Introduction to Unix CS 21

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Introduction to Unix CS 21

• Lecture 2

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Lecture Overview

• Starting out in Unix
• Getting help when needed
• Looking around a little
• Environment and Environment Variables
• The Unix file structure
• Organization
• Navigation

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Logging In (And Logging Out)

• Your Unix experience begins with a login prompt
• Login name and password
• Always be sure to log out
• Why?
• Malicious people shouldnt have access to your
  account
• Locking the screenis it safe?
• Well, maybe, maybe not

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Good Password Versus Bad Password

• What makes a bad password?
• Any word in a dictionary
• Any word from a sci-fi movie/show
• 30 sec 8 minutes to break
• What makes a good password?
• Easily interpreted and remembered by user
• Looks like random gibberish to others
• passwd normally changes your password

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Im In, Now What?

• No tour of Unix like Windows has
• Youre pretty much on your own
• Unix assumes you know what you are doing
• You are given just enough leverage to either do
  everything well or really mess everything up

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Where To Get Help

• man
• The most important command in all of Unix
• RTM
• info
• More complete descriptions of certain packages
• help
• Shell specific commands
• apropos
• whatis

(RTFM)
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Man Page Sections And What They Mean

• 1 User commands
• 2 System Calls
• 3 C library routines
• 4 Administrative files
• 5 Miscellaneous
• 6 Games
• 7 I/O and special files
• 8 System administration commands

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More on man

• Other sections exist (such as the perl reference
  manual and the tcl reference manual)
• -a will show all the man pages that match
• First is shown by default
• How would you find out more about man?
• man man

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Example
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How Am I Supposed To Read This?

• Name
• The name of the program and a brief description
• Synopsis
• How to run this command
• Description
• Listing of all flags and what they do
• See also
• Relevant related commands

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What Do You Do If There Isnt A Man Page?

• Option 1 run the program and hope it doesnt
  destroy anything
• Option 1b run the program with random flags and
  see what exactly it destroys
• Option 2 do a little snooping
• Check for man pages or documentation on the web
• See if there are any environment variables that
  it might use

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Who is logged on and what are they doing?

• users
• Shows a list of all currently logged on users
• who (and whoami)
• Shows a list of all currently logged on users and
  where they are logged on from
• w
• Shows a list of all currently logged on users and
  what they are running
• finger
• Tells a little more information about users

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Privacy issue?

• You can tell when someone else is logged on and
  exactly what they are doing
• Everyone else can see what you are doing
• Is this acceptable?

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One User To Rule Them All

• The superuser root
• Root has the permission to do anything
• Including wiping the entire system clean
• The system will allow root to do just about
  anything
• Comparable to Administrator in Windows
• Of course, any user in Windows XP can be
  designated an Administrator and install software

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No, You Cant Get Root

• For obvious reasons, you will not be given root
  access
• Dont feel bad though, very few people at school
  have root
• You can always install your own machine at home
  to play around with
• Knoppix

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Environment And Environment Variables

• A set of preset variables that all programs run
  from that shell can see
• In order to see them, run the command printenv
• Set environment variables with the command
  export VARVALUE
• Clear environment variables with unset VAR

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Example
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Stupid Analogy Of The Day
Environment Variables

• Each contain useful information
• Every program can check them out and read them
• If someone changes them, the next person to check
  them out will see those changes

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Exporting Or Not

• If you dont export the variable, it is a local
  variable and not accessible to everyone
• Get access to a local variable by using the
  dollar sign (VAR)

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A Very Important Environment Variable

• PATH
• Tells Unix where to look in order to find
  programs to run
• Very important hint
• When typing a command, you should be able to hit
  the tab key in order to complete it
• Called tab completion, this is a most invaluable
  tool
• The system searches your PATH for any program it
  can run that matches what you have typed so far

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Files, Files Everywhere

• Unix treats almost everything as a file
• Standard text files and binaries
• Directories
• Links
• Even devices!
• Your terminal is a file, your keyboard is a file,
  your hard drive is a file, everything is a file!

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What Was That?

• To repeat, Everything in Unix is a file!
• Why?
• This goes back to the design of Unix trying to
  keep everything simple
• Treating everything the same allows for a simple
  interface to interact with everything in the same
  way

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Unix Directory Structure

• Everything is a file, and the directory structure
  is like a filing cabinet

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Inverted Tree Structure
/
/usr
/bin
/home
/var
/lib
/home/csmajs
/home/csgrads
/home/csmajs/undergrads
/home/csgrads/graduates
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What Goes Where?

• /
• Root of the entire system
• Comparable in Windows to C\
• Windows, however, has several roots depending on
  what partition and device you are looking at
• /bin
• Commonly used binaries (programs)

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And

• /sbin
• More programs to run
• Statically linked
• Still should run if you tinker around and mess
  things up
• /usr
• User related commands as well as a whole bunch of
  random stuff

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And

• /lib
• Libraries go in here
• /dev
• All devices are located in here
• /home
• Traditionally, this is where user accounts are
  stored
• /etc
• Startup files and configuration files for daemons
  and other programs

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And

• /var
• Various files go in here
• Traditional location of mailboxes
• /var/spool/mail
• /proc
• Special files that contain information about the
  system or info from running programs

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Navigating The Directory Structure

• You are always located at one location in the
  tree
• ls
• Provides a listing of the current directory
• All files and directories are shown
• cd
• Change directory
• cd / Moves your current directory to /
• cd without any arguments moves you to your home
  directory

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Creating And Removing Directories

• mkdir
• Makes a directory
• mkdir myDir
• rmdir
• Removes an empty directory
• rmdir myDir

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A Couple Of Notes

• Unix is case-sensitive
• myDir is different from MyDir and mydir
• Not all files are shown with ls
• ls a will show all files
• Hidden files in Unix always start with a .
• Example .hidden
• .plan

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Command Line Options

• Almost all Unix commands have options that change
  what they do
• Read the man pages for a listing
• They have the form -a
• Depending on the command, they may be lumped
  together
• -a l is the same as -al

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How Different?
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Special Directories

• .
• This stands for the current directory
• ..
• This stands for the directory directly above the
  current directory
• Your home directory
• These directories can be stacked
• ../.. stands for two directories directly above
  the current directory

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Relative Versus Absolute Pathnames

• Unix supports two different styles for
  directories
• Absolute pathnames
• Always start from / and contain the entire route
• /home/csgrads/villarre
• Relative pathnames
• Assumed to be relative to current directory
• Doesnt start with a /
• classes/cs21

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Advanced Navigation Through The Directory
Structure

• pushd
• Push a directory onto a stack
• popd
• Pop a directory from a stack
• A Stack is whats known as a LIFO structure
• Last in, first out

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How Does A Stack Work?
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How Does A Stack Work?
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How Does pushd and popd Work?
Current Directory
pushd /var/spool
/home/csmajs
/var/spool
/home
/var/spool
/home/csmajs
pushd /home
popd
popd
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Today In Lab

• Put all of the material from today into practice
• Lab will be posted later today at
  www.cs.ucr.edu/villarre/cs21/

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Reading Assignment

• Read chapters 2-4
• I will assign a homework beginning of next week
• First quiz will be a week from next Tuesday