What is an Operating System?
- define OS:
"control program for computer"
- OS functions:
- allocate resources to users of system:
-
storage: memory, disk, tape
file system
printers
terminals
- schedule process tasks
- control access to CPU (another resource)
- allow timesharing: single CPU handles multiple task
apparently concurrently
- CPU only does one thing at a time
Evolution of Operating Systems
- load and run:
- batch:
if multiple users, each waits for turn
multiple jobs wait in queue for execution
- single user interactive, single tasking
- single user interactive, multitasking:
- multi-user interactive:
Why Unix?
- portable:
written in C, non-proprietary
- scalable:
runs on PCs through supercomputers
- powerful and flexible user interface: shell, GUI, "turnkey"
- simple interface for multiple processes from user commands
- flexible heirarchichal file system:
- strong support for networking and communications
- simplicity maintained through I/O redirection and pipes:
- chain simple, single-purpose tools to perform complex tasks
- large library of user supported and contributed software, all
in source form
History of Unix:
- developed at Bell Labs in 1969 by Ken Thompson
- rewritten in C for port to PDP-11 (with help from Dennis Ritchie)
- lineage:
Version 7 --> System V and BSD
Unix in the real world
- runs on most computer systems
Unix at UNF:
System V:
1
4
1
1
12
AT&T 3B15
(unf5)
AT&T 3B2/310 (unf1 - unf4)
AT&T 3B2/500 (unf7)
Sequent Balance (unf6)
Sun 386i
BSD:
12 Sun 386i
1 Sequent Balance (unf6)
Networking Under Unix
- TCP/IP:
the "standard" Unix network
- developed by DARPA
- runs on 10-megabit/sec Ethernet
- allows:
- transparent access to remote files (files on other computers
on the network appear "local")
- file transfer
- electronic mail
- remote login
- remote command execution
- allows network of different machines from different vendors to
share resources in a transparent and consistent manner - even IBM
The Unix environment:
- telling Unix your terminal type
- the shell: Bourne shell, Korn shell, C Shell, bash, tcsh
- Unix is case sensitive
Unix files:
- file names - max 14 characters (SysV), 256 characters (BSD)
- any characters (even non-printing ones) are legal
- all files are just a stream of ASCII characters (bytes)
- if text file, lines terminated by NL (ascii 10)
- no end-of-file character stored in file
Commands to manipulate files:
- cat, pg (SysV), more (BSD, less (both), cp, mv, ln, rm, vi
The Unix File System
- Unix has a heirarchical file system (tree-structured)
- subdirectories are just files containing information about other
files
- current working directory
- home directory
- specifying files (relative, absolute paths)
- . and ..
- each file has a unique inode (index node) number
- inode contains pointers to data on disk
- since inodes are unique, file can have multiple name (names
are just mapped to inode
- directory entries just contain names and inode numbers
- special files: devices (in /dev directory)
- character, block devices, fifos
Commands to manipulate directories:
- ls, cd, pwd, mkdir, rmdir, mv
File permissions:
- 3 levels:
read, write, execute
user, group, other
- chmod
Electronic mail: mail
The Shell - the Unix command interpreter:
- Command-line syntax: $ command [-options] [arg1 [arg2 [...]]]
<return>
- command:
name of an executable file
- each command executing is a process:
"Standard" I/O:
output
I/O Redirection:
standard input, standard output, standard error
you can "redirect" standard output:
$ ls -l >dirfile
puts the output of the 'ls' command into the file "dirfile"
- if dirfile did not exist, it is created
- if dirfile did exist, its previous contents are replaced
by the output of the 'ls' command
$ ls -l >>dirfile
puts the output of the 'ls' command into the file "dirfile"
- if dirfile did not exist, it is created
- if dirfile did exist, the output of the 'ls' command is
appended to it
- you can redirect standard input: < > >>
Pipes - you can tell one command to use as its input the output from
another command: |
Filename generation: "wild cards": * ? [...]
Unix commands:
- where are files: /bin /usr/bin /usr/lbin /usr/local/bin
- shell PATH variable:
- others: touch, tail, file, chown, chgrp, who, w,
lpr, lpq, lprm, date, echo, diff, du, ps,
kill, nice, nohup, grep, telnet, rlogin, rsh, rcp, man
Background Processing:
&
- stdout and stderr unchanged, process is "unlinked" from stdin
- jobs:
list background jobs for this terminal session with
brief ID #
Miscellaneous shell stuff
- Redirecting stderr:
2>, 2>&1
- command separation: ';' separates commands on a line
- command grouping - (command-list):
runs command-list in subshell
- line continuation - '\' at end of line continues the current line
- command history and editing
Shell Scripts - file containing a list of shell commands
- have the same effect as typing the commands in to the shell
- sh scriptfile, make executable,
. scriptfile
- Shell Variables:
- all are string variables
- unless exported, not passed to subprocesses
- all are created when first assigned a value
- all are referenced by preceding name with '$'
- command substitution:
- exporting variables:
- environment:
$ DATEVAR=`date`
the environment
A string storage space, a copy of which is
given
to every process started by the shell
- to place a variable in the environment, export it
- programs (shell scripts and otherwise) can look at the env.
and modify their behavior based on it's contents
- many programs use environment variable to "learn" about what
the user expects them to do
Readonly shell variables:
$0, $1 - $9, $*, $#, $$, $?
- shift, set
Flow control in shell scripts:
-test or [ ... ]:
More shell stuff:
if, while, for, case
-f,-d,-r,-w,-x,-n,-z,=,!=,-eq,-ne,etc.
echo, read, exit
-"here" documents, <<
- Shell Functions
Trapping signals - prevent user from breaking out of shells: trap
Word processing under Unix:
WYSIWYG vs. embedded command:
- runoff, roff, nroff/troff, TeX, LaTeX
- Documenter's Workbench - nroff/troff macros: mm, ms, man
- preprocessors: tbl, grap, eqn, pic
- Writer's Workbench:
spelling, style, grammar, readability
Writer's Workbench: diction, gram, punct, spellwwb, proofr, wwb
Introduction to C programming:
First systems-programming language that wasn't machine specific
(assemb)
A First C program:
#include <stdio.h>
- include or header files contain info for
- compiler about functions used in this
- program
/* this is a comment */
- standard form for comments, may cross
lines
main()
function
{
- every program has exactly one main
- and this is its definition -- it's the
- entry point for the program
- body of function enclosed in {}
printf("Hello world.\n");
- call to puts function with string as
arg
}
- end of function, end of program
- Compiling:
$ cc -o exec-name source-name
Data in C:
char, short, int, long, float, double, void
- scalar variables
- modifiers: signed, unsigned
- sizeof() operator:
returns size of object in bytes
- statements, expressions, operators
- expressions:
constants, strings, function calls
- expressions have a single value of the appropriate type
- lvalue is a special type of expression which can be assigned a
value
- operators act on expressions
- compound statements are enclosed in {}
- the null statement is ";"
- Constants: constants are expressions, have a value, can be
operated
on by operators
- constants have a type
- string constants:
any characters in double quotes
- "special" characters: \n, \t, \b, \r, \f, \\, \?, \', \",
\0
\ooo, \xhh
Functions in C:
take arguments and return values:
- functions have type:
default is int
- type is the type of the value returned by function
- functions must be declared like variables
- system and library functions are declared in appropriate
header (include) files
- function arguments must be declared in "formal argument list"
- arguments passed are assumed to match declared type - not checked
The printf() function - produces formatted output on stdout
- also scanf(), putchar(), getchar()
Operators:
- arithmetic: + - * / %
- assignment: = ++ -- += -= *= /= %=
- comparison: == != < > <= >=
- boolean:
! || &&
- bitwise:
~ | & ^ >> <<
if and while
for
cast
Pointers:
& *
- NULL
- pointer declarations
Parameter passing:
call by value