CS 497C – Introduction to UNIX Lecture 3: Inside UNIX Chin-Chih Chang

Linux and GNU Although UNIX finally turned commercial, Richard Stallman and Linus Torvalds had different ideas. Torvalds is the father of Linux, the free UNIX that has swept the computer world. Stallman runs the Free Software Foundation (formerly known as GNU – a recursive acronym that stands for “GNU’s NOT Unix”)

Linux and GNU Linux is distributed under the GNU General Public License which makes it mandatory for developers and sellers to make the source code public. Linux flavors: Red Hat, SuSE, Caldera, TurboLinux, Mandrake, Debian. These distribution include a plethora of software.

Inside UNIX The kernel and the shell are the masters of UNIX. The shell interacts with the user and the kernel with the machine’s hardware. The kernel is the center of the operating system – a collection of programs mostly written in C. The kernel is loaded into memory when the system is booted.

The Kernel The kernel manages the system resources, allocates time between users and processes, decides process priorities and performs other tasks. Other programs access the services of the kernel through a set of functions called system calls.

The Shell The shell takes a command from the user, deciphers the special characters, and finally communicates with the kernel. The shell is an interface between the user and the kernel.

Inside UNIX UNIX is a multiuser system. In UNIX, a single user can also run multiple tasks concurrently. UNIX tools singly aren’t all that powerful. Using a supported interprocess communication feature, the shell can arrange for a command to pass on data to another command. UNIX doesn’t really care to know the type of file.

Inside UNIX UNIX supports a pattern matching feature. Apart from files, a special group of commands use a generalized pattern (called a regular expression). The UNIX shell is also a programming language. The language features of the UNIX shell are used to design shell scripts. A shell script is a program including UNIX commands.

Inside UNIX UNIX is written in C. Commands use system calls to invoke the kernel. man – on-line help.

man Problem in CS Computers man may not work on all accounts because of an incorrect MANPATH environment variable. If man does not work for you, please edit your.cshrc.linux and.login.linux files and remove any line having to do with MANPATH. pico - simple text editor in the style of Pine Composer pine – a program for Internet News and .

General Features of a Command A UNIX command consists of a single word generally using alphabetic characters. Commands are essentially files representing programs – mainly written in C. These files are stored in certain folders known as directories. For instance, the ls command is also a program located in the directory /bin.

General Features of a Command A UNIX command file doesn’t need to have a special extension like.exe and.com. There’s hardly a practical restriction on length either; a command can be up to 255 characters long – a limit set for any file of the file system. UNIX is sensitive to case. $ LS LS: Command not found.

Coming Next Understanding the UNIX Command Review Chapter 1. Do Self-Test of Chapter 1. Homework 1