The UNIX Operating System

kernel + shell + system calls
provides access to H/W resources of computer (kernel)
manages memory and CPU (kernel)
provides command-line interface to services (the shell)
provides method-call interface to services (system calls)

Also: libraries which provide a higher-level view/API than do the system calls. (API = application programming interface)

Overview of UNIX portion of course

We'll (try to) cover:

basic concepts and terminology
files and directories
processes
interprocess communication
- - signals and signal handling
- - pipes
- - shared memory, messages, semaphores
- X terminal i/o
- - sockets
standard i/o library

Basics: Files and Processes

info stored in files
while files have logical structure to programs that create, use them, at UNIX level, they are just a sequence of bytes (no record terminators, no file terminators, no special file types)
names < 256 characters ( < 14 to be backward compatible)

Directories, pathnames

directories have hierarchical, tree-like structure
each directory can contain files and subdirectories
full names of UNIX files are pathnames -- include directories


Example:

	/users/faculty/eileen/intro.notes  (absolute pathname)

	if _current working directory_ is /users/faculty/eileen,

	then can use 

		intro.notes	(relative pathname)

	if _current working directory_ is /users/faculty

	then can use

		eileen/intro.notes (also a relative pathname)

Ownership, permissions

each file has an owner, that owner is a member of a _group_
each file has 3 sets of permissions: read/write/execute
one set for owner, one set for group, one set for public

Devices

UNIX treats devices like files. That is, a filname exists that represents a device like a keyboard or printer. To write to the printer, you can just write to the file that represents it...

	for example:

		cat fileX > /dev/rmt0

causes the conents of fileX to be written to the tape drive associated with the rmt0 file in the /dev directory

Processes

process = instance of an executing program

When you type:


> ls

at the command line, the shell process creates a process to run ls. UNIX is multitasking -- more than one process can run at the same time. (multiple processes share the CPU)

IPC - Interprocess communication

mechanisms that allow processes to send each other info. Methods differ in type/amount of info, number of processes, whether processes need to exist at the same time, be on same machine, etc.

pipes - output of one program is input of another

shell supports this:


	> ls | more
	
	output of the ls program is input to the more program


	> ls | grep notes | more

	output of the ls program is input to the grep program
	notes is a command line parameter to grep
	output of grep is input to more

signals

processes that exist at same time on same machine can send integer signals

shared memory

processes that exist at same time can share variables

semaphores

to control access to shared memory

sockets

processes that exist at same time on same or different machines can communicate arbitrary info

System calls and library functions

In this course, you'll learn about the "system call interface"

kernel

memory resident program, deals with process scheduling and i/o control

system calls

the interface to the kernel and the resources it controls. Are invoked like library subroutines (but typically more efficient, lower level, run in 'kernel mode' rather than 'user mode'). Library routines are a layer between user code and system calls. Example:


library call:

	nread = fread(inputbuf, OBJSIZE, numerobjs, fileptr);

system call:

	nread = read(filedes, inputbuf, BUFSIZE)