Run Level (Init)

For being the Linux user one basic thing that should know is the level of running service called run level. There are 7 run level from 0 through 6 can be found.

Run level	Description
0	Halt the system, especially used by Administrators to shut down the system  immediately
1	Single-user mode, usually used for interactive filesystem maintenance when other services like network, web server and file sharing services arestopped
2	Multi user mode
3	Default multi user mode on Red Hat
4	Typically unused
5	Full multiuser mode with GUI login
6	Reboot  the system, especially used by Administrators to reboot the system  immediately

Basic Command

There are more than a thousands of command Linux shell can process. Don’t worry, you can find the command in shell prompt by using “press tab” key or may be either input the initial character and then press “TAB” key to find the required command. But a typical power user is familiar with just a handful. These would include:

• cat:  Prints the contents of a file. Any text file can be viewed, with the default output being displayed on the monitor.
• cd:  Change directories. When this command is used without any options, it changes from the current directory to the home directory of the user.
• chmod:  Change modules (file access permissions).
• cp:  Copies files and directories.
• du:  Displays disk usage.

Shell!

Every computer systems have the human interface component. In Linux, there is a command prompt to communicate between user and kernel which is known as Shell. There are several shell found in the system such as Bourne shell (sh), Bourne-again shell (bash), Korn shell (ksh), Z shell (zsh) and C shell (csh).  Bourne Shell (sh) is the original shell for the Unix system written by Steve Bourne. In Linux, the default shell is Bourne-again shell (bash) which is a GNU variant of Bourne Shell (sh).