Unix Overview

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Unix Overview

• CSCI 2467 System Programming Concepts
• Computer Science Department
• University of New Orleans
• Instructor Michael Ruth
• mruth_at_cs.uno.edu

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Topics

• Unix Philosophy
• Basics
• Key Features
• Multi-tasking
• Multi-users
• Networking built-in
• Internal Boundaries
• File Processing
• Memory Management
• Unix Structure
• Kernel
• Shell
• Utilities
• Groups of utilities

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The Philosophy of Unix

• Make each program do one thing well.
• To do a new job, build afresh rather than
  complicate old programs by adding new features.
  (KISS Concept)
• Expect the output of every program to become the
  input to another, as yet unknown, program. (Pipe
  Concept)
• Don't clutter output with extraneous information.
  
• Avoid stringently columnar or binary input
  formats. (Textual Concept)
• Don't insist on interactive input.
• Design and build software, even operating
  systems, to be tried early, ideally within weeks.
  (prototyping concept)
• Don't hesitate to throw away the clumsy parts and
  rebuild them.
• Use tools in preference to unskilled help to
  lighten a programming task, even if you have to
  detour to build the tools and expect to throw
  some of them out after you've finished using
  them.(Lazy Concept)

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Key Features of Unix

• Multi-tasking
• Multi-users
• Networking Capabilities
• File Processing
• Virtual memory
• Other Features

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Multi-tasking

• The ability of the operating system to perform
  more than one task at a time
• Essential for
• Real-time performance
• Multi-User Systems
• Asynchronous systems
• Additionally, these tasks can communicate if
  necessary

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Multi-Users

• The ability of the Unix operating system to allow
  more than one user at a time
• The handling of multiple users is also very
  important
• Notion of sharing
• Notion of guarded access
• Preventing Interference

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Accessing the Network

• Unix was built to communicate
• With other computers (via a host of network
  protocols)
• BSD Unix created the notion of the Sockets API
• Which is still in use today
• Inter-process Communication
• Tasks are able to communicate freely
• Notion of piping

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File Processing

• Everything is a file
• Allows for every I/O subsystem to be handled
  uniformly
• Mouse, printer, text file, etc
• Unix files have neither record structure nor
  attributes
• Everything can and should be treated as ASCII
• The file system is hierarchal

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Virtual memory

• The UNIX operating system has virtual memory, or
  swap space.
• Vital for
• Applications that use very large amounts of data
• Systems which handle multiple users/tasks in
  parallel
• Paging, Swapping, Thrashing

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Other Features

• Portability
• Unix is very portable (thanks to C)
• Flexibility
• Unix is very customizable
• Available Software
• Tons of freely available software available right
  now
• Case Sensitivity
• Almost everything in Unix is CaSe-SeNsItIvE!

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Unix Structure

• Unix is composed of layers
• These layers exist between the user and the
  hardware
• And consists of three major components
• Kernel
• Shell
• Utilities

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Unix Kernel

• The core of the Unix OS
• Performs all low-level operations which create
  the Unix environment on a particular system
• Kernel interacts directly with the hardware
• The part of the kernel which deal with I/O
  directly (AKA drivers) must be adapted to each
  new system
• Porting a UNIX system is the process of adapting
  the hardware dependent parts of the kernel
• You do not need to know anything about the kernel
  in order to use a UNIX system (and shouldnt)

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Kernel Function Types

• Kernel functions are of two broad types
• Autonomous those functions which are performed
  without being explicitly requested by user
  processes
• Responsive those functions which are initiated
  by requests from processes.

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Functions of the Kernel

• File Management and Security
• I/O Services
• Date and Time Services
• Interrupt and Error Handling
• System Accounting
• Memory Management
• Process Scheduling Management

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Unix System Calls

• Requests from processes come in the form of
  system calls.
• A system call can be thought of as a low level
  request to the operating system
• The Unix Kernel supports over 60 system calls
  which taken together form the Unix API
• This is one of the reasons Unix is so very
  portable

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Shell

• Whenever you login to a Unix system you are
  placed in a shell program.
• The shell's prompt is usually visible at the
  cursor's position on your screen.
• To get your work done, you enter commands at this
  prompt.
• The shell is a command interpreter it takes each
  command and passes it to the operating system
  kernel to be acted upon.
• It then displays the results of this operation on
  your screen.

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Shell Types

• Several shells are usually available on any UNIX
  system, each with its own strengths and
  weaknesses.
• Different users may use different shells.
• Initially, your system adminstrator will supply a
  default shell, which can be overridden or changed
• The most commonly used shells are
• Bourne shell (sh)
• C shell (csh)
• Korn shell (ksh)
• TC Shell (tcsh)
• Bourne Again Shell (bash)

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Shell Programming

• Each shell also includes its own programming
  language.
• Command files, called "shell scripts" are used to
  accomplish a series of tasks
• Additionally there are scripting languages that
  perform the same thing
• Perl
• Python
• Sed
• Awk
• Tcl
• Scheme

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Utilities

• UNIX provides several hundred utility programs,
  often referred to as commands.
• Accomplish universal functions
• editing
• file maintenance
• printing
• sorting
• programming support
• online info
• Modular single functions can be grouped to
  perform more complex tasks

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References

• Unix Overview (USAIL Unix System Administration
  Independent Learning)
• http//www.uwsg.iu.edu/usail/index/concepts.html
• The Art of Unix Programming (online book E.
  Raymond)
• http//www.faqs.org/docs/artu/
• Intro to Unix/Linux (Canada National Research
  Council)
• http//rcsg-gsir.imsb-dsgi.nrc-cnrc.gc.ca/document
  s/basic/basic.html
• Intro to Unix (Maui High Performance Computing
  Center)
• http//www.mhpcc.edu/training/vitecbids/UnixIntro/
  UnixIntro.htmlOverview
• Overview of Unix OS (Bell Labs)
• http//www.bell-labs.com/history/unix/tutorial.htm
  l
• Understanding Unix A Conceptual Guide
• ISDN978-0880223430
• Library book (very very old -gt Do NOT buy this
  book!!!)

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Future Classes

• Unix File System
• Unix Permission System
• Text/File Processing
• Processes and Job Control
• Shell Programming Regular Expressions
• Other important Unix Commands

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Questions?