Transcript 128509655X_397019x

Understanding Operating Systems
Seventh Edition
Chapter 13
UNIX Operating System
Learning Objectives
After completing this chapter, you should be able to
describe:
• The goals of UNIX designers
• The significance of using files to manipulate devices
• The strengths and weaknesses of competing
versions
• The advantages of command-driven user interfaces
Understanding Operating Systems, 7e
2
Brief History
• 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
Understanding Operating Systems, 7e
3
Brief History (cont’d.)
• Research project originally in 1965
– Joint venture between Bell Laboratories (AT&T
research and development group), General Electric,
and MIT
• Bell Laboratories withdrew in 1969
– Ken Thompson and Dennis Ritchie continued the
project
Understanding Operating Systems, 7e
4
(table 13.1)
The historical roots of UNIX. For current information about the many versions
of UNIX, see www.theopengroup.org.
© Cengage Learning 2014
Understanding Operating Systems, 7e
5
The Evolution of UNIX
• First official version
– Design
• Do one thing well
– Ran on a popular minicomputer
– Widely adopted: competitive in the market
• Thompson and Ritchie: version 3
– New programming language (C language)
Understanding Operating Systems, 7e
6
The Evolution of UNIX (cont'd.)
• AT&T forbidden to sell software
– Universities and developers advanced software
• Commercial transformation
• Berkley “bsd” version: 1973-1975
Understanding Operating Systems, 7e
7
The Evolution of UNIX (cont'd.)
• 1991: “The Open Group” formed
– Owns UNIX trademark
• Key UNIX features preserved
– Expanded commands and controls: increased to meet
new needs
• Standards established
– Improve program’s portability from one system to
another
Understanding Operating Systems, 7e
8
Design Goals
• Thompson and Ritchie vision
– UNIX operating system
• Created by programmers for programmers
– Fast, flexible, and 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 speed or sophistication
Understanding Operating Systems, 7e
9
Design Goals (cont'd.)
• Long-term goal
– Portability
• Reduces conversion costs
• Application packages not obsolete with hardware
changes
– Achieved with UNIX version 4: device-independent
• POSIX
– Portable Operating System Interface for Computer
Environments
• IEEE standards defining portable operating system
interface
Understanding Operating Systems, 7e
10
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
Understanding Operating Systems, 7e
11
Memory Management (cont'd.)
• Typical internal memory layout (single user)
– Program code
– Data segment
– Stack
Understanding Operating Systems, 7e
12
(figure13.2)
The historical roots of
UNIX. For current
information about the many
versions of UNIX, see
www.theopengroup.org.
© Cengage Learning 2014
Understanding Operating Systems, 7e
13
Memory Management (cont'd.)
• 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
Understanding Operating Systems, 7e
14
Memory Management (cont'd.)
• 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 section: process information saved when
process interrupted
– Nonsharable section of memory
Understanding Operating Systems, 7e
15
Memory Management (cont'd.)
• UNIX kernel
– Implements “system calls”
• Memory boundaries for process coexistence
– System calls
• File Manager interaction and I/O services request
– Implements most primitive system functions
• Permanently resides in memory
– Employs least recently used (LRU) page replacement
algorithm
• Network PCs, single-user, and multi-user systems
– Applies same memory management concepts
Understanding Operating Systems, 7e
16
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
Understanding Operating Systems, 7e
17
Process Management (cont'd.)
• 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 (1)
• CPU-bound job
Understanding Operating Systems, 7e
18
Process Management (cont'd.)
• Process scheduling algorithm (cont'd.)
– 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
Understanding Operating Systems, 7e
19
Process Management (cont'd.)
• Process scheduling algorithm (cont'd.)
– 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
Understanding Operating Systems, 7e
20
Process Table Versus User Table
• Simple processes (nonsharable code)
• Process table and user table
– 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, and code segment
• Swapped as needed
Understanding Operating Systems, 7e
21
Process Table Versus User Table
(cont'd.)
• Each process table entry contains:
– Process identification number
– User identification number
– Process memory address or secondary storage
address
– Process size and scheduling information
• Process table existence
– Set up when process is created
– Deleted when process terminates
Understanding Operating Systems, 7e
22
Process Table Versus User Table
(cont'd.)
• Text table
– Sharable code processes
– Process table maintains
• Text table 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 = zero (0): implies code no longer needed
Understanding Operating Systems, 7e
23
Process Table Versus User Table
(cont'd.)
• User table
– Allocated to each active process
– Stored in transient memory area
• User table contains:
– User and group identification numbers
• Determine file access privileges
– Pointers to system’s file table
• Every file that the process uses
– Pointer to current directory
– List of responses for various interrupts
– All information is accessible during process
Understanding Operating Systems, 7e
24
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
Understanding Operating Systems, 7e
25
Synchronization (cont'd.)
• fork
– UNIX command: executes one program from another
program
– Gives second program all first program attributes
(open files)
– Saves first program in original form
– Splits program: two copies
• Both run from statement after fork command
– fork executed
• “Process id” (pid) generated
• Ensures each process has unique ID number
Understanding Operating Systems, 7e
26
Synchronization (cont'd.)
(figure13.4)
When the fork
command is
received, the
parent process
shown in (a)
begets the child
process shown in
(b) and Statement
2 is executed
twice.
© Cengage
Learning 2014
Understanding Operating Systems, 7e
27
Synchronization (cont'd.)
• wait
– UNIX command: synchronizes process execution
• Suspends 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
Understanding Operating Systems, 7e
28
Synchronization (cont'd.)
(figure13.5)
The wait command used in
conjunction with the fork
command will synchronize
the parent and child
processes. In (a) the
parent process is shown
before the fork, (b) shows
the parent and child after
the fork, and (c) shows the
parent and child during the
wait.
© Cengage Learning 2014
Understanding Operating Systems, 7e
29
Synchronization (cont'd.)
• exec
– exec commands
• execl, execv, execls, execlp, and execvp
• Start new program execution from another program
– 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
– Create parent-child relationship
• Call fork, wait, and exec in that order
Understanding Operating Systems, 7e
30
Synchronization (cont'd.)
(figure13.6)
The exec command is
used after the fork and wait
combination. In (a) the
parent is shown before the
fork, (b) shows the parent
and child after the fork, and
(c) shows how the child
process (Process 2) is
overlaid by the ls program
after the exec command.
© Cengage Learning 2014
Understanding Operating Systems, 7e
31
Device Management
• Device independence to applications
– I/O device treated as special file type
• Device files given name
– Descriptors called inodes
• Identify devices, contain device information, and stored
in the device directory
• Device drivers
– Subroutines working with operating system
– Supervise data transmission
• Between main memory and peripheral unit
Understanding Operating Systems, 7e
32
Device Management (cont'd.)
• 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 device switch)
• cdevsw (character device switch)
Understanding Operating Systems, 7e
33
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
Understanding Operating Systems, 7e
34
Device Classifications (cont'd.)
(figure13.7)
When a process sends an I/O request, it goes to the UNIX kernel where the
char and block device drivers reside.
© Cengage Learning 2014
Understanding Operating Systems, 7e
35
Device Classifications (cont'd.)
• Class: block or character
– Each has configuration table
• Array of entry points into device drivers
– Major device number
• Array index accessing 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)
Understanding Operating Systems, 7e
36
Device Classifications (cont'd.)
• Character class devices
– Handled by device drivers 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 block and character
classes
– Examples: disk drives and tape drives
Understanding Operating Systems, 7e
37
Device Drivers
• Special section in kernel
• Disk drive’s device drivers
– Use seek strategy to minimize arm movement
• Kept in set of files
– Included as needed
– Small changes due to peripheral upgrades
• Linked into the kernel to keeping the operating system
informed of new features and capabilities
– Kept in /dev directory by default and convention
Understanding Operating Systems, 7e
38
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
Understanding Operating Systems, 7e
39
File Management (cont'd.)
• Ordinary files
– Users’ stored 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
Understanding Operating Systems, 7e
40
File Management (cont'd.)
• Files stored as sequences of bytes
– No structure imposed
• Structure of files
– Controlled by programs using them: not by system
• Text files
– Character strings
• Lines delimited by line feed or new line character
• Binary files
– Sequences of binary digits
• Grouped into words as they appear in memory during
program execution
Understanding Operating Systems, 7e
41
File Management (cont'd.)
• Organizes disk into blocks
• Divides disk into four basic regions
– First region (address 0): reserved for booting
– Second region (superblock): contains disk size and
other regions’ boundaries
– Third region: includes file definitions called i-list
– Remaining region: holds free blocks available for file
storage
• Files stored in contiguous empty blocks
– Simple allocation and no need to compact
Understanding Operating Systems, 7e
42
File Management (cont'd.)
• 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, and file size
Time of creation, last use, and last update
Number of links
File type
• Directory, ordinary file, or special file
Understanding Operating Systems, 7e
43
File Naming Conventions
•
•
•
•
Case-sensitive filenames
255 character length
Special characters allowed, except colon (:)
No file naming conventions
– Some compilers expect specific suffixes
• Supports hierarchical tree directory structure
– Root directory identified by slash (/)
Understanding Operating Systems, 7e
44
(figure13.9)
File hierarchy seen as an upside-down tree with the forward slash (/ ) as the
root, leading next to the directories and subdirectories, and then to the files
shown in dashed line boxes.
© Cengage Learning 2014
Understanding Operating Systems, 7e
45
File Naming Conventions (cont'd.)
• Absolute path name: starts at the root directory
• Relative path name: does not start at the root
directory
• Path name rules
– Path name beginning with a slash starts at the 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)
Understanding Operating Systems, 7e
46
Directory Listings
• “long listing”
– Eight information pieces for each file
• Access control, number of links, name of the group and
owner, byte size of the file, date and time of last
modification, and filename
Understanding Operating Systems, 7e
47
Directory Listings (cont'd.)
(table 13.2)
This table shows the list of files stored in the directory journal from the system
illustrated in Figure 13.9. The command ls -l (short for “listing-long”)
generates this list.
© Cengage Learning 2014
Understanding Operating Systems, 7e
48
Directory Listings (cont’d.)
• 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)
Understanding Operating Systems, 7e
49
(figure 13.10)
Graphical depiction of a list of file and directory permissions in UNIX.
© Cengage Learning 2014
Understanding Operating Systems, 7e
50
Directory Listings (cont'd.)
• 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 is no longer
needed: can be deleted
Understanding Operating Systems, 7e
51
Data Structures
• File information divided into parts between:
– Directory entries: filename and device’s physical
addresses
– i-nodes: all other information
• i-nodes stored in reserved part of device
– Where directory resides
Understanding Operating Systems, 7e
52
Data Structures
• i-node has 13 pointers (0–12)
–
–
–
–
First 10 pointers
Eleventh entry
Twelfth entry
Thirteenth entry
► direct blocks
► indirect block
► double indirect block
► triple indirect block
• When a file is created
– i-node is allocated to it
– Directory entry with filename and i-node number is
created
Understanding Operating Systems, 7e
53
Data Structures (cont'd.)
• When a file is linked
– Directory entry created with the new name and the
original i-node number
– Link-count field in the i-node is incremented by one
• When a shared file is deleted
– Link-count field in the i-node decremented by 1
• When the count reaches 0, the directory entry is erased
and all disk blocks allocated to the file are deallocated
Understanding Operating Systems, 7e
54
User Interfaces
• Menu-driven systems and command-driven
interfaces
– Both are nearly identical in UNIX and Linux
• User commands in command driven system
– Very short
• One character or a group of characters (acronym)
– Cannot be abbreviated or spelled out
– Must be in correct case: generally only lowercase
– May combine on single line: additional power and
flexibility
Understanding Operating Systems, 7e
55
(table 13.3)
Small sample of UNIX (and Linux) user commands which are entered in all
lower case. Check your technical documentation for proper spelling and
syntax.
© Cengage Learning 2014
Understanding Operating Systems, 7e
56
(table 13.3) (cont’d.)
Small sample of UNIX (and Linux) user commands which are entered in all
lower case. Check your technical documentation for proper spelling and
syntax.
© Cengage Learning 2014
Understanding Operating Systems, 7e
57
User Interfaces (cont'd.)
• 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
Understanding Operating Systems, 7e
58
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
Understanding Operating Systems, 7e
59
Script Files (cont'd.)
• 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
Understanding Operating Systems, 7e
60
Redirection
• Send output to file or another device
– Angle bracket ( > ) inserted between command and
destination
– Examples:
• ls > myfiles
• cat chapter_1 chapter_2 > section_a
• cat chapt* > section_a
– Double bracket symbol ( >> ) appends new file to an
existing file
– Example:
• cat chapter_1 chapter_2 >> section_a
Understanding Operating Systems, 7e
61
Redirection (cont'd.)
• Reverse redirection ( < )
– Takes input for program from existing file instead of
keyboard
– Example:
• mail Chris Anthony Ida Roger < memo
Understanding Operating Systems, 7e
62
Redirection (cont'd.)
• 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
Understanding Operating Systems, 7e
63
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
Understanding Operating Systems, 7e
64
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
• Command with filename given: contents of file sorted
and displayed on screen
• Example: sort wk_names > sortednames
– Input file sorted and written to another file
Understanding Operating Systems, 7e
65
Filters (cont'd.)
• Obtain numerical sort in ascending order
– sort -n wk_names > sortednumbs
• Obtain numerical sort in descending order
– sort -nr wk_names > sortednumbs
• Obtain sort on the third field in alphabetical order
– sort +2f empl > sortedempl
Understanding Operating Systems, 7e
66
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 mail-list
• grep -v Pittsburgh mail-list
• grep -c Pittsburgh mail-list
Understanding Operating Systems, 7e
67
Additional Commands (cont'd.)
• grep (cont'd.)
– Can be combined with who command
– Example: who | grep bob
• Bob’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 old_largefile new_largefile &
Understanding Operating Systems, 7e
68
Additional Commands (cont'd.)
• nice
– Allows lowering program priority
– Example:
nice cp old_largefile new_largefile &
Understanding Operating Systems, 7e
69
Conclusion
• UNIX: far reaching impact on the computing world
– Has revolutionized development of operating systems
• Power, speed and flexibility
• Widespread adoption on computing devices of every
size
– Reasons for success
• Spare, flexible, and powerful command structure
• Recent addition of several standard graphical user
interfaces: easier to issue commands
• Adoption by Apple: basis for Mac OSX operating
system: extended the user base
Understanding Operating Systems, 7e
70