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At reboot, all Linux system will enter a specific startup environment as dictated by a number in the configuration file /etc/inittab known as the run level. The following shows all possible values of run levels and their meanings.

0: Halt
1: Single user mode
2: Basic multi-user mode
3: Full multi-user without GUI (X Windows)
4: Not Used
5: Mutli-user with GUI (X Windows)
6: Reboot

The following example alters the network file system service to start at the next boot time only in run level 3:

[brucelee@ ~]$ chkconfig -level 3 nfs on

The following example shows how to start, stop, and restart an apache web server (httpd). Most services require root authority to complete the required task.

[brucelee@ ~]$ service httpd stop;
rm: cannot remove `/var/run/httpd.pid': Permission denied [FAILED]

[brucelee@ ~]$ su
[root@ brucelee]# service httpd stop;
Stopping httpd: [ OK ]
[root@ brucelee]# service httpd start;
Starting httpd: [ OK ]
[root@ brucelee]# service httpd restart;
Stopping httpd: [ OK ]
Starting httpd: [ OK ]
[root@ brucelee]#

To see a list of all currently running and stopped services, use the following command.

[brucelee@ ~]$ service -status-all

Use grep to list only services that are running or services that are stopped. Following is a partial listing.

[brucelee@ ~]$ chkconfig -list | grep running
sm-client (pid 2015) is running...
smbd (pid 3134 3133 3131) is running...

[brucelee@ ~]$ chkconfig -list | grep stopped
nfsd is stopped
rpc.rquotad is stopped
nmbd is stopped
nscd is stopped
ntpd is stopped

The following table shows typical services which are available in many popular Linux distributions. Services that are underlined in the table are either required or strongly recommended for typical commercial dedicated server deployment. All other services should be disabled if not needed. Each service has a brief description to help you decide which ones should be disabled. If disabling a service causes issues to your server performance, you can always restarting the stopped service manually. When in doubt, you should carefully review the documentation of the service you want to disable, and check it out on a local test server. A service can be permanently removed from running during the next reboot with chkconfig --delete option. Additional or customized services can also be added with chkconfig --add option.

Service Description
acon Specifically for the right to left written languages: Arabic, Farsi and Hebrew.
acpi Advanced Configuration and Power Interface event daemon. acpid is designed to notify user-space programs of ACPI events. acpid should be started during the system boot, and will run as a background process, by default. It will open an events file (/proc/acpi/event by default) and attempt to read whole lines. When a line is received (an event), acpid will examine a list of rules, and execute the rules that match the event.
alsa Advanced Linux Sound Architecture. The only sound system actually included in the kernel.
anacron can be used to execute commands periodically, with a frequency specified in days. Unlike cron, it does not assume that the machine is running continuously. Hence, it can be used on machines that are not running 24 hours a day, to control daily, weekly, and monthly jobs that are usually controlled by cron.
arpwatch Ethernet ip address pairing monitor. monitors and logs remote IP addresses with hostnames.
atd run jobs queued (by at) for later execution.
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