Interacting with a UNIX computer in the MathCS Lab: Listing the content of directories

Introduction

The computers in the MathCS lab are using the UNIX Operating System
Most students have not used a computer that is controlled by the UNIX Operating System

Important notice:

It is extremely important that you attend all labs
If you don't understand some part of the lab, please ask the Teaching Assistant for help
They are paid to explain the material to you; so take advantage of this service.
The labs is an essential part of the course and supplements the lecture materials that are best taught hands on (such as the material in this webnote)
If you miss a lab, make sure you do (most) the lab on your own before coming to the next lab
And if you don't understand some part of the lab, ask the Teaching Assistant for help during the next lab

Log in
- Log in procedure:
- After a successful login, you should see a desktop that look something like this:
  (You can see a larger picture by clicking here: click here )
  The (white) window in the center of the desktop is a Terminal application
  This is a UNIX "shell" program that allow you to interact with the computer through command lines typed from the keyboard

Launching a new "Terminal" application from the Launch bar

To launch another Terminal application, do the following:

Left click on the Launch button in the lower left corner of the desktop
Left click on the Application entry
Left click on the Utilities entry
Finally, left click on the Terminal entry

Graphically:

(You can see a larger picture by clicking here: click here )

Result:
A faster way is to click on the Terminal icon if it available on your quick launch bar menu:

Launching a new "Terminal" application from a Terminal window
- To launch another Terminal application using a command line (that must be entered in a Terminal window), type the following command in a Terminal window:
  and hit the ENTER key
  
  Graphically:
****************** Lab exercise: launch a terminal
- Perform this task now:

********************* Lab exercise: launch firefox and send email

Perform the following tasks:

Launch the firefox web browser application on your lab machine
Note:
Use the firefox web browser application to visit your favorite web mail site (e.g., Yahoo, GMail, Hotmail, etc)
Send a short email message to the class account of your section
The email accounts of the different sections of CS170 are:

Directories and their usage
- A directory is a computer folder that can contain (hold):
- Files are electronic documents that contain information
- Directories are electronic folders that let the user organize his files
Identifying a directory
- We need to identify a directory before we can perform an operation (like delete) on the directory.
  (There is a need to identify things stored inside a computer uniquely.
  Just like using Social Security Numbers to identify people uniquely)
- The mean to identify a directory unique inside a UNIX file system is:
- There are 2 kinds of directory paths:
  We will now explain the absolute directory path and discuss the relative path later.

Concept: absolute directory path

Absolute directory path:

An absolute directory path tells the computer how to find a directory starting from the root directory
An absolute directory path of a particular directory x consists of a list of directory names starting from the root directory "/" to the directory x
The list of directory names is separated by the "/" symbol

Example 1:

The directory path for the highlighted directory is:

/bin

Note:

The exact path should be: "//bin".
- The first "/" is the symbol for the root directory.
- The second "/" is a separating "/" symbol.
The root "/" (the first "/") can be omitted for brevity

Example 2:

The directory path for the highlighted directory is:

/home/bin

Example 3:

The directory path for the highlighted directory is:

/home/km/bin

Some facts:

There may be multiple directories with the same name (but not inside the same directory)
An absolute directory path will uniquely identify one particular directory.
(In fact, the absolute directory path is the "official" name of a directory)
An absolute directory path always starts with the symbol /
A directory path is case-sensitive

****************** Lab exercise: list some directories by their absolute paths
- Enter the following commands in the terminal window and observe the outputs
  The outputs of the commands are the content of each directory (i.e., what's inside that "directory folder")
Concept: the home directory
- Each user in UNIX is assigned a unique user ID
- Each user in UNIX is assigned a home directory (folder) where he/she can store his/her files.
- The (absolute) directory path of the user with the user ID "X" is:
  Examples:

Reminder: how to make a computer do things for you

We make a computer do a task (for us) by:

Also:

People have written down computer programs (applications) to do frequently performed tasks
Some well known computer applications:

Now it's time to learn how to use (i.e. run) some of these applications to become a skillful computer operator

(You need to become a skillful computer operator before you can become a skillful computer programmer)

Tasks that are performed very frequently

Frequently performed tasks: (you will practice these in Lab 1)

Operations on directory (folder):

Print the path of the current (working) directory
List the content of a directory
Navigate the directory tree
Create a directory
Delete a directory
Rename a directory
Move a directory to somewhere else

Operations on files
Create a file must be done through an editor application which will be explained in a later lab

Remember:

You will be executing a computer program to perform each of the task above
Some times, you will be executing the same program to do different tasks
(Because some programs are like a Swiss army knife that can do many different tasks)

We will also need to learn some concepts (and terminology) associated with these tasks

Prelude to the examples
- The concepts and command are illustrated from the user with user ID cheung
  (That's because Prof. Cheung has developed this course material)
  Caveat:
- The directory structure that is relevant to the example is as follows:
  This is what you would see when you browse the /home/cheung/cs170 directory:
- Some facts about the user "cheung":

Concept: Current directory (a.k.a. working directory)

Computer applications often uses information (= data) stored in some file
Files in a computer systems are stored in some directory
Every computer application has a working directory (= the directory where the application is working in)

Print the path of the current (working) directory:

The application that instruct the computer to print the path of the working directory is:
You can run this application from a Terminal window by typing in the name of the application
Example:

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

Task: List the content of a directory

The command (= application) that is used to list the content of a directory:
(ls is an abbreviation of the word list)
The ls command (application) will list the names of the files and the directories stored in the specified directory in alpha-numerical order

Note:
Example 1: " ls /" --- give the list of directory and files stored in the root directory /
Example 2: " ls /etc" --- give the list of directory and files stored in the directory /etc

Listing vs. Browsing a directory:

Browsing is slow
(You need to click a lot before you can reach the desired directory that you want to browse)
I can type in a directory path a lot a faster than I can click...
Browsing produce a nicer output
(If you become a Computer Scientist, you will learn that only information matters, not a nicer look)
Bottom line: geeks prefer the speedier directory listing method

****************** Redo the Lab exercise: list some directories by their absolute paths
- Enter the following commands in the terminal window and observe the outputs
  Now you should understand what is going on. If not, please ask questions !!!

Concept: Relative directory paths

Suppose Prof. Cheung wants to list the content of the cs170 directory in his home directory
He would execute this command: ls /home/cheung/cs170
Example:
If the current directory is his home directory (/home/cheung), then he can use a shorter form by using a relative directory path

Relative directory path:

An relative directory path tells the computer how to find a directory starting from the current (working) directory
An relative directory path a particular directory x consists of a list of directory names starting from the current directory to the directory x
A relative path does not start with the "/" symbol
(If a path starts with a "/" symbol, it is assumed to be an absolute directory path)
The list of directory names is separated by the "/" symbol

Example:

Suppose that current directory = /home/cheung
I want to list the content of the directory /home/cheung/cs170
Starting from /home/cheung, the relative path to the directory /home/cheung/cs170 is:
We need to add the blue part (/cs170) to get to the target directory
So we use this command to list the cs170 directory:

Actual execution: