Linux Services, Runlevels, and RC.D scripts

Reference:

[1] Https://www.linux.com/news/introduction-services-runlevels-and-rcd-scripts

A Linux Service is an application (or set of applications) that runs in the background waiting to being used, or carrying out Essential Tasks.

In the DIRECTORY/ETC/RC.D, you'll find either a set of files named rc.0, Rc.1, Rc.2, Rc.3, Rc.4, Rc.5, and Rc.6, or a s Et of directories named RC0.D, RC1.D, rc2.d, RC3.D, RC4.D, RC5.D, and RC6.D. You'll also find a file Named/etc/inittab including info about runlevels. The system uses these files (and/or directories) to control the services to be started. The boot process uses these parameters to identify the default RunLevel and the files that would be used by that runlevel.

About RunLevel, your might assume that this refers to different levels, the system goes through during a boot up. Instead, think of the runlevel as the point at which the system is entered. Runlevel 1 is the most basic configuration (simple single user access using a text interface), while Runlevel 5 is the MO St Advanced (multi-user, networking, and a GUI front end). Runlevels 0 and 6 is used for halting and rebooting the system. The following table shows a generic list of configurations (and some examples of different distros) taken from Linux- the complete Reference (R.peterson, Osbourne/mcgraw-hill).

Run level	Generic	Fedora Core	Slackware	Debian
0	Halt	Halt	Halt	Halt
1	Single-user mode	Single-user mode	Single-user mode	Single-user mode
2	Basic multi-user mode (without networking)	User definable (Unused)	User definable-configured the same as RunLevel 3	Multi-User mode
3	Full (text based) multi-user mode	Multi-User mode	Multi-User Mode-default Slackware RunLevel
4	Not used	Not used	X11 with KDM/GDM/XDM (session managers)	Multi-User mode
5	Full (GUI based) multi-user mode	Full multi-user mode (with a x-based login screen)-Default RunLevel	User definable-configured the same as RunLevel 3	Multi-User mode
6	Reboot	Reboot	Reboot	Reboot

The different operating levels (run level) in a Linux system represent the different operating states of the system, such as running Level 3 when the Linux server is running normally, multi-user mode capable of providing network services, and running level 1 allowing only administrators to operate through a single console of the server host, i.e. " Single-user mode.

Linux Boot Process

When you boot your computer, the first thing, the It'll do is load the bootloader--either GRUB or LILO in most cases. The bootloader would then load the Linux kernel-the core operating system. Next, a process called init starts. This process reads the file/etc/inittab to establish of the runlevel to use. The RunLevel is the start mode for the computer.

Once Init knows the runlevel it'll look for the appropriate files or directories as defined In/etc/initab.

Init would then either run the script files defined By/etc/initab, or run the script files under the directories defined by/ Etc/initab (depending on the set up of your system).

Finally, Init would present you with the logon mode, which you ve selected.

Linux Services, Runlevels, and RC.D scripts