UNIX
By
Nick Gomez, Chris Rees, Brian Hernandez, Hector Rodriguez,
Chris Sosa, and Joshua Burbridge
What Is UNIX?

An operating system originally created in 1969

Provided many ground-breaking features

Completely self supporting

Originally designed in assembly code

Rewritten in C in 1973

The foundation of modern day Linux and BSD operating systems
Why UNIX?
UNIX incorporated many special features that were either new or uncommon
to operating systems of the time. UNIX revolutionized operating system design,
and many of it’s principles are used in modern operating systems, such as
Windows, OS X, and Linux.
Features
 Hierarchical File System
 Compatible File, Device, and Inter-Process I/O
 Asynchronous Processes
 System Command Language
 Supports Multiple Users
The File System
Let’s see why the UNIX file system was so unique for its day!
Three Types of Files
1. Ordinary Files
 Binary (executable) files
 Plain text files
 Symbolic links
Three Types of Files
2.
Directories
 Provides mapping between file names and files
 Allows paths (e.g. /home/uname/Desktop)
 The base directory called root (e.g. / )
 . and .. indicate current and parent directory respectively: /usr/local/.. same
as /usr
Three Types of Files
Important Note
Files do not exist inside directories. Rather, directories store file names and
pointers to the information that describes the file.
Three Types of Files
3.
Special Files
 Represents a particular device
 Read from and write to like normal files
 Unlike normal files, it interacts with associated devices
 Stored in the /dev directory
Why Special Files?
 Provides little distinction between files and devices
 Programs expecting files to read/write to can also accept devices without
needing to differentiate the two
 Devices are subject to the same protection mechanics that files have
Removable File System
Another unique feature of the UNIX file system is removable file systems. Using
the system command mount, you can attach removable volumes to a leaf in the
file structure. There is virtually no distinction between the removable volume
and the permanent file system.
Protection
Another innovation of UNIX was access control on files.
 Files created with owner name
 6 bits for read/write/execute permissions for owner and others
 7th bit can allow other users to temporarily become owner of the file
Super-User
 Regular users have limited access on a machine in some form or another
 Super-user is the highest privileged user on the machine, able to perform
any action
 Usual constraints on the machine do not apply to super-user
 Should only allow one person access
Implementation Of File System
 Directories contain file names and pointers
 Pointer is called an i-number (index number) of the file
 I-number references the information of the file in the system table
File Information
 Owner name
 Protection bits
 Physical address of location on disk
 Size
 Time of modification
 Number of links to file
 Directory bit
 Special file bit
 “Large” or “small” bit
Efficiency of File System
 File system provides satisfactory performance
 7621 line test program
 Total time of 35.9 seconds, 212 lines per second
 Time taken for each element:
63.5% Assembler execution
20.0% Disk wait time
16.6% System overhead
I/O Calls
 Eliminate differences between different devices
 open(name, flag) and Create
 No user-visible automatic locks on files, unnecessary and insufficient
 Sufficient internal interlocks
 Files are sequential
 read(filep, buffer, count), write(filep, buffer, count)
 seek(filep, base, offset)
Special Files
 Unique feature of UNIX
 Integral to I/O devices
 Acts as ordinary file
 Read/Write requests activate device
 File and device I/O treated similar as possible
 Same syntax and meaning
 Same protection
Processes and Images
 Image – execution environment
 Core Image
 Register Values
 Open Files
 Curr. Directory
 Curr. state
Processes
 Execution of image
 Image resides in core
 pID = fork(label)
 Splits into two processes
 Contain original image
 Parent/child processes
 Parent Process
 Control returns from fork
 Child Process
 Control passed to label
Pipes
 filep = pipe()
 Returns file descriptor
 Creates interprocess channel
 Passed from parent to child through fork
 Read/Write
 Read waits, data transferred
 Setup by common ancestor of processes
Execution of Programs
 execute(file, arg1, arg2, …, arg-n)
 arg1 usually same as file
 Read in/execute
 Creates process
 Code in process replaced
 Only returns on error
 “jump” instead of subroutine
Process Synchronization
 processid = wait()
 Suspend execution
 Wait for child process
 Error if no children
 Can contain status from further descendants
Termination
 exit(status)
 Terminates process
 Destroys image
 Closes open files
 “Obliterates”
 Returns to parent or higher
The Shell
 Aids in communication with UNIX
 Command line interpreter
 Command + args
 Delimit to strings, search for command
 Execute
 Return to Shell
 If command not found:
 /bin/ and execute
Standard I/O
 Open file
 Descriptor 1 – typewriter, write to 1
 Descriptor 0 – Read from 0
 Can use “>” and “<“
 “>” print/out to file after
 “<“ read/in from file after
 ls > there
 ed < script
 Interpreted directly by the Shell
Filters
 Commands can direct to each other
 Use |
 ls | pr -2| opr
 Simplifies commands to the Shell
 Makes commands look less “clumsy”
 Filter – copies standard input to standard output with processing
 Character transliteration, sorting of input, encryption/decryption
Command Separators: Multitasking
 Provided by the Shell
 Commands separated by ;
 ls ; ed
 Commands followed by & do not wait for termination
 as source > output &
 Use parenthesis for order of operations
 (date ; ls) > x &
Shell as Command: Command Files
 Shell is a command
 Can be called recursively
 File contains following lines:
as source
mv a.out testprog
testprog
 Command sh < filename
 Can also substitute parameters
Implementation of the Shell
 Shell mostly waits for user input
 New-line character causes read to return
 Read broken down into form capable of execute
 fork() called
 Child process attempts to execute with arguments
 Parent waits for child to die
Command is finished. Shell waits for next command
 Main loop of Shell never terminates
 Except at EOF
Initialization
 Instances of Shell that user sees are child processes
 UNIX creates process of init on startup
 1 for keyboard, 0 for printer file descriptions
 Creates Shell when user logs in
 When Shell executes, init waits
Other Programs as Shell
 User’s entry in password file can contain name of program
 Programs can be executed at login
 Can immediately begin working
 User’s can be prevented from invoking programs
 Limit users to ONLY single program
 Ex: Chess
Log in as player, only play game
Traps
 Programs stuck in loop can be terminated
 Interrupt signal
 “delete” character
 Cease execution, do not produce core image file
 Quit signal
 Force core image to be created
 Shell ignores quits
 Prevent user from being logged out by quit
Perspective
 UNIX was not created to meet any needs
 Creators dissatisfied
 Created more “hospitable” environment for PDP-7
 Written to easily write, test, run programs
 System was able to maintain itself
 Easily modified to fit new ideas/technology
Influences
 Success of UNIX comes from exploitation of specific ideas
 Examples include
 Fork operation from Berkely
 I/O System calls from Multics
 Shell name and function from Multics