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Welcome to the spoken tutorial on Basics of Shell Scripting.

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In this tutorial, we will learn about

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* System variables

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* User defined variables and

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* Accepting user input via keyboard

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To follow this tutorial you should be familiar with Linux Operating System

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If not, for relevant tutorials please visit our website which is as shown

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For this tutorial I am using

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* Ubuntu Linux 12.04 Operating System and

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* GNU Bash version 4.1.10

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Please NoteGNU Bash version 4 or above is recommended for practice.

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Let us start with a introduction to variables

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* Bash variables provide temporary storage for information

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* These variables can be used within the lifespan of the program.

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* There are two types of variables

 System variables

 User defined variables

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System variables, These are created and maintained by Linux Bash Shell itself.

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They are defined by Capital letters.

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Commonly used system variables are

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* BASH_VERSION,

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* HOSTNAME,

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* HOME etc

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Let us open the terminal by pressing Ctrl Alt and T keys simultaneously  on your keyboard.

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Now type set and press Enter.

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This will display all the system variables.

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Alternately, you can type env or printenv, to view all the system variables.

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Let me clear the prompt

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Now, type echo space within double quotes dollar sign HOSTNAME

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and Now press Enter.

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The hostname of the system will be displayed.

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Now let's find out the full path of homedirectory.

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Type  echo space within double quotes dollar sign HOME(in Capital)

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Press Enter.

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The full path of user's home directory will be displayed.

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Now, type

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echo space within double quotes HOME (in capital)

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Press Enter.

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This will display only the HOME not the value of HOME variable.

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So it is neccessary to use dollar sign( '$') at the beginning of every variable, to display its value.

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Let us switch back to our slides

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User Defined Variables

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* These variables are created and maintained by users.

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* It is always a good idea to avoid uppercase for the names of user defined variables.

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* This makes it easy to differentiate between user defined and system variables.

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Switch back to our terminal

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Type username equal to sign sunita

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Please note that there should not be any blank space between username, equal to sign and sunita.

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Now press Enter.

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To display the value of variable username

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Type echo space within double quotes dollar sign username

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press Enter

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This will display sunita on your terminal.

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The value of a variable can be unset,

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Let us switch back to our slide

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unset,  the value of variable can be unset by using the unset command

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The syntax for this is unset variablename

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Let's use the previous example where username is our variable.

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Switch to the Terminal.  Now type unset space username press Enter

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Let us check Type echo space within double quotes dollar sign username  press Enter.

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Nothing will be displayed on the terminal.

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This means that the value of variable username has been removed.

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Now switch back to our slide

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Global and local variables.

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* In Shell script, user defined variables can be declared globally or locally.

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* By default, all variables are global.

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* This means, their values remains the same inside and outside the function.

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Let us learn how to declare variables globally and locally.

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Switch to the terminal and type

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gedit space g_(underscore)variable.sh space & (ampersand sign)

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gedit is the text editor g_(underscore) variable.sh is our file name

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and & (ampersand) is use to free up the prompt.

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press Enter.

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Type the code as shown here, in your g_(underscore)variable.sh file.

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Let me explain the code now.

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The first line, with the hash and exclamation symbol, is a shebang or a bang line.

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username=sunita is the userdefined variable and it is declared globally.

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echo will display the string outside function:  And

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dollar username will print the value of the variable username.

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This is how we defined a function in BASH script.

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We will discuss about functions in detail, in later tutorials.

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This is the body of the function.

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Here another message inside function will be displayed, along with the value of username.

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Here, we call the function

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This is our code. Now let's execute it.

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Come back to our Terminal.

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Let me clear the prompt

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First we need to make our file executable.

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Type chmod space plus x space g_(underscore)variable.sh  press Enter

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Now type dot slash g_(Underscore)variable.sh

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Press Enter

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Observe the output.

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Outside the function, username takes the value sunita.

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Inside the function also, username takes the same value sunita.

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This is because username was declared globally outside the function.

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Next, let us learn how to declare a variable locally.

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Type gedit space l_(Underscore)variable.sh space & (ampersand sign)

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Press Enter.

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Type the code as shown here, in your l_(underscore)variable.sh file.

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Let me explain the code.

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The code is the same as before, except for an extra line of code inside the function.

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Inside the function block, we have a line,local space username equals to jack

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This assigns a new value for the variable username locally.

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Now switch to the Terminal.

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Let's make file executable

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By Typing chmod space plus x space l_variable.sh

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Press Enter.

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Type dot slash l_variable.sh

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Press Enter.

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The output is displayed

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Outside the function, username takes the value sunita.

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Whereas inside the function, username takes the value jack.

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This is because username is assigned this value locally, within the function.

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Now let us quickly see how to get user input via keyboard.

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The read command is used to accept input from the keyboard.

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It can also be used to assign an input value to a user defined variable.

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The syntax of read command is

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read space hyphen p space within double quotes PROMPT

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Please note that PROMPT is just a string, that waits for user input.

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You may replace it with your own string.

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Now Switch to the terminal

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Typegedit space read.sh space &  (ampersand sign)

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Press Enter.

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Type the code as shown, in your read.sh file.

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Let me explain the code.

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In this example, input is given from the keyboard by the user.

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This is the bang line.

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Here -p displays the prompt, without a newline and takes input from the keyboard.

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The user input will be stored in the variable username.

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echo command displays the message

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Hello and the name entered by the user, via the keyboard.

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So, let us  execute the programme.

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Come back to our terminal

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Type chmod space plus x space read.sh

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press Enter.

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Typedot slash read.sh  press Enter.

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Here it is displayed Enter username:

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I will type ashwini  press Enter.

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The message Hello ashwini is displayed.

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ashwini was assigned as an input value to the user defined variable username.

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Let us go back to our slide and summarise.

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In this tutorial we learnt,

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* System variables

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* User defined variables

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* Accepting user input via keyboard

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As an assignment

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Write a simple Bash program to get the following system variables

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* pwd  and * logname

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Write a simple Bash program

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* To ask username from user

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* To exit the program, if user does not enter anything, within 10 seconds

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* {Hint: read -(Hyphen)t 10 -(Hyphen)p}

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Watch the video available at the link shown below.

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It summarises the Spoken Tutorial project

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If you do not have good bandwidth, you can download and watch it

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The Spoken Tutorial Project Team Conducts workshops using spoken tutorials Gives certificates to those who pass an online test

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For more details, please write to contact@spoken-tutorial.org

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Spoken Tutorial Project is a part of the Talk to a Teacher project

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The script has been contributed  by FOSSEE and Spoken Tutorial Teams.

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This is Ashwini Patil from IIT Bombay.Signing off.Thank you for joining.