How Linux Boots - page 2
Let's take stock; the system is up and running. All those services are ready. Can the user use all that power yet? No, you must login.
There are plenty of services (like web access via a WWW client) that work without login. But many services thatLinux offers are best accessed via the shell. To start a shell the user must type a user name and password. This is checked against the system password file, and then a shell is started. A shell is similar to the MSDOS command prompt, but is more powerful, as literally hundreds of commands are available. The shell runs a number of startup files on login. These include system-wide shell startup files and files that can be altered by the individual user with affecting everyone else
Linux startup happens surprisingly quickly despite the apparent complexity. Also each stage of the startup is fully documented, so it is easy to trace things happening via the system logs. Most users need not worry about the system startup. Packages that need to run as the system powers up will have scripts provided by the package manager to install them correctly. The shell initialization files or window manager configuration files are usually the place for tweaking your system settings. If you have to alter some internals of the Linux system, the whole setup and sequence are easy to follow.