Linux with Operating System Concepts

configurable
. Configuration 
is usually handled through configuration files, which are often stored in the /etc direc-
tory (or a subdirectory). Finally, services can be running or stopped, and you (the system 
administrator) can control which services are running or stopped and which services are 
automatically started at system initialization time based on the runlevel (see the discussion 
of the rc script from Section 11.4.3).
There are several files related to every service. These are the services themselves (execut-
able programs, typically stored in /usr/sbin, /sbin, or in some cases, /usr/bin), configura-
tion and data files found in or under /etc, and the service controlling scripts. The service 
controlling scripts are found in /etc/init.d. It is these scripts that the symbolic links from 
the /etc/rc#.d directories reference (recall Figure 11.2). We examine below how to use these 
controlling scripts and we examine some of the configuration files in Section 11.7.
11.5.1 Categories of Services
Each Linux service provides a different form of support to software, the user, the file sys-
tem, the network, or some other aspect of the operating system. The Linux services can be 
broken down into several different categories. Some services support a range of requests 
while other services must combine to perform their task. Here, we will break the types of 

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Linux with Operating System Concepts
services into the following categories. It should be noted that other Linux administrators 
and authors may select a different categorization of services.
• Boot
• File system
• Hardware
• Language support
• Logging
• Network, web/Internet
• Power management
• Scheduling
• System maintenance
Notice the separation between network and web/Internet services. While network ser-
vices must be running to communicate with the Internet or with websites, the various 
web/Internet services do not have to be running to communicate within the local area net-
work. For instance, the network service itself is necessary to communicate via network and 
rdisc is used during system initialization to discover your computer’s local subnet router. 
The service certmonger maintains website security certificates, keeping them up-to-date. 
Without this service, the user is still able to access the network and the Internet although 
not necessarily in a secure fashion. The iptables/ip6tables services are the Linux firewall. 
As with certmonger, if these services are not running, network and Internet access are still 
available but not securely. Other service categories above could be broken into a finer grain 
such as with hardware services as there are different services to support different devices 
such as a Bluetooth service, services to support USB devices, services to support printing, 
and so forth. Table 11.3 illustrates many of the services, including their name, their role, 
and their type.
11.5.2 Examination of Significant Linux Services
In CentOS 6.0, there are over 60 services available. Different Linux distributions will use 
different services. There are nearly 80 in Ubuntu version 12. Many overlap and some go 
by slightly different names (e.g., cron vs. crond). Here, we briefly examine several services.
• abrtd—this is the automated bug report daemon. When a piece of applications soft-
ware crashes, this daemon will collect information about the software and the crash 
such as the core dump and the command line instruction, and perform some action. 
The file /
etc/abrt/abrt.conf
contains the configuration information for this 
daemon. Specifically, it includes rules that dictate which software should be reported 
and what action(s) taken by abrtd for that software. Actions are primarily report 

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System Initialization and Services
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