Unix by Nick Gomez, Chris Rees, Brian Hernandez, Hector Rodriguez

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