoremove <package-name>

+ Update and upgrade packages
To see what package can be upgraded

$ sudo yum check-upgrade

$ sudo yum update



6.4 : Work with package module streams (AppStream)

They allow to designate multiple versions of packages to group together as modules

To see available modules
$ sudo yum module list

To get information about a particular module
$ sudo yum module list nodejs

yum will always download the package with the default module tag
To get different version and profile
$ sudo yum module install nodejs:14/development

To take a look at which ones got installed
$ sudo yum module list --installed nodejs

To reset the module settings to the default
$ sudo yum module reset nodejs