RC SCRIPTS IN LINUX

RC SCRIPTS

1.     The /etc/inittab file specifies which scripts to run when runlevels change.

2.     These scripts are responsible for either starting or stopping the services that are particular to the runlevel.

3.     Because of the number of services that need to be managed, rc scripts are used.

4.     The main one, /etc/rc.d/rc, is responsible for calling the appropriate scripts in the correct order for each runlevel.

5.     Such a script could easily become extremely uncontrollable to keep this from happening, a slightly more elaborate system is used.

6.     For each runlevel, a subdirectory exists in the /etc/rc.d directory.

7.     These runlevel subdirectories follow the naming scheme of rc X .d, where X is the runlevel. For example, all the scripts for runlevel 3 are in /etc/rc.d/rc3.d.

8.     In the runlevel directories, symbolic links are made to scripts in the /etc/rc.d/init.d directory.

9.     Instead of using the name of the script as it exists in the /etc/rc.d/init.d directory, however, the symbolic links are prefixed with an S, if the script is to start a service, or with a K, if the script is to stop (or kill) a service.

10. These two letters are casesensitive. You must use uppercase letters, or the startup scripts will not recognize them.

11. In many cases, the order in which these scripts are run makes a difference. For example, you can’t start services that rely on a configured network interface without first enabling and configuring the network interface.

12. To enforce order, a two-digit number is suffixed to the S or K. Lower numbers execute before higher numbers; for example, /etc/rc.d/rc3.d/ S10network runs before /etc/rc.d/rc3.d/S55sshd (S10network configures the network settings, and S55sshd starts the Secure Shell [SSH] server).

13. The scripts pointed to in the /etc/rc.d/init.d directory are the workhorses i.e. they perform the actual process of starting and stopping services.

14. When /etc/rc.d/rc runs through a specific runlevel’s directory, it invokes each script in numerical order.

It first runs the scripts that begin with a K and then the scripts that begin with an S. For scripts starting with K, a parameter of stop is passed. Likewise, for scripts starting with S, the parameter start is passed.