Tips and Examples

• Be Sure You Know Where You Are
• Finding Errors in Homework Scripts

Review

• The Hierarchical Filesystem
• Unix Files and Directories
• Filenames
• Case Sensitivity
• Filename Extensions
• Current Directory
• Your Home Directory
• Navigating the Hierarchical File Systems

New Material

• Hidden Filenames
• Startup Files
• The . and . . Directory Entries
• Pathnames
• Absolute Pathnames
• Tilde, ~, in Pathnames
• Relative Pathnames
• Relative Pathnames in Your Current Directory
• Relative Pathnames in a Subdirectory
• Relative Pathnames above the Current Directory
• Relative Pathnames Neither above Nor below the Current Directory
• Standard Directories
• Your Home Directory versus /home
• The Two Uses of /
• The Two Special Uses of .

Studying

• Keep Each Study Period Short

Homework 4

I have posted Homework 4 here.

As usual, it will be due next Sunday at 11:59 PM.

Quiz 2

I have posted the answers to Quiz 2 here.

Let's take a look.

Questions

Are there any questions before I begin?

Tips and Examples

Be Sure You Know Where You Are

• Whenever you are working at the command line ...
• you always have a current directory
• It is important that you always know your current directory
• If you think you are in one directory ...
• but you really are in another ...
• your command may not work
• Every semester I get at least one email from a student ...
• saying some file has disappeared
• Linux will never delete a file unless you tell it to
• The file is always somewhere
• But it may not be where you think it is
• Often the file is in your home directory
• When you think a file has disappeared look for it in other directories
• Another problem can arise if you are in the wrong directory
• If you try to save a file and get error message about permission ...
• it means you are in the wrong directory
• The owner of a directory decides who can do what in that directory
• If you are in a directory where you do not have write permission ...
• you won't be able to create anything
• A text editor like nano will usually open a file whatever directory you are in
• But you won't be able to save it ...
• unless you have write permission in that directory
• We'll talk about permission in a future class

Finding Errors in Homework Scripts

• There are three kinds of errors you can have in a homework assignment
  • Unix errors
  • Step Comment errors
  • Using the wrong commands in a step
• Unix errors occur when you make a command line mistake
• For example, cp take two arguments
• If you only give it one argument you will get a Unix error

  $ cp ../foo.txt 
  

• You can find these errors in your script by running bash in a special way
• If you want to look for any Unix errors in the script hw4.sh you should run

  bash hw4.sh > /dev/null

• If you do you will not see the normal output of the script
• You will only see the Unix error messages your scripts creates
• Step errors occur when you do not have proper Step comments in your script
• The script step_comment_check.sh will show you any Step comment errors in your script
• You can run this command from any directory like this

  ~ghoffman/it244_test/step_comment_check.sh

• But you need to supply two arguments
  • Your Unix username
  • The number of the homework assignment
• To check my my hw3.sh for Step comment errors I would write

  $ ~ghoffman/it244_test/step_comment_check.sh ghoffman 3
  # Step 1
  Could not find # Step 2
  # Step 3
  # Step 4
  # Step 5
  # Step 6
  # Step 7
  # Step 8

• In this script I have proper Step comments for 7 of 8 Steps
• But not for Step 2
• The last kind of error is when the Unix commands in your script ...
• do not do what the homework Step requires
• For example if the Step told you to go to my home directory ...
• and your command was

  cd

• This would be an error
• I have scripts to check for this, but they are not available to you
• Why?
• Homework assignments are meant to be like what you might do in industry
• There you would not have script that can check each of your commands
• You would have to rely on your own understanding of Unix commands
• The homework assignment are meant to give you a taste of this real world experience

Review

The Hierarchical Filesystem

• Unix uses a hierarchical filesystem
• This means there is one directory at the top called the root directory
• The root directory is indicated by a simple slash character, /
• All other directories are contained within the root directory ...
• or one of its many subdirectories
• So the shape of the hierarchical filesystem is like a pyramid

• This structure is called a tree because it looks like a tree turned upside down
• A hierarchical filesystem also resembles a family tree
• So we often use terms that describe family members when talking about directories
• The directory up one level from your current directory is called the parent directory
• All directories above the current directory are called ancestors
• All directories inside the current directories are called child directories
• All directories inside or below the current directory can be called descendants
• All directories and files within the same parent directory are called siblings

Unix Files and Directories

• Files are linear arrangements of data on a storage device
• There are several types of files
  • Program files
  • Text files
  • Data files
  • Configuration files
• Directories are simply containers that hold files and other directories
• On Windows and the Mac they are called folders

Filenames

• When you ask Unix for a file you must give it two pieces of information
  • The name of the file
  • The location of the file in the hierarchical file system
• Every file has a filename
• The maximum number of characters in a filename varies from one Unix to another
• Most Unix flavors allow file names of up to 255 characters
• It is best to keep filenames short
• This makes typing and remembering them easier
• Never use a space in a file or directory name
• This is a bad idea
• Use an underscore, _ , instead of a space in file names
• To avoid problems, only use the following characters in file names
  • Lowercase letters (a-z)
  • Digits (0-9)
  • Underscore _
  • Dash -
  • Period .
• You cannot have two files with the same name in the same directory

Case Sensitivity

• Unix is case sensitive
• This means that "Foo", "foo" and "FOO" are three different things
• At least as far as Unix is concerned
• Unix utility and program names are always lowercase
• Some operating systems do not distinguish between UPPERCASE and lowercase characters
• Windows is such a system
• Make life easy for yourself
• Use only lowercase characters in Unix filenames

Filename Extensions

• Extensions are strings of characters that appear at the end of the filename after a period, .
• Extensions are not recognized by the Unix filesystem
• As far as Unix is concerned they are just legal characters that are part of the filename
• Some Unix programs expect their files to have certain extensions
• For example, the C compiler, gcc expects the filenames of source code files to end in .c
• The Java compiler, javac expects Java source files to have .java at the end of the filename
• The extensions are required by these programs
• But not by Unix

Current Directory

• The way a Unix command works depends on your environment
• One of the most important parts of your environment is your current directory
• The pwd (print working directory) command will always tell you your current directory

Your Home Directory

• Whenever you log in to a Unix host ...
• you will always find yourself in your home directory
• This a directory that belongs to your Unix account only
• You have full control of everything inside this directory
• If you use cd with no arguments it will take you to your home directory

  $ cd
  
  $ pwd
  /home/ghoffman

• Your home directory contains a number of hidden files ...
• which customize your environment
• Like .plan
• On most Unix systems, home directories are found inside the /home directory
• On a Mac, home directories appear inside /Users
• The name of your home directory is the same as your Unix username

Navigating the Hierarchical File Systems

• Any file or directory in the filesystem will be one of four positions ...
• relative to your current directory
• It can be inside your current directory
• It can be below your current directory
• It can be above your current directory
• It can be off to the side of your current directory
• In this last case, you must go up before you can go down to reach this file

Attendance

New Material

Hidden Filenames

• A file whose filename begins with a period, .   is a "hidden" or "invisible" file
• ls does not display these files unless you use the -a option
• These files are used to configure your Unix environment
• Two special hidden files . and .. appear in every directory

  $ ls  -a
  .  ..  hw3  work

• We'll discuss them shortly

Startup Files

• The files .login and .bash_profile are startup files
• Startup files contain Unix commands ...
• that are run just before you get the first shell prompt
• They customize your Unix environment
• They only work when placed inside your home directory
• They can set variables which can help you with your work
• We'll talk more about this in a future class

The . and . . Directory Entries

• Every directory has at least two entries: . and . .
• When a new directory is created, these entries are always there
• . stands for the current directory
• . . stands for the parent directory
• The parent directory is immediately above your current directory
• It is the directory that contains your current directory

  $ pwd
  /home/ghoffman
  
  $ cd ..
  
  $ pwd
  /home

• . is most often used in two circumstances
  • To run a program in your current directory
  • To move or copy a file to your current directory

  $ ls
  it244  notes.txt  work  work2
  
  $ cd  work
  
  $ ls
  bletch.txt  foo.txt
  
  $ cp  ../notes.txt  .
  
  $ ls
  bletch.txt  foo.txt  notes.txt

Pathnames

• Every file has a pathname which is used to access the file
• A pathname has two components
  • A path to reach the file
  • The name of the file
• A pathname is like an address on a letter
• A name and directions to get there
• The name of the file is always at the end of a pathname
• What comes before the filename is the path to the directory that holds the file
• A path is the list of directories you must go through to get to the file
• A slash, / , at the beginning of a pathname ...
• refers to the root directory
• Other slashes separate the directory name ...
• from whatever follows it
• There are two types of pathnames
  • Absolute
  • Relative

Absolute Pathnames

• The directory at the top of the filesystem is called the root directory
• The root directory is represented by a single slash, /
• It can stand alone ...
• or appear as the first character before a directory name
• An absolute path is a list of directories starting with the root directory ...
• and ending with the directory that holds the file
• When you add the filename to the end of an absolute path ...
• you have an absolute pathname
• The absolute pathname of my startup file, .bash_profile ...
• in my home directory ...
• is /home/ghoffman/.bash_profile
• This means that my home directory, ghoffman ...
• is under the directory named home ...
• and this directory is under the root directory, /
• The advantage of an absolute pathname ...
• is that it can be used from any part of the filesystem
• It does not depend on where you are at a given moment
• The disadvantage is that absolute pathnames tend to be long
• It is easy to make a mistake when typing an absolute pathname
• An absolute pathname always begins with either a slash, / or a tilde, ~

Tilde, ~, in Pathnames

• Tilde, ~, is one of the characters that have special meaning to Unix
• Whenever you type ~ on the command line ...
• Bash replaces ~ with the absolute pathname of a home directory
• ~ by itself is replaced by the absolute pathname of your home directory
• You can see this most clearly when you run echo on ~

  $ echo ~
  /home/ghoffman

• If you follow ~ with a Unix username ...
• it is replaced with the pathname of that user's home directory

  $ echo ~it244gh
  /home/it244gh

• If what follows the ~ is not a valid Unix username no substitution will happen

  $ echo ~xxxxx
  ~xxxxx

Relative Pathnames

• Absolute pathnames are useful because you can use them anywhere
• But they are long and easy to mistype
• It is easier to use relative pathnames
• In a relative pathname the path starts from your current directory
• In an absolute pathname the path starts from the root directory, /
• The only difference between an absolute and a relative path ...
• is where they start
• An address on a letter is like an absolute pathname
• If you address a letter to me at

  Glenn Hoffman
  40 Central Street
  Somerville, Massachusetts 02143
  USA

  you can mail it to me anywhere in the world
• It will get to me
• If someone asked me the directions to the bathrooms ...
• from the my office I would say

  Go out the door
  Turn right
  Walk until you come to a corridor on your left
  Turn left into the corridor
  The bathroom doors will be on the left

• These directions only work from the my office and a few others
• This is like a relative path
• While all absolute pathnames start with a slash, /, or a tilde, ~ ...
• relative pathnames never do
• If the first character is not a slash, /, or a tilde, ~, ...
• it is a relative pathname
• As far as Unix is concerned it makes no difference ...
• whether you use and absolute or relative pathname
• Relative pathnames are more convenient and are most often used
• There are four types of relative pathnames
  • When the file is in your current directory
  • When the file is in a subdirectory of your current directory
  • When the file is in a directory that is above your current directory
  • When the file is in a directory that is neither above or below of your current directory

Relative Pathnames in Your Current Directory

• Using a relative pathname inside your current directory is easy
• The relative pathname is simply the name of the file or directory
• The "path" part of the relative pathname is nothing
• Because the file or directory you want is inside your current directory

  $ ls
  notes.txt  it244  work  work2
  
  $ cat notes.txt 
  foo
  foo
  foo to you

• Notice that all I need to use the file as an argument ...
• is the file name
• It's the same for directories

Relative Pathnames in a Subdirectory

• Things get a little more complicated when you are dealing with a file in a subdirectory
• n this case you must list every directory ...
• between your current directory and the file you want

  $ ls work
  bletch.txt  foo.txt  notes.txt
  
  $ cat work/notes.txt 
  foo
  foo
  foo to you

• After each directory name in a path, you must have a a slash, /
• / separates a directory name from what comes after it
• There is no slash, /, or tilde, ~ before the name of the first directory
• In a relative pathname

Relative Pathnames above the Current Directory

• Every directory has a . . entry
• . . refers to the directory immediately above the directory
• If I wanted to list the contents of the parent directory I would use

  ls ..

• You can use many . . entries to refer to a directory several levels up
• Use one  . .  for each directory up the chain of directories ...
• with a / between each . .

Relative Pathnames Neither above Nor below the Current Directory

• What if the file is neither above nor below?
• Here you have to go up to a common ancestor directory ...
• and then go down to the directory that holds the file
• Use as many  . .  as you need to get to a common ancestor directory
• The common ancestor directory is the first directory ...
• that both your current directory and the other directory ...
• have in common
• Once you get to the common ancestor ...
• you go down to the directory that holds the file

Standard Directories

• Each Linux distribution stores its important files in different directories
• This makes it hard to document programs
• It also mades things hard for developers ...
• working with different flavors of Linux
• Fortunately, we now have something that can help
• It is called the FHS
• It stands for Linux Filesystem Hierarchy Standard
• It has been adopted by most major Linux distributions
• The standard does not say where to find different kinds of files
• Instead it says that if you have a certain directory ...
• what kind of files you should see in that directory
• Most Linux distributions do not use all these directories
• But when they do, they use them for the type of files specified by the standard

Your Home Directory versus /home

• Most Unix accounts have a home directory
• But not all
• The Ubuntu command that creates an account ...
• has an option to not create a home directory
• On most Linux systems home directories are contained in the directory /home
• This is directly under the root directory
• Which is written as /
• On the Mac, home directories are contained in /Users
• Also directly under the root directory
• Do not confuse /home with your home directory

The Two Uses of /

• / is used in two different ways when writing pathnames
• When / appears at the beginning of a pathname ...
• it refers to the root directory
• The directory at the top of the filesystem
• The root directory is the only directory ...
• that is not contained in another directory
• Here is an example of this use of /

  ls /home

• When the / is not at the beginning of a pathname it is used a separator
• A character to mark where a director name ends ...
• and another name begins
• Here is an example of this use of /

  ls it244/hw

• This is the relative pathname of the hw directory inside the it244 directory
• A pathname can use the / in both ways

  ls /home/ghoffman

• Here the first / means the root ...
• but the second is used to separate the home from ghoffman

The Two Special Uses of .

• Another character that has more then one use when writing pathnames is .
• When . appears alone it means the current directory
• So the following command copies a file to the current directory

  cp /home/ghoffman/test.txt .

• When . appears as the first character in a filename ...
• that file is hidden
• You can only see it if you run ls with the -a option
• These "invisible" files are used to configure Linux ...
• and Linux programs
• Examples are .forward and .plan
• Unlike /, . can be used as an ordinary character in a file or directory name

  $ touch a.b.c.d....
  
  $ ls
  a.b.c.d....

Studying

• Keep Each Study Period Short

Class Exercise

Class Quiz