Introduction to UNIX

1. CS465 Introduction to UNIX

2. What is UNIX? (1) • UNIX is an Operating System(OS). • An operating system is a control program that allocates the computer's resources, schedules tasks, and helps the user communicate with the computer. • Most popular PC operating systems: Windows 95/98/2000/XP -- proprietary, single-user OS • UNIX was developed long before Windows, about 30 years ago at AT&T Bell Labs (95% written in “C” programming language).

3. What is UNIX? (2) • UNIX was designed as an operating system for experts, used on high-end workstations, servers and hosts. • UNIX provides some powerful features: • Interactive, multi-user support • Security - private and shared files • Inter-process communication • Extensive network support

4. History of Unix • 1970-1974 • Early stages, developed on PDP-11 machines • Unix is not an acronym, but a weak pun on MULTICS • 1976, first licensed release, Version 6 • 1978, first portable version, Version 7 • 1979, Berkeley 3 BSD • 1983, System V as industry standard • 1984, Microsoft releases Xenix • 1986, BSD 4.3, AT&T Version 9 • 1987, SVR4, Mach, … • 1993, Linux

5. UNIX Compared with other OS • Mature and stable OS • Large user community • Vendor-independent OS • Network-portable window system (X) • Runs on range of hardware (PC - Cray) • Common Open System Environment (COSE) • Source code is available to view and modify

6. Multi-user, Multi-tasking • More than one user can run at the same time and each user can run more than one task at the same time • In Unix, each program is started as a process. • A process is a program in execution. • Usually only one copy of a program, but there may be many processes running the same program. • Each interactive user may have: • only one process in foreground • may have several processes in background

7. Most Important Feature of UNIX • Most important feature of UNIX:STABILITY • 30 years to get the bugs out • Important in shared environments and critical applications • Shared Environments Example: University • Windows NT crashes at least once a day in labs • UNIX servers crash about once a semester (usually due to hard disk failure) • UNIX more than 100 times more reliable than Windows!

8. UNIX Versions • Two main types of UNIX: • BSD (Berkeley Software Distribution)/OSF • System V (developed at AT&T) our book • Different versions of UNIX for different hardware: • Sun Microsystem’s Solaris (and SunOS) • Hewlett-Packard’s HP-UX • IBM’s AIX • SGI’s IRIX

9. Cornerstones of the Unix Philosophy • Clean minimal design (nothing unnecessary) • Make each program do one thing well. • Combine existing utility programs to perform more complex tasks, and allow users to build their own utility programs.  • Use terse commands and messages to reduce typing and screen output. • Allow users to select what command language they will use (shell selection).

10. Additional Unix Philosophy • Make file, device, and interprocess I/O compatible. • All three are treated like files • Open access: • Minimal restrictions to the ways of doing things • User can be very creative (and frustrated). 8. Major parts of the Unix OS: • Kernel • Shell • File System

11. Unix Operating System Structure User • OS mediates between the user and the computer Application Programs Shell Kernel Hardware

12. The Unix Kernel • Loaded at system startup (boot up) and is memory-resident • Directly controls the hardware • Manages all devices • Schedules work done by the CPU • Manages memory and allocates it to each process • Gets instructions from shell and carries them out • Enforces access permission on the file system

13. The Shell • Command interpreter • To get your work done, you enter commands at the shell prompt • Can edit the command line • Shell interprets the command and passes it to the kernel for execution • Allows you to create a customized environment • Stores and allows manipulation of command history • Used to write shell scripts

14. Shells (1) • Bourne Shell: the default shell (sh) • original Unix shell, written by Steve Bourne • does not have interactive features of newer shells • widely used for writing shell scripts • available on ALL Unix systems • C Shell (csh): originally written for BSD • with syntax similar to the C language • with many enhancement over the Bourne shell.

15. Shells (2) • Korn Shell (ksh): AT&T’s answer to C Shell • Superset of the Bourne shell • Also includes many C shell features • very efficient, written by Dave Korn • TC Shell (tcsh) • Superset of C shell with EMACS-like command line editing • Bourne Again Shell (bash) • Public domain shell written by the Free Software Foundation

16. Why has Unix been successful? • UNIX is portable, because it was written in C. • Provides hardware independence. • Open System • Underlying operating system source code is available •  Provides a productive environment • Allows multi-tasking and sharing of data • Excellent C development environment is built-in • Networking capabilities are built in

17. Unix Disadvantages • UNIX is not as user-friendly as some operating systems • Command names are often cryptic • User help is not great • UNIX does not error check user commands to protect users from hurting themselves or the system • Example: Request to copy a file over an existing file will overwrite the existing file with no warning.

18. Unix Disadvantages • UNIX is less secure than some operating systems • Developed to be used as a software development environment, in which all users are working together cooperatively. Security was traded for more convenience and flexibility. • Well-documented open code makes hacking easier. • UNIX's portability also makes it less efficient on any particular hardware. • Proprietary operating systems are optimized for that hardware.

19. I hope that you will decide that Unix’s advantages outweigh its disadvantages!