Basic UNIX Concepts

Why We Need an Operating System (OS)

• OS interacts with hardware and manages programs.

• A safe environment for programs to run is required.

• Programs not expected to know which hardware they will run on.

• Must be possible to change hardware without changing the programs.

In Computing Today It Is Desirable To Be

•Multiprogramming: Multiple programs can be in memory.

•Multiuser: Multiple users can be on system.

•Multitasking: One user can run multiple programs.

What an OS Does When a Program Runs on a Computer

• OS loads program from disk and allocates memory and CPU.

• Instructions in program are run on CPU and OS keeps track of last

instruction executed.

• If program needs to access the hardware, OS does the job on its behalf.

• OS saves the state of the program if program has to leave CPU temporarily.

• OS cleans up memory and registers after process has completed execution.

A Little UNIX History

• 1969 Richie/Thompson at Bell Labs (AT&T) invent UNIX

• BSD UNIX from Berkeley created from AT&T version

• Fragmentation grows: Solaris, AIX, HP-UX, Tru64 UNIX

• System V from AT&T (SVR4) attempts unification

• AT&T -> Novell -> X/OPEN -> The Open Group (own trademark)

A Little UNIX History (continued)

• Early 80’s: BSD UNIX contains TCP/IP spurring growth of Internet

• Early 80’s: MIT develops X Windows, a windowing system for UNIX

• 1991: Linus Torvalds develops Linux, a free UNIX implementation

• IEEE POSIX – Portable Operating System Interface (for UNIX)– Defines standardized UNIX computing environment– OSs based on UNIX can claim conformity

• 2001: Single UNIX Specification Version 3 (SUSV3)

What Makes Unix Special

• It is written in a high-level language, C.

• Everything in the system is represented as a file – containers for information .

• Work gets done by processes – representations of programs in execution.

• Workload shared by two separate programs, the kernel and the shell

•Kernel - the core OS

•Shell – the interface between user and kernel (command line interpreter)

• Kernel uses system calls to do most of the work.

What Makes Unix Special (continued)

• It is multiprogramming, multitasking, and multiuser.

• UNIX ships with many useful tools (e.g., compilers, interpreters, filters, …).

• “Small is beautiful” philosophy. Pipes connect output of one command to the

input of another command (e.g., who | wc).

• UNIX provides strong pattern matching features (e.g., *.c)

•The shell is also a programming language (shell scripts)

• Online help is provided through the man command..

UNIX Architecture: The Kernel

• Loaded by the bootstrap program at startup.

• Program always resides in memory.

• Has direct access to the hardware.

• Handles file I/O.

• Manages processes.

• One copy shared by all users.

UNIX Architecture: The Shell

• Everyone has a login id/ password assigned by an administrator.

• After the user logs in, a shell program is invoked.

• The shell accepts user input, options, and parameters.

• The shell makes calls to the kernel to execute the user request.

• At least one shell is invoked by every user.

• User has a choice of shells: Bourne, C, Korn, Secure, Bash, …

A Typical UNIX Session