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Interacting with a UNIX computer: Navigating through the directory tree

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• Convenience of the current directory
  • We working with some file(s), it would be nice that we can access the file with the least number of key strokes possible

  • Fact in UNIX:

    All applications in UNIX can access a file by its name when the file is in the current directory (That's because the filename itself is the relative path to the file - which will be explained later)

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  • In order to take take advantage of this convenience, we must change the current directory to the directory that contains the desired file(s)

    We will learn how to do this right now...

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• Task: Changing the current (working) directory / Navigate the directory tree
  • The command (= application) that is used to change the current directory to another directory is:

    cd NEW-Current-DIRECTORY-PATH

    cd is an abbreviation of the words change directory

    The New-Current-DIRECTORY-PATH can be an absolute directory path or a relative directory path

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  • Example of the "cd" command:    cd /home/cheung/cs170   (absolute path)

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  • Example of the "cd" command:    cd cs170   (relative path)

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• ****************** Lab exercise
  • Perform these tasks now:

    List the content of your current directory Make sure the directory cs170 is in the listing If the directory cs170 is not in the listing, tell the Teaching Assistant !!! Change the current working directory of the terminal window to your cs170 directory (do this only if you see cs170 in the listing)

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• Concept: Arguments of a command
  • In the command below:

    cd /home/cheung/cs170

    The first word cd is the name of the command (is also the name of the computer application that will be run)

    The second "word" /home/cheung/cs170 is the argument of the command

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  • Number of arguments:

    Some commands have 1 argument (E.g.: cd) Other commands have no (0) argument (E.g.: pwd) Yet other commands have multiple (2 or more) argument

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  • Purpose of arguments:

    Arguments provide information to the command (application) It allows the command (application) to be more flexible (can do more)

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• Concept: Default argument
  • Some commands (applications) have an assumed argument when the command is not given any argument

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  • Default arguments of the commands that you have learned so far:

    Command (application)	Default argument
    ls	the current directory
    cd	the home directory

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  • Example: default argument for ls

    When ls is not given any argument, it will list the content of the current directory

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  • Example: default argument for cd

    When cd is not given any argument, it will change the working directory of the home directory

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• Concept: Special directories
  • There are a number of special directories in UNIX that have special notations

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  • Special directories:

    Symbol	Directory represented by the symbol
    /	The root directory
    ~	The home directory of the user
    .	The current (working) directory
    ..	The parent directory of the current directory

    The parent directory is the directory that is immediately above the current directory in the directory tree.

    The parent directory of the root directory / is itself (/)

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  • Examples:
    • Suppose the home directory is /home/cheung and
    • current (working) directory is /home/cheung/cs170

    then:

    Command	The directory used as argument of the command
    ls /	root directory
    ls ~	/home/cheung   (= home directory of the user)
    ls .	/home/cheung/cs170   (= current (working) directory)
    ls ..	/home/cheung   (= the parent directory of the /home/cheung/cs170)
    ls ../..	/home
    ls ../../..	/
    ls ../../../..	/   (because the parent directory of the root directory / is itself

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• Exercise: Practice "ls" and "cd"
  • Exercise:

    Practice the cd and the ls command in the window below. You are user bob and your current directory (highlighted in green) is /home/bob Exercise: There are several tasks specified. Type the correct cd or ls command at the "command input" box and hit the ENTER key to accomplish the task (The answer is given a little lower in the webpage, but try not to look at the answers all the time - or else you will miss this opportunity to solidify what you have learned)

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  • ls Exercise:

    I don't know java

    (The current directory = /home/bob) List the content of the current directory List the content of bob's "photos" directory using a absolute path List the content of bob's "photos" directory using a relative path List the content of sally's home directory using a absolute path List the content of sally's home directory using a relative path

    Answers:

    I don't know java

    (Type in these commands in the "command input" box and hit "ENTER") ls ls /home/bob/photos        ls photos ls /home/sally ls ../sally

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  • cd Exercise:

    I don't know java

    (You starts with current directory = /home/bob) Starting in bob's home directory, use a absolute path to change the current directory to sally's home directory Now list the content of the current directory From sally's home directory, use a relative path to change the current directory to bob's home directory Now list the content of the current directory From bob's home directory, use a relative path to change the current directory to sally's "music" directory Now list the content of the current directory From sally's "music" directory, use a relative path to change the current directory to bob's "photos" directory Now list the content of the current directory

    Answers:

    I don't know java

    (Type in these commands in the "command input" box and hit "ENTER") cd /home/sally ls cd ../bob ls cd ../sally/music ls cd ../../bob/photos             ls

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• Task: Create a directory
  • The command (= application) that is used to create a new directory:

    mkdir PATH-of-a-NEW-DIRECTORY

    You can use an absolute or a relative directory path in the mkdir command.

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  • Example: create a sub-directory named hw1 inside /home/cheung/cs170

    Output of the ls command	Browsing the directory with a File Explorer

    Before the mkdir command, the cs170 directory contains: myFile1 and myFiles2

    After the mkdir command, the cs170 directory also contains: hw1

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• ****************** Lab exercise
  • Perform this task now:

    Create a directory named lab1 inside your cs170 directory.          You will know if you are successful is the following command: ls ~/cs170 prints the listing lab1

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• Concept: command options
  • Many of the UNIX commands (applications) have options that can modify the behavior of the command (application)

  • Options for a UNIX command are always specified in the following manner:

    Options are usually single character items Options are specified immediately after the command name You can specify more than one options The arguments for the command are given after the list of options Options are preceded by the "−" symbol

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  • Example: what a UNIX command looks like

    command -a -b -c arg1 arg2 arg3 arg4

    The options of the command are: "-a -b -c" (3 options)
    The arguments of the command are: "arg1 arg2 arg3 arg4" (4 arguments)

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• Some commonly used options of the ls command
  • Some useful options of the ls command:

    Option	Effect of the option
    -t	List files ordered by the time of creation (latest first)
    -r	List files in the reverse order
    -l	Long listing - give detailed information on files and directories
    -F	Append "/" to a directory name and "*" to the name of an executable application

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    Example:

    Output of different ls commands	Browsing the directory with a File Explorer

    You can see that the output format is changed by the various options specified to the ls command

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• ****************** Lab exercise
  • Perform this task now:

    Show the long listing of your home directory (~)          Notice the permissions of your cs170 directory It show read like this: drwxrwx--- .... cs170 If your cs170 permissions is not "drwxrwx---", tell the Teaching Assistant !!!

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• The cd command has no options
  • The cd command does not have any options....

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• Finding out the options and arguments of a command

  • To find out how to use a command, you can access the manual page for a command by running the man application.

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  • Example: ls

    man ls

    You will see:

    User Commands ls(1) NAME ls - list contents of directory SYNOPSIS /usr/bin/ls [-aAbcCdeEfFghHilLmnopqrRstuvVx1@] [file]... DESCRIPTION For each file that is a directory, ls lists the contents of the directory. For each file that is an ordinary file, ls repeats its name and any other information requested. The output is sorted alphabetically by default. When no argument is given, the current directory (.) is listed. When several arguments are given, the arguments are first sorted appropriately, but file arguments appear before directories and their contents.

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  • The synopsis shows you how to use the application (what options and arguments you can specify).

    You need to remember that the format of every UNIX command is:

    command-name -options arguments

    Now let's read the synopsis:

    /usr/bin/ls [-aAbcCdeEfFghHilLmnopqrRstuvVx1@] [file]... meaning: 1. The absolute path of the ls application is /usr/bin/ls 2. The list -aAbcCdeEfFghHilLmnopqrRstuvVx1@ are the options The [] around [-aAbcC...] means that you may specify the options (and you may leave it out) For the meaning of each option, you must look (search) in the manual page... 3. The word file is the argument. The [] around [file] means that you may specify the "file" as an argument (and you may leave it out) The ... after "[file]" means that you may repeat the argument again (with another name for file) or you may stop. Some possible forms that you can run ls as: ls or: ls A or: ls -l A or: ls -l -t A or: ls A B or: ls -l A B

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• Task: Delete a directory
  • The command (= application) that is used to delete a directory is:

    rmdir DIRECTORY-PATH

    You can use an absolute or a relative directory path.

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  • Example: delete the sub-directory named hw1 inside /home/cheung/cs170

    (I made a mistake in the first attempt, I tried to remove the directory /home/cheung/hw1 which did not exists. I forgot the cs170 part)

    Mistakes are less likely by using a relative path:

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  • Note:

    The UNIX system will not allow you to delete a directory if the directory is not empty So if you have created some files inside the hw1 directory, you cannot delete it. You will need to delete all files (and sub-directories) from a directory before you can delete that directory. Later, you will learn how to delete files from a directory.

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• ****************** Lab exercise
  • Perform these tasks now:

    Change your working directory to   ~/cs170 (Skip this step if your current directory is already ~/cs170) Create a directory named junk inside your cs170 directory.          Then delete that directory (named junk) from your cs170 directory.

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• Task: Rename a directory
  • The command (= application) that is used to rename a directory is:

    mv Old-Name New-Name

    mv is the acronym for move

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  • Extremely important note:

    The directory New-Name must not exist Otherwise, you will perform a entirely different operation !!! (Explained below)

    It is advisable to use a relative directory path to rename a directory (you will know why very soon)

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  • Example: rename the sub-directory named hw1 inside /home/cheung/cs170 to proj1

    (I used the −F option to let you see which names are files and which are directories)

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• ****************** Lab exercise
  • Perform these tasks now:

    Change your working directory to   ~/cs170 (Skip this step if your current directory is already ~/cs170) Create a directory named not-junk inside your cs170 directory.          Next rename the directory named not-junk to the new name junk          Then delete the directory (now named junk) from your cs170 directory.

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• Task: Move a directory to somewhere else
  • Suppose you want to create a directory handouts inside /home/cheung/cs170/proj1

    But instead, you created the directory inside /home/cheung/cs170 (inside a wrong folder (directory))

    Graphically:

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    Solution:

    move the directory handouts into the folder (directory) proj1

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  • The command (= application) that is used to move a directory X into some directory D is:

    mv X D

    (This has the same effect by using a file browser and do this:

    drag the item X into the folder D
    )

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  • Extremely important note:

    The directory D must already exist Otherwise, you will perform a rename operation !!! (You would have renamed the directory X to D)

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  • Example: move the directory named handouts inside /home/cheung/cs170/proj1

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• ****************** Lab exercise
  • Perform these tasks now:

    Change your working directory to   ~/cs170 (Skip this step if your current directory is already ~/cs170) Create a directory named junk inside your cs170 directory.          Create another directory named junk2 inside your cs170 directory.          Next move the directory named junk2 inside the junk directory.          List the current directory List the directory junk List the directory junk/junk2 Try to delete the directory junk (with rmdir junk) It will fail because the directory is not empty ! Delete the directory junk/junk2          Now delete the directory junk (with rmdir junk)

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• Command line vs. Drag and drop
  • Drag and drop is easy to learn

    But it takes a long time go to one place and drag the thing into another folder.

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  • Using a command line is hyper fast, but: only after you have become familiar with the Computer System

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  • Bottom line

    If you want to become a Computer Expert, you have to learn to do things fast (or else, your career working with a computer will be painfully slow and unproductive) You will need to learn command line interaction with a computer if you want to be a Computer Expert