1. WHAT IS LINUX??? Linux is an operating system. It is the software on a computer that enables applications and the computer operator to access the devices on the computer to perform desired functions. The operating system (OS) relays instructions from an application to, for instance, the computer's processor. The processor performs the instructed task, then sends the results back to the application via the operating system.

3. FILE SYSTEM " On a UNIX system, everything is a file; if something is not a file, it is a process." This statement is true because there are special files that are more than just files (named pipes and sockets, for instance), but to keep things simple, saying that everything is a file is an acceptable generalization. A Linux system, just like UNIX, makes no difference between a file and a directory, since a directory is just a file containing names of other files. Programs, services, texts, images, and so forth, are all files. Input and output devices, and generally all devices, are considered to be files, according to the system.

4. FILE SYSTEM LAYOUT

5. THE KERNEL The kernel is the heart of the system. It manages the communication between the underlying hardware and theperipherals. The kernel also makes sure that processes and daemons (server processes) are started and stoppedat the exact right times. The kernel has a lot of other important tasks, so many that there is a specialkernel-development mailing list on this subject only, where huge amounts of information are shared. It wouldlead us too far to discuss the kernel in detail. For now it suffices to know that the kernel is the most important file on the system.

9. Access rights:First line defense The Linux security model is based on the one used on UNIX systems, and is as rigid as the UNIX security model (and sometimes even more), which is already quite robust. On a Linux system, every file is owned by a user and a group user. There is also a third category of users, those that are not the user owner and don'tbelong to the group owning the file. For each category of users, read, write and execute permissions can be granted or denied.

10. The file mask When a new file is saved somewhere, it is first subjected to the standard security procedure. Files without permissions don't exist on Linux. The standard file permission is determined by the mask for new file creation.Instead of adding the symbolic values to each other, as with chmod, for calculating the permission on a new file they need to be subtracted from the total possible access rights.