Introduction to Unix CS 21

1
Introduction to Unix CS 21

• Lecture 4

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Lecture Overview

• cp, mv, and rm
• Looking into files
• The file command
• head and tail
• cat and more
• What weve seen so far

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Homework and Quiz

• Homework 1 now available at http//www.cs.ucr.edu
  /villarre/cs21/homework1.html
• Due next Thursday (January 20th) at the beginning
  of class
• Quiz 1 next Thursday (January 20th)
• Everything covered up through today will be fair
  game
• Homework assignments, lab assignments, reading
  assignments, and lecture material

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The First Wildcard

• (asterisk)
• Wildcards are just like wild cards in poker
• They can be anything (within reason)
• In Unix, this means that can be replaced by
  anything in the current directory
• actually gets replaced by everything in the
  current directory

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Example Of Usage
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Why Is This Useful?

• At first glance, this doesnt seem to mean much
• After all, ls without the will list all the
  files anyway
• You can specify a little more than every file
• .txt will match all files that end in .txt

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Example Of Advanced Usage
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Moving Files Around

• There are a couple of essential commands to move
  files around
• cp
• mv
• Remember, you can use touch to create an empty
  file to play around with

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cp Usage

• The cp command makes a copy of a file
• Usage cp OLDFILE NEWFILE

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Examples Of cp
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Problems With cp?
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Commonly Used Flags With cp

• -i flag
• Asks if you want to write over a file that
  already exists
• -r flag
• Recursively will copy all files and subdirectories

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What Does Recursion Mean?

• The same program gets called over and over again
• In this context, cp gets called on all files in
  all subdirectories
• Parent directories and other files above the
  current directory are not affected

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A Graphical Representation Of Recursion
cp r /usr /temp
/
/home
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Keep In Mind All Of The Permissions

• You will only be able to copy a file that you
  have read permission on
• You will only be able to create a file in a
  directory that you have write permission in

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mv Usage

• The mv command will move a file from one location
  to a new location
• Usage mv OldFile NewFile
• You can also think of this as a rename command

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Examples Of mv
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How Does This Compare To Windows?

• Windows will let you drag and drop files from one
  location to another
• Right click if you would like to copy
• A little progress bar shows up animating the file
  being moved over to the new folder
• Other than that, it is exactly the same

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rm Usage

• The rm command will remove a file
• This doesnt normally include directories
• Usage rm filename

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Examples Of rm
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Commonly Used Flags

• -i
• Verify the delete
• -f
• Force the removal of a file
• If you have permission, the file is gone,
  regardless of any warnings that might pop up
• Yeah, yeah, just do it
• Overrides the i flag
• -r
• Recursively delete files

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Dangers Of rm

• Unix Is missing something you are probably used
  to
• rm is probably one of the most dangerous commands
  in Unix

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Once Its Gone, Its Gone

• There is no way to get a file that has been
  removed back
• Only run rm if you are absolutely sure you want
  to remove a file
• The i flag provides a little protection
• Prompts the user if they are really sure they
  want to delete the file

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Extreme Dangers Of rm

• rm rf
• Say Bye, bye to your home directory
• rm rf /
• You wont have permission to delete much, but
• If you are root, say goodbye to the entire
  system!
• rm rf .
• rm rf
• Dont look as dangerous, but you have to be
  absolutely certain you know where youre at

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So Why Use rm rf Then?

• The most destructive command in Unix, why would
  you ever want to use it?
• Just so happens that you WILL want to remove
  large portions of your files at some time (most
  likely many times)
• Much easier to run rm rf than delete each
  file individually

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Avoiding The Dangers Of rm

• The best way is to make sure you are always using
  the i flag and only use f when you are certain
• Always check where your current directory is so
  you dont delete the wrong file
• Make copies of important files just in case

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Unix, The Multichoice OS

• Whats the difference?
• rm rf subdirectory/
• rmdir subdirectory/

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Multichoice Again

• Whats the difference?
• mv fileA fileB
• cp fileA fileB
• rm fileA

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Examining Files Closer

• As previously stated, everything in Unix is a
  file
• But different files have different uses
• How do you tell what type a file is?
• Example In Windows, a .doc file is a Word
  Document
• No such restriction is enforced in Unix
• A .doc file might even be an executable!

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The file Command

• A helpful command to get you started in your
  quest for knowledge file
• Checks the first few bytes of the file in
  question and takes its best guess as to what type
  of file it is
• Sometimes file gets it wrong, but most of the
  time it is pretty good

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Example Different Types Of Files
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What If I Want To Actually Look Inside The File
Myself?

• If the file is a text file, several options exist
• head
• tail
• cat
• more
• If the file is a binary (executable), you dont
  want to read it! (trust me)

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The head Command

• Print out the first few lines of a text file
• 10 by default
• Provides a quick way to see if this is the file
  youre looking for
• Doesnt bombard you with a million line file
  scrolling off the screen
• Usage head FILE

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Common Flags For The head Command

• -6
• Only print out the first 6 lines
• Actually, any number works here the same way
• What counts as a line?
• Everything up until the new line terminator

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Examples Of head Usage
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The tail Command

• Pretty much the opposite of head
• Prints out the last few lines of a file
• 10 by default
• Usage tail FILE
• Just like head, -NUM
• Prints out the last NUM lines

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Examples Of tail Usage
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The cat Command

• The cat command will print out an entire file to
  the screen
• Usage cat FILE
• Cat?
• Short for concatenate
• This command can be used to print out multiple
  files one right after the other
• cat FILE1 FILE2

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Problem With cat

• If the file is very large, it will scroll off the
  screen too fast to read
• No way to read a scrolling file without stopping
  the program
• Cntrl-C will kill a running program

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The more Command

• Works exactly like cat, but doesnt automatically
  scroll the screen
• Usage more FILE
• This is the first truly interactive program weve
  seen
• You can control how the program runs while it is
  running

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More Example

• How do you scroll to the next screen?
• Hit the space bar

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Advanced more usage

• Return will move you one line
• NUM followed by return will move you NUM lines
• Example 5 Return displays the next 5 lines
• q will quit more without finishing

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less Is more

• A better version of more exists less
• Allows all of the same options as more
• Allows easier moving through the file
• Arrow keys and page up, page down will move you
  both forward and backward
• Which you choose to use is, of course, up to you

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What About Binary Files?
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You Now Should Have Enough Info To Cause Some
Damage

• You now have the ability to
• Wander about the system
• Create simple files and change all the properties
  of these files
• Copy and move files around
• Check out what type of files you are looking at
  and read the interesting ones
• Delete files you no longer want

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The Class So Far

• History of Unix and the hacker connection
• Logging on and getting help
• man
• Environment variables
• The Unix Directory Structure
• ls
• cd
• pushd and popd
• Relative and absolute pathnames
• . And ..

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Class Summary Continued

• Disk usage
• du
• Compressing files
• Symbolic Links
• Ownership and permissions
• chmod
• umask

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The Class So Far Continued

• Moving files
• cp
• mv
• Checking the contents and type of files
• file
• head and tail
• cat and more

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In Lab Today

• You will practice setting permissions and the
  effect they have
• chmod and umask
• Start creating simple files and moving them
  around the system
• Read files using head, tail, cat, and more

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Next Week

• Things really start to get interesting as we
  start getting programs to work together and tie
  everything together
• Well look at more uses of wildcards and start
  making Unix sentences with pipes and filters
• Well look at the oddly named but strangely
  powerful command grep