Why UNIX?• Superior Initial Design

– Multi-user, Multiprogramming, Protected mode, Many Support many Devices

– Written to optimize the use of memory and maximize efficiency– Remained consistent to its original design– Not driven by profit motives– Source code compatible across platforms

• Other Operating Systems– IBM MVS: initially designed for a batch punch card environment

with proprietary monitors– DOS: initially ran on very single user, limited memory computers.

Multiprogramming was added on as an afterthought.

UNIX downside: Geared for power users using a command line interface

OS Benchmarks (A bit dated)

OS Loop Write Seek Read

FreeBSD 3.0 21 7.3 17.8

Linux 3.0 14 16.3 18.2

SCO (Unix) 2.4 34 16.6 24.5

Window 3.1 6.2 190.4 26.6 56.8

Linux 3.0 15.97 11.98 13.95

Window 95 6.65 125.94 11.79 47.45

Why C• Fast

– Designed to produce code nearly as fast as assembly code.

– Designed to be able to support Operating System development.

• Portable– Recompiling will cause code to work across platforms– POSIX provides a standard OS interface

• Powerful: Able to perform low level functions• Small: C code is smaller than other high level languages• Popular: C is a very popular programming language

Downside: With power comes danger. You have to know what you are doingDownside: Not object oriented. You have to be disciplined

Language Comparisons

• Assembly – small, fast, not portable• C – almost as small and fast, portable• Fortran, Pascal – Slower, specific purpose• C++, PL1 – Complex, larger and slower• Java, Basic – Byte oriented, even slower• Scripting languages – interpreted, slow

Note: JIT technology makes byte code languages more speed competitive

Executable Memory

C 3811 820kb

C++ 18033 1052 kb

Memory Use Example

History1. 1960s was the era of batch processing

• Punch cards, paper tape, Magnetic tape• System operators

2. Multics attempt at an OS• general purpose, multi user, time sharing• project died: technology of the time could not support the effort

3. 1969 – AT&T Bell Labs (Ken Thompson) writes first version of Unix in BCPL

4. 1972 – Dennis Ritchie creates "C", UNIX written in C5. 1984 – Richard Stallman starts the free Software

Foundation (FSF) with goal to develop a free version of UNIX (GNU – Not Unix)

6. 1991 – Linus Torvalds writes Linux Kernel (GNU+ Kernel)7. 2000 – User Friendly UNIX (Mac OS X) interface8. Academia likes UNIX based systems; they are free

"Pigs get fat, Hogs get slaughtered"• AT&T develops UNIX at Bell Labs• Berkeley gets a copy and makes enhancements• AT&T commercializes UNIX and makes it proprietary• Richard Stallman was an AI researcher at MIT

– He objected to the trend towards proprietary software. – He formed GNU (GNU's not UNIX) – GNU to replace the UNIX shells and utilities

• Berkeley objected to AT&T licenses, and developed BSD• Andrew Tananbaum created Minimal Unix (MINIX)• Linus Torvalds (CS student) coded the Linux kernel• Open source produces lots of high quality software• No open source movement implies no Internet

UNIX Structure• The Kernel is the part of

the OS always in memory• Includes

– File system– Device manager– memory management– process scheduler– well defined interface

• GUI: Point and click (many wrappers to the X-Window interface)

• Shells: command line

Users

GUI's Shells

Hardware

Kernel

Shells• A command-line interface that allows users to directly

access operating system capabilities

• There are more than one shells possible in UNIX: csh, bash, tsch, ksh, etc

• Text only commands, without using the mouse. “Finger never leaves keyboard.”

• Groups of commands can be strung together in unique ways to make difficult tasks possible, without creating specialized programs

Comparison between shells

• sh (bourne shell)– small– good scripting capability– popular with system

administrators

• csh (c-shell)– extends sh– uses a c-like syntax– created by Bill Joy– popular with UNIX

users

• bash – "bourne-again" shell– extends sh with some

features form csh– The linux default– We will use bash

• tcsh– extension of csh

Where do we find a UNIX

• Virtual “Linux” servers provided by IT– Log In (Putty program) creating a secure shell (SSH)

with your standard username/password– See a shell prompt (>, &, $, etc.)– Type your command– The shell will execute the command and return with

another prompt• Mac/OS provides a UNIX-like system• You can install LINUX on windows systems

using a dual boot• You can install CYGWIN on a windows system

The Shell language• Remember

– Shell commands are case sensitive– The general form of commands are

• Syntax: <cmdName> [-flags] [args]

Note: <> indicates something that is required, [] indicates something that is optional

Example: >echo –n I love UNIX1. > is the shell prompt2. echo is a UNIX command 3. -n is a flag4. I love UNIX are three arguments (space separators). 5. This command outputs the arguments: I love UNIX6. The –n inhibits a new line after the output

Display prompt Enter command from command line execute/process command

Some basic commands>clearclears the screen>wholists the users currently logged in>logout or >exit (Exit is recursive)exit the shell, and wait for another log in>echo whatever I wantoutputs the data to the right>man <manName>display UNIX manual for the command, manName

Note: Use ctrl c to abort commands (abbreviation ^c)

Shell Variables• Example: $<varName>

• The shell has some built in variables$TERM represents the terminal type

$SHELL represents which shell is active

$USER user name

$PATH places to look for executables

• What do you suppose the following does?>echo I have a $TERM with $SHELL shell

• Printenv outputs environment variables, set outputs environment and shell variables

Environment variables: those common to all shells

The File System• The file system is a hierarchical tree structure of

regular files and directories (folders)

• Within the file system, there are individual files for programs and data, and directories that contain other files

• Directory Examples– The root directory is / (analogous to c: on Windows)

– A users home directory is ~– /faculty/harveyd/public_html/classes/cs367/ppt

Important: File names are case sensitive. Name only with numbers, letters, and underscores. Punctuation characters can conflict with shell commands

Explore Linux (partial directory list)

• /root : super user’s home directory

• / : system root directory (like Window’s c: )

• /tmp : temporary sharable files available to all users

• /bin : executables required for kernel

• /sbin : executables requiring super user access

• /etc : system configuration files

• /var : system changeable files

• /opt : optional installs

• /home : contains all user’s home directories

• /mnt or /media : mount points

• /dev : system devices

• /boot : contains linux kernel

• /usr/bin : applications coming with distribution

• /usr/sbin : applications installed later

File Commands>cdChange directory

Example using 'absolute' pathnames (cumbersome)/faculty/harveyd/public_html or ~/dir1/dir2/dir3

Example using relative pathnames>cd ~ (go to the users home directory)>cd public_html (public_html directory relative to working directory)>cd .. (change to parent directory above the working directory)

>pwdDisplays the 'absolute' pathname from the root with '/' separators

Note: >cd . refers to the current yNote: The shell variable $PWD contains what pwd displays

more/ less and cat commands>cat <filename>Display the contents of the file

Problem: too much to displaySolution: >more fileName

Explanation: more is a program that displays one page at a time and then waits for you to enter a commandExplanation: less is an enhancement to more (“less is more”)

Note: The man command uses more commands, and displays a page at a time

Command Action

Space or f or page down

Go forward one page

b or page up Go back one page

/keyword Find first page with keyword

/ or n Find next occurrence of keyword

Up or down arrows

Move up or down one line

return Go forward one line

q Quit

List contents of a directory• List all files that match the pattern

>ls <pattern> … <pattern>• A pattern can be: a combination of letters, asterisks, and

question marks. – Asterisks mean any sequence of characters– ? is any single character– Example: *cat?dog* refers to any file name having the word cat

separated by one character frm the word dog anywhere in the file name

– [abc] means any single character between the brackets (but cannot use / or . characters in the list.

• Flags (refer to the man command for a complete list>ls –l lists files in a long format>ls –a lists all files including hidden ones (starting with a period)>ls –al or ls –a –l lists files combining the flags

man <anycommand> gives information how to use it

File links• Hard link (ln myFile myLink)– Points directory to the file’s data– We can delete either the file or the link– The actual data is removed when the last link is removed– It is illegal to link to directories

• Symbolic links (ln –s myfile mySymbolicLink)

– References file’s directory entry by name– myfile does not even need to exist (link broken)– Deleting a link, never deletes the file– Equivalent to Windows shortcuts– Circular links cause errors when access is attempted

Additional Commands• Rename file: >mv <oldName> <newName>• Copy file: >cp <file> .. <file> <destination>• Secure copy to remote computer:

– scp <file>...<file><destination– Example: scp cat.c harveyd@sou.edu:drop

• Delete file: >rm <file> .. <file>• Make a directory: >mkdir <directory>• Execute allowed as superuser: >sudo command• Which version of a command: >which <command>• Type of command : >type <command>• Who’s logged in: >whoami• Detailed documentation: >man <command>• Brief description: >whatis <command>• Description of shell built-in commands: info <command>

Note: Most commands have many flags. For example –r is often used to access all subdirectories (ex: rm –r <dir>

More on flags• cp –vfirl or rm –rifv --help

– Note that we can string multiple flags together– v = verbose, f = force, i = interactive, r = recursive, l =

link instead of copy– Most also have long english type versions preceded

by double dashes. For example: cp –recursive

• ls –dlaRr– d = directory (not contents), l = long form, a = all files,

R = recursive, r = reverse order

• mkdir –p (create parent directories if not found)• man –k term (same as apropos) to find all

entries related to term