Introduction to UNIX

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CS465

• Introduction to UNIX

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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 ATT Bell Labs (95 written in C
  programming language).

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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

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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, ATT Version 9
• 1987, SVR4, Mach,
• 1993, Linux

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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

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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

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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!

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UNIX Versions

• Two main types of UNIX
• BSD (Berkeley Software Distribution)/OSF
• System V (developed at ATT) ?our book
• Different versions of UNIX for different
  hardware
• Sun Microsystems Solaris (and SunOS)
• Hewlett-Packards HP-UX
• IBMs AIX
• SGIs IRIX

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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).

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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

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Unix Operating System Structure

• OS mediates between the user and the computer

User
Application Programs
Kernel
Shell
Hardware
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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

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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

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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.

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Shells (2)

• Korn Shell (ksh) ATTs 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

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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

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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.

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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.

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• I hope that you will decide that
• Unixs advantages
• outweigh
• its disadvantages!