Linux RHCSA Part 4 - Runlevels - managing services, running processes - services

In every operating system, the ability to manage running services and daemons is a crucial aspect. System stability and reliability depend on the ability to manage and configure services. Services may malfunction due to very simple reasons, such as an incorrectly set system time, or due to something more complex, like sharing a file in a network environment for users using NFS. Knowledge of service management is a large part of the Red Hat exam and is also a critical skill for an administrator in real-world work with Linux environments. In Linux, services are also called daemons. The reason some services are named this way is that a single service is actually composed of many other daemons. For example, NFS. Usually, a service we call a daemon is a service whose name ends with the letter "d". For example, the SSH service is a service named "sshd", or the Apache web server is named "httpd".

Let's see what differences in running services exist between individual runlevels. Let's start with mode 3 (runlevel 3 - text mode):

# ls /etc/rc.d/rc3.d/ 

The contents of this directory are symbolic links to scripts in the system, usually located in # /etc/rc.d/init.d. 

If we take a look, most links have a syntax resembling: # [K | S]XXservice_name. One can guess that this refers to "K" for "Kill" and "S" for "Start", and "XX" denotes a number between 01 and 99, including duplicates. So these are service names. The digits indicate the order in which services start after entering a given mode, for example, the services S10network and S55sshd, where it is logical that the network must be up first, after which one can communicate via SSH. The same applies to stopping processes. That is, starting and stopping in order according to the dependencies of one service on another. Therefore, one must pay special attention when editing the contents of runlevel directories (rc.d), because if the order of starting or stopping dependent services is not maintained, it can lead to unpredictable system behavior or the shutdown of certain services. 

Now that we know the differences between services started and stopped in different runlevels, we can see how the management tool used for enabling or disabling selected services for a given runlevel works. Imagine we have to make an entry in the previously mentioned services, i.e., S10network and S55sshd, for all runlevels, which means 2x6 = 12 locations where changes must be made. And what if we had to change something in all services? How long would that take? It is easier to manage services across all runlevels using the chkconfig tool. 

Syntax of the chkconfig command: # chkconfig [options] service_name

chkconfig options (Use two dashes before attributes.):

Task 1

What is the status of the SSH service at which level?

Ans: # chkconfig --list sshd as a result we get:  

sshd 0:off 1:off 2:on 3:on 4:on 5:on 6:off

The result obtained is very simple to interpret. The service is started in levels 2, 3, 4, 5 and stopped in levels 0, 1, and 6. 

Task 2

How to stop the SSH service from starting at all levels?

Ans: # chkconfig sshd off  then we check what the status looks like after the change # chkconfig --list sshd :

sshd 0:off 1:off 2:off 3:off 4:off 5:off 6:off

Now the sshd service will not start at any level. 

Task 3 

Allow the service to start at the levels it was previously enabled on. 

Ans: # chkconfig sshd on  and to verify # chkconfig --list sshd :

          sshd 0:off 1:off 2:on 3:on 4:on 5:on 6:off

Besides using the chkconfig command, we also have a graphical tool available, which is launched by running the ntsysv command. The only difference between the tools is the fact that ntsysv provides a menu with a list where we can select or deselect chosen services.

# cat control-alt-delete.conf
# control-alt-delete - emergency keypress handling
#
# This task is run whenever the Control-Alt-Delete key combination is
# pressed. Usually used to shut down the machine.
start on control-alt-delete
exec /sbin/shutdown -r now “Control-Alt-Delete pressed”


You can see where the input for this task comes from, i.e., pressing the Ctrl+Alt+Delete keys. Exec defines the command that will be executed then. Besides tasks, let's also see the definition of an event: 



# cat ck-log-system-start
# Upstart event
# ck-log-system-start - write system start to log
#


start on stopped rcS
console output
exec /usr/sbin/ck-log-system-start


This event creates a log file recording the moment when the system is started correctly. At this point, one might wonder how to trigger a task or event? For this, we use a command similar to service: initctl  


Syntax of the initctl command: # initctl [command] 

Options for initctl:

• start         - Start a task
• stop          - Stop a task
• restart      - Restart a task
• reload      - Send HUP signal to a task
• status       - Query task status
• lists           - List known tasks
• emit          - Trigger a task

With subsequent system releases, more and more services are coming under the control of the new tool, which is Upstart.

If you are reading this course in order, at this point your knowledge should be expanded with topics regarding the GRUB bootloader, system services, and their management. These topics are repeated many times in the Red Hat Prep Guide, which is a clear sign that this is knowledge you should focus on.