How to find and configure the services which start when a Linux system boots up
A typical Linux system can be configured to boot into one of 5 different runlevels. During the boot process the init process looks in the /etc/inittab file to find the default runlevel. Having identified the runlevel it proceeds to execute the appropriate startup scripts located in the /etc/rc.d sub-directory.
For example if you have a runlevel of 5 configured then the init process will work through the list of startup scripts located in /etc/rc.d/rc5.d. These startup scripts start either with the letter "S" or "K" followed by a number and then a (hopefully) description word. For example the startup script for NFS (Networked File System) is typcically S60nfs whilst the stratup script for YUM system might be called K01yum.
Scripts that start with an "S" are invoked before those prefixed with a "K". The number in the filename controls the order in which the script will be executed with that group (either "S" or "K"). You wouldn't, for example, want to start NFS before the basic networking is up and running. It is also worth noting that the files in the rc.d sub-directories are not the actual scripts themselves but rather symbolic links to the actual files located in /etc/rc.d/init.d.
There are number of ways to control what services get started wihtout having to delve into the /etc/rc.d sub-directories yourself.
The command line tool chkconfig (usually located in /sbin) can be used to list and configure which services get started at boot time. To list all service settings run the following command:
/sbin/chkconfig --list
This will display a long list of services showing whether or not they are started up at various runlevels. You may want to narrow the search down using grep. For example to list the entry for the HTTP daemon you would do the following:
/sbin/chkconfig --list | grep httpd
which should result in something like:
httpd 0:off 1:off 2:off 3:on 4:off 5:off 6:off
Alternatively you may just be interested to know what gets started for runlevel 3:
/sbin/chkconfig --list | grep '3:on'
chkconfig can also be used to change the settings. If we wanted the HTTP service to start up when we at runlevel 5 we would issue the following command:
/sbin/chkconfig --level 5 httpd on
A number of graphical tools are also available for administering services. On RedHat 9 you can run the following command:
redhat-config-services
The equivalent command on RedHat Fedora Core is:
system-config-services
The above graphical tools allow you to view which services will start for each runlevel, add or remove services for each runlevel and also manually start or stop services.
Another useful tool if you do not have a graphical desktop running or access via a remote X server is the ntsysv command. ntsysv resides in /sbin on most systems. Whilst a convenient tool when you don't have an X server running the one draw back of ntsysv is that it only allows you to change the settings for the current runlevel.
