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An operating system is the program that controls all other parts of a computer system, both the hardware and the software. It allows you to use all facilities provided by the system. Every computer has an operating system.
The UNIX operating system consists of three important parts: a kernel, a shell and a filesystem.
This is the core of each UNIX system and is loaded each time the system is started up - the computer is booted. It manages the whole computer system. You don't need to know anything about the kernel to use the system. If you want to know more about the kernel there are several books to read.
When you login to a Unix system you are placed in a program called the shell. You can see its prompt (usually % or $) at the left of your screen or window. You can enter commands after this prompt. The shell takes each command and passes it to the operating system kernel to be acted upon and the results can be seen on your screen. The shell also provides you with possibilities to create your own user environment, define command aliases, write shell scripts, edit the command line and much more depending of which shell you are using. All student accounts are using a shell called tcsh.
A file system is a method to organise and store large amounts of information so it is easy to manage. The smallest unit in which information is stored is a file. The file system consists of directories, files and linkes to files.
When you login to an Unix system you will start in your homedirectory, it is referred to: ~loginname (e.g ~kristina). Each file you create will be placed in your homedirectory. After a while there will be a lot of files and maybe you will need to sort the files. You can create your own directories. To create a new directory or move to another directory than your home directory you have to use Unix-commands.