Case Study UNIX | novice-vikas

Aim or Objective :
 To study basics of UNIX Operating System .

The Shell

• The shell is the interface between the command language user and the OS
  
• The shell is a user interface and comes in many forms (Bourne Shell, sh; Berkeley C Shell, csh; Korn Shell, ksh; Restricted Shell, rsh)
  
• User allowed to enter input when prompted ($ or %)
  
• System supports all shells running concurrently. Appropriate shell is loaded at login, but user can usually (except in sh, rsh) dynamically change the shell
  
• A UNIX command takes the form of
  
  executable_file [-options] arguments
  
• The shell runs a command interpretation loop
  
  
  • accept command
    
  • read command
    
  • process command
    
  • execute command
• Executing the command involves creating a child process running in another shell (an environment within which the process can run). This is done by Forking.
  
• The parent process usually waits for the child to terminate before re-entering the command interpretation loop
  
• Programs can be run in the background by suffixing the command-line entry with an ampersand (&). Parent will not wait for child to terminate

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The Processing Environment

Input and Output

• UNIX automatically opens three files for the process
  
  
  STDIN - standard input (attached to keyboard)
  STDOUT - standard output (attached to terminal)
  
  STDERR - standard error (attached to terminal)
  
• Because UNIX treats I/O devices as special types of files, STDIO can be easily redirected to other devices and files
  
  who > list _of _users

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The Kernel

• Central part of the OS which provides system services to application programs and the shell
  
• The kernel manages processes, memory, I/O and the Timer - so this is not the same as the kernel that we covered in Lecture 3!
  
• UNIX supports multiprogramming
  
• Processes have their own address space - for protection
  
• Each process's process environment is composed of an unmodifiable re-entrant text (code) region, a modifiable data region and a stack region.
  
• The text region is shareable
  
• Processes can modify their environment only through calls to the OS

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The File System

• UNIX uses HDS with root as its origin
  
• A directory is a special UNIX file which contains file names and their i-nodes (index nodes)
  
• Subdirectories appear as file entries
  
• Directories cannot be modified directly, but can are changed by the operating system when files and subdirectories are created and deleted
  
• File and Directory names must be unique within a particular directory (i.e., the path name must be unique)
  
• The File System is a data structure that is resident on disk
  
• It contains a super block (definition of the file system); an array of i-nodes (definition of the files in the system); the actual file data blocks; and a collection of free blocks
  
• Space allocation is performed in fixed-size blocks

The i-node

• Contains
  
  
  the file owner's user-id and group-id
  protection bits for owner, group, and world
  
  the block locator
  
  file size
  
  accounting information
  
  number of links to the file
  
  file type
  

The Block Locator

• Consists of 13 fields
  
• First 10 fields points directly to first 10 file blocks
  
• 11th field is an indirect block address
  
• 12th field is a double-indirect block address
  
• 13th field is a triple-indirect block address

Permissions

• Each UNIX file and directory has 3 sets of permission bits associated with it
  
• These give permissions for owner, group and world
  
• System files (inc. devices) are owned by root, wizard, or superuser (terminology!)
  
• Root has unlimited access to the entire installation - whoever owns the files!

Setuid

• When you need to change your password, you need to modify a file called /etc/passwd. But this file is owned by root and nobody other than root has write permission!
  
• The passwd command (to change passwords) is owned by root, with execute permission for world.
  
• The setuid is a bit which when set on an executable file temporarily gives the user the same privileges as the owner of the file
  
• This is similar in concept to some OS commands executing in Supervisor mode to perform a service for an otherwise unauthorised process

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Process Management

• Description of Process Management in SunOS

Scheduling

• Priority-based pre-emptive scheduling. Priorities in range -20 to 20. Default 0.
  
• Priorities for runnable processes are recomputed every second
  
• Allows for ageing, but also increases or decreases process's priority based on past behaviour
  
• I/O-bound processes receive better service
  
• CPU-bound processes do not suffer indefinite postponement because the algorithm `forgets' 90% CPU usage in 5*n seconds (where n is the average number of runnable processes in the past 60 seconds)

Signals

• Signals are software equivalents to hardware interrupts used to inform processes asynchronously of the occurrence of an event

Interprocess Communication

• UNIX System V uses semaphores to control access to shared resources
  
• For processes to exchange data or communicate, pipes are used
  
• A pipe is a unidirectional channel between 2 processes
  
• UNIX automatically provides buffering, scheduling services and synchronisation to processes in a pipe line
  
• The presence of a pipe causes the processes in the pipe line to share STDIO devices
  
  who | grep cstaff
  
• The output from who is directed to a buffer. grep will take its input from this buffer. The output from grep will be displayed on the terminal

Timers

• UNIX makes 3 interval timers available to each process
  
• Each counts down to zero and then generates a signal
  
• The first runs continuously
  
• The second runs while a process is executing process code
  
• The third runs while the process executes process code or kernel code

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Memory Management

Address Mapping (Virtual Storage) - Paged MMS

• Virtual address V is dynamically translated to real address (P, D)
  
• Direct Mapping is used, with the Page Map held in a high-speed RAM cache
  
• Each Page Map Entry contains a modified bit, an accessed bit, a valid bit (if the page is resident in PM) and protection bits
  
• The system maintains 8 page maps - 1 for the kernel (not available to processes) and 7 for processes (contexts)
  
• 2 context registers are used - one points to the running process's page map and the other to the kernel's page map
  
• The replacement strategy replaces the page that has not been active for longest (LRU)

Paging

• SunOS maintains 2 data structures to control paging
  
• The free list contains empty page frames
  
• The loop contains an ordered list of all allocated page frames (except for the kernel)
  
• The pager ensures that there is always free space in memory
  
• When a page is swapped out (not necessarily replaced) the system judges whether the page is likely to be used again
• If the page contains a text region, the page is added to the bottom of the free list, otherwise it is added to the top
  
• When a page fault occurs, if the page is still in the free list it is reclaimed

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I/O

Data

• All data is treated as a byte stream
  
• UNIX does not impose any structure on data - the applications do
  
• So data can be manipulated in any way - but programmers must explicitly structure the data

Devices

• A device is just a special type of file
  
• These files can have protection bits, so that a printer, e.g., cannot be read
  
• Permission to use sensitive devices, e.g., magnetic disk, is restricted to root and all other users have to use system calls to executable files which have their setuid bit set

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Summary

• Explained how UNIX is constructed with reference to material we have covered in the course
  
• The UNIX File System
  
• Process Management in UNIX
  
• UNIX Memory Management
  
• UNIX I/O Device Independence
  
• Command Interpretation Loop

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