Understanding Operating Systems Fifth Edition Chapter 13 Unix Operating System

Understanding Operating SystemsFifth Edition

Chapter 13

Unix Operating System

Understanding Operating Systems, Fifth Edition 2

Learning Objectives

• The goals of UNIX designers

• The significance of using files to manipulate devices

• The strengths and weaknesses of having competing versions of UNIX

• The advantages of command-driven user interfaces

• The roles of the Memory, Processor, Device, and File Managers in UNIX


Overview

• Three major advantages of UNIX– Portability

• Code written in high-level language (C language) – Powerful utilities

• Brief, single operation commands• Combinable into single command

– Application device independence• Configurable to operate on any device type

• Disadvantage– No single standardized version– Brief, cryptic commands difficult for novice learner


History

• Research project originally in 1965– Joint venture between Bell Labs, AT&T, General

Electric, and MIT• Goal

– Develop MULTICS for GE-645 mainframe• MULTICS ambition

– Serve diverse user group needs• Too intricate, complex, large for commercial value

• Bell labs withdrew in 1969• Ken Thompson and Dennis Ritchie continued the

project


History (continued)


The Evolution of Unix

• Original language– DEC PDP-7 assembly language

• First official version: 1971– Design

• Do one thing well

– Ran on DEC PDP-11– No pipes or filters

• Added in version 2

• Thompson and Ritchie: version 3– New programming language (C language)


The Evolution of Unix (continued)

• AT&T forbidden to sell software– Universities and developers advanced software

• Commercial transformation

• Berkley BSD version: 1973-1975

• 1984: government deregulation– AT&T personal computer with UNIX System 4

• Contained additional Berkley version features

• AT&T System 4 promotion as standard fails

• 1990: two dozen versions



• 1991: AT&T UNIX system laboratories– Develops System V release 4– Features

• System V release 3, BSD 4.3, SunOS, Xenix

• “The Open Group” formed– Owns UNIX trademark

• 1993: Berkeley– 4.4 BSD: based on AT&T’s UNIX (AT&T license)– Novell acquires UNIX from AT&T



• Current releases– Modify “do one thing well” position– Commands more difficult to use

• Pipelines preserved– Adaptable to new situations with ease

• Meet new user needs– Full local area network support– Comply with international standards– Security improved– Uses Common Desktop Environment (CDE)– ISO/IEC 9945:2003 Standard


Design Goals

• Thompson and Ritchie vision– UNIX operating system

• Created by programmers for programmers– Fast, flexible, easy-to-use

• Immediate goals– Support software development

• Included utilities for customized code• Utilities designed for simplicity: do one thing well• Small manageable sections of code

– Keep algorithms simple• Based on simplicity, not sophistication


Design Goals (continued)

• Long-term goal– Portability

• Reduces conversion costs• Application packages not obsolete

– Achieved with UNIX version 4• Hardware independent

• POSIX – Portable operating system interface for computer

environments• IEEE standards defining portable operating system

interface• IEEE STD. 1003.1 (2004 edition)


Memory Management

• Multiprogramming systems– Swapping (small jobs)

• Entire program in main memory before execution

• Program size restriction

• Round robin policy

– Demand paging (large jobs)• More complicated hardware

• Increases system overhead

• Thrashing (under heavy loads)

• Advantage: implements virtual memory concept


Memory Management (continued)

• Typical internal memory layout (single user)– Program code– Data segment– Stack



• Program code– Sharable portion of program– Reentrant code

• Physically shared by several processes

• Code protected: instructions not modified during normal execution

• Data references: without absolute physical address

– Space allocation• Program cannot release until all processes completed

• Text table: tracks processes using program code



• Data segment– After program code

• Grows toward higher memory locations – Nonsharable section of memory

• Stack– Starts at highest memory address

• Grows downward• Subroutine calls and interrupts add information • Main memory • Process information saved when process interrupted

– Nonsharable section of memory



• UNIX kernel– Implements “system calls”

• Memory boundaries for process coexistence

– System calls• File Manager interaction and request of I/O services

– Implements most primitive system functions• Permanently resides in memory

– Uses LRU page replacement algorithm

• Network PCs, single-user, and multi-user systems– Use same memory management concepts


Process Management

• Handles– CPU allocation– Process scheduling– Satisfaction of process requests

• Kernel maintains tables– Coordinates process execution– Device allocation

• Uses predefined policies– Select process from READY queue– Begin execution

• Give time slice


Process Management (continued)

• Process scheduling algorithm– Selects highest priority process to run first– Priority value: accumulated CPU time

• Processes with large CPU time get lower priority

– Compute-to-total-time ratio• System updates for each job every second

• Total time process in system divided by used process CPU time

– Ratio = one• CPU-bound job



• Process scheduling algorithm (continued)– Processes with same computed priority

• Handled by round robin

– Interactive processes: low ratio (no special policies)– Balance I/O-bound jobs with CPU-bound jobs

• Keeps processor busy

• Minimizes waiting processes overhead



• Process scheduling algorithm (continued)– Loading process from READY queue

• Process with longest secondary storage time

– Swap out process• Process waiting longest (disk I/O, idle )

– When processor becomes available• Process selected may not be ready (waiting on I/O)

• Determine inactive but ready for execution

• Process priorities recalculated

• Handled dynamically


Process Table Versus User Table

• Simple processes (nonsharable code)

• Tables– Keep system running smoothly

• Process table– Always resides in memory– Maintains text table

• User table– Resides in memory while process is active– User table, process data segment, code segment

• Swapped as needed



Process Table Versus User Table (continued)

• Process table

• Each entry contains: – Process identification number– User identification number– Process memory address or secondary storage

address– Process size and scheduling information

• Set up when process is created

• Deleted when process terminates



• Text table– Sharable code processes– Process table maintains

• Contains:– Memory address or secondary storage address of

text segment (sharable code)– Count: tracks number of processes using code

• Increased by one when process starts using code

• Decreased by one when process stops using code

• Count = 0: implies code no longer needed



• User table– Allocated to each active process– Stored in transient memory area

• Contains:– User and group identification numbers

• Determine file access privileges– Pointers to system’s file table

• Every file process uses– Pointer to current directory– List of responses for various interrupts– All information accessible when process running


Synchronization

• UNIX– True multitasking operating system

• Requires processes wait for certain events– Each event represented by integers

• Equal to address of table associated with event

• Race occurs – Event happens during process transition decision

• Wait for event and entering WAIT state


Synchronization (continued)

• fork– Execute one program from another program – Second program

• Given all first program attributes (open files)

– Save first program in original form– Split program: two copies

• Both run from statement after fork command

– fork executed• “Process id” (pid) generated

• Ensure each process has unique ID number





• wait– Synchronize process execution

• Suspend parent until child finished

– Program IF-THEN-ELSE structure• Controlled by pid value

• pid > zero: parent process

• pid = zero: child process

• pid < zero: error in fork call





• exec– Start new program execution from another program

• execl, execv, execls, execlp, and execvp

– Successful exec call • Overlay second program over first

• Only second program in memory

– No return from successful exec call• Parent-child concept: does not hold

– Each exec call• Followed by test ensuring successful completion




Device Management

• Device independence to applications– I/O device treated as special file type

• Device files given name– Descriptors called “iodes”

• Identifies devices, contains device information, stored in device directory

• Device drivers– Subroutines working with operating system – Supervise data transmission

• Between main memory and peripheral unit– Most common drivers included in UNIX


Device Management (continued)

• Device driver kernel incorporation– During system configuration

• Recent UNIX versions– Program called config– Automatically creates conf.c

• For any hardware configuration– conf.c contains parameters controlling resources

• Number of internal kernel buffers and swap space size– conf.c contains two tables

• bdevsw (“block I/O switch”)• cdevsw (“character I/O switch”)


Device Classifications

• Divide I/O system– “Block I/O” system (“structured I/O” system) – “Character I/O” system (“unstructured I/O” system)

• Physical device identification– Minor device number– Major device number– Class: block or character


Device Classifications (continued)



• Class: block or character– Each has configuration table

• Array of entry points into device drivers– Major device number

• Index to array to access appropriate code (specific driver)

– Minor device number• Passed as an argument to device driver• Access one of several identical physical devices

– Block I/O system• Devices addressed as 512-byte block sequences• Allows device manager to buffer (reduce I/O traffic)



• Character class devices– Device drivers handle implementing character lists– Example: terminal

• Typical character device

• Two input queues and one output queue

• I/O procedure synchronized through hardware completion interrupts

• Some devices belong to both classes– Examples: disk drives, tape drives


Device Drivers

• Special section in kernel– Includes instructions

• Allows operating system communication with device

• Disk drive’s device drivers– Use seek strategy to minimize arm movement

• Kept in set of files– Loaded as needed

• Case of seldom used devices– Kept in memory all the time

• Loaded at boot time– Kept in /dev directory by default and convention


File Management

• Three file types – Directories– Ordinary files– Special files

• Each enjoys certain privileges

• Directories– Maintain hierarchical structure of file system– Users allowed to read information in directory files– Only system allowed directory file modification


File Management (continued)

• Ordinary files– Users store information– Protection based on user requests

• Related to read, write, execute, delete functions performed on file

• Special files– Device drivers providing I/O hardware interface– Appear as entries in directories– Part of file system (most in /dev directory)– Special filename indicates type of device association



• Files stored as sequences of bytes– No structure imposed

• Text files– Character strings

• Lines delimited by line feed, new line, character

• Binary files– Sequences of binary digits

• Grouped into words as they appear in memory during program execution

• Structure of files– Controlled by programs using them: not by system



• Organizes disk into blocks of 512 bytes each

• Divides disk into four basic regions – First region (address 0): reserved for booting– Second region: contains disk size and other regions’

boundaries – Third region includes: file definitions called “i-list” – Remaining region: free blocks available for file

storage

• Files stored in contiguous empty blocks– Simple allocation and no need to compact



• “i-node”

• Each entry in i-list called an “i-node” (or inode)– Contains 13 disk addresses

• Contains specific file information– Owner’s identification– Protection bits, physical address, file size– Time of creation, last use, and last update– Number of links– File type

• Directory, ordinary file, or special file


File Naming Conventions

• Case-sensitive filenames

• 255 character length

• No file naming conventions– Some compilers expect specific suffixes

• Supports hierarchical tree file structure– Root directory identified by slash (/)


File Naming Conventions (continued)


File Naming Conventions (continued)

• Path name rules– Path name starting with slash

• Root directory

– Path name • One name or list of names: separated by slashes

• Last name on list: filename requested

– Two periods (..) in path name• Moves upward in hierarchy (closer to root)

• Only way to go up hierarchy

• All other path names go down tree

– Spaces not allowed within path names


Directory Listings

• “long listing” – Eight pieces of information for each file

• First column – Shows file type and access privileges for each file

• First character: nature of file or directory

• Next three characters: access privileges granted file owner

• Next three characters: access privileges granted other user’s group members

• Last three characters: access privileges granted to users at large (system-wide)


Directory Listings (continued)

• Second column– Indicates number of links (number of aliases)

• Referring to same physical file

• Aliases– Important UNIX feature: support file sharing

• Several users work together on same project– Shared files appear in different directories belonging

to different users– Filename: may be different from directory to directory– Eventually number will indicate when file no longer

needed: can be deleted


Directory Listings (continued)


Data Structures

• File descriptors divided into parts– Hierarchical directories

• Contain filename and i-number

• Pointer to another location: i-node

– i-node • Contains rest of information

– i-nodes stored in reserved part of device• Where directory resides

– i-node has 13 pointers (0–12)


Data Structures (continued)



• When file opened– Device, i-number, read/write pointer stored in system

file table and indexed by i-node

• When file created– i-node allocated to it– Directory entry with filename and i-node number

created



• When file linked– Directory entry created with new name – Original i-node number and link-count field in the i-

node incremented by one

• When shared file deleted– Link-count field in i-node decremented by one– When count reaches zero

• Directory entry erased

• Deallocate all disk blocks and allocate i-node block to file


User Interface

• Command-driven system

• User commands– Very short

• One character or a group of characters (acronym)

– Cannot be abbreviated or spelled out – Must be in correct case

• System prompt very economical– Only one character: ($) or (%)

• Error messages quite brief


User Interface (continued)



• General syntax of commands– command arguments file_name

• “command”– Any legal operating system command

• Interpreted and executed by shell

• “arguments“– Required for some commands, optional for others

• “file_name” – Relative or absolute path name




Script Files

• Automate repetitious tasks– Command files

• Often called shell files or script files

• Each line of file– Valid instruction

• Executed by typing sh and script file name

• Also executed by defining file as executable command– Type filename at system prompt


Script Files (continued)

• Script file example

setenv DBPATH /u/lumber:.:/zdlf/product/central/db

setenv TERMCAP $INFODIR/etc/termcap

stty erase `^H’

set savehistory

set history=20

alias h history

alias 4gen infogen -f

setenv PATH /usr/info/bin:/etc


Redirection

• Send output to file or another device– Symbol: > (between command and destination)– Examples:

• ls > myfiles• cat chapt1 chapt2 > sectiona,• cat chapt* > sectiona

– Symbol >> appends new file to an existing file– Examples:

• cat chapt1 chapt2 >> sectiona• cat chapt* >> sectiona


Redirection (continued)

• Reverse redirection (<)– Takes input for program from existing file instead of

keyboard– Example:

• mail ann roger < memo


Redirection (continued)

• Redirection (>)– Combined with system commands to achieve any

desired results– Example: who > temporary

• Store in “temporary” file: all user names logged on

• Interpretation of < and >– Carried out by shell – Not by individual program

• Input and output redirection– Used with any program


Pipes

• Provide possibility to redirect output or input to selected files or devices– Connect output from one program to input of another– No need for temporary or intermediate files– Example: who | sort

• Pipeline– Several programs simultaneously processing same

I/O stream– Example: who | sort | lpr


Filters

• Program– Read some input, manipulate it, generate output– wc (word count):

• Example: wc journal• System response: 10 140 700• File journal has 10 lines, 140 words, 700 characters

– sort: Contents of file sorted and displayed on screen• Example: sort sortednames


Filters (continued)

• Sort list in alphabetical order ignoring letter case– sort –f > sortednames

• Obtain numerical sort in ascending order– sort -n > sortednums

• Obtain numerical sort in descending order– sort -nr > sortednums


Additional Commands

• man– Displays operating system online manual– Example: man cmp

• Displays page for compare (cmp) command

• grep– “global regular expression and print”– Look for specific character patterns– Examples:

• grep Pittsburgh maillist• grep -v Pittsburgh maillist• grep -c Pittsburgh maillist


Additional Commands (continued)

• grep (continued)– Can be combined with who command– Example: who | grep sam

• Sam’s name, device, date and time he logged in

– Example: ls -l / | grep '^d‘ • Displays subdirectories list (not files) in root directory

• nohup– Log off the system without program completion– Example:

nohup cp oldlargefile newlargefile and


Additional Commands (continued)

• nice– Allows lowering program priority– Example:

nice cp oldlargefile newlargefile and


Summary

• UNIX: major force in operating system field– Written by programmers for programmers– Quite popular among programmers

• Advantages– Spare user interface, device independence,

portability, lack of verbosity, powerful command combinations

• Disadvantages– Learning command-driven interface, brief commands

• Graphical interface and point-and-click surfacing

Documents

Understanding Operating Systems Fifth Edition Chapter 13 Unix Operating System