Difference between revisions of "BASH/C2/Basics-of-Shell-Scripting/English-timed"

| Welcome to the spoken tutorial on '''Basics of Shell Scripting.'''

| In this tutorial, we will learn about

|* '''System variables'''

|* '''User defined variables''' and

|* Accepting user input via keyboard

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

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

| For this tutorial I am using  

|* '''Ubuntu Linux 12.04''' Operating System and  

|* '''GNU Bash''' version '''4.1.10'''

| Let us start with a introduction to variables

| * '''Bash variables''' provide temporary storage for information

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

|* There are two types of variables  

# '''System variables'''

# '''User defined variables '''

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

|They are defined by Capital letters.

|Commonly used '''system variables '''are

|* '''BASH_VERSION, '''

 

|* '''HOSTNAME, '''

|* '''HOME etc'''

| Let us open the terminal by pressing '''Ctrl Alt and T''' keys simultaneously  on your keyboard.

| Now type '''set''' and press '''Enter.'''

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

|Let me clear the prompt

| Now, type '''echo''' space within double quotes '''dollar sign''' '''HOSTNAME'''

|and Now press '''Enter.'''

|The '''hostname''' of the system will be displayed.

| Now let's find out the full path of '''home'''directory.

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

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

| Now, type

 

| 02:27

|'''echo''' space within double quotes '''HOME''' (in capital)

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

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

|Let us switch back to our slides

|'''User Defined Variables'''

|* These '''variables''' are created and maintained by users.

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

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

| Switch back to our terminal

| Type '''username equal to sign sunita'''

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

|Now press '''Enter.'''

 

| To display the value of variable '''username'''

|Type '''echo''' space within double quotes '''dollar sign username'''  

| press '''Enter'''

| The value of a variable can be unset,

|Let us switch back to our slide

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

|The syntax for this is '''unset variablename'''

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

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

|Now switch back to our slide

|'''Global''' and '''local variables.'''

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

|* By default, all '''variables''' are '''global.'''

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

| Let us learn how to declare '''variables''' '''globally''' and '''locally.'''

| Switch to the '''terminal '''and type

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

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

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

| press''' Enter.'''

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

| Let me explain the code now.

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

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

| '''echo '''will display the string '''outside function: ''' And

|'''dollar username''' will print the value of the variable '''username.'''

|This is how we defined a '''function''' in '''BASH''' script.  

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

| Here, we call the function

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

|Let me clear the prompt

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

|Now type '''dot slash g_(Underscore)variable.sh'''

|Press '''Enter'''

| Observe the output.

 

|Outside the function, '''username '''takes the value '''sunita.'''  

| This is because '''username''' was declared '''globally '''outside the function.

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

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

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

| Let me explain the code.

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

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

| This assigns a new value for the variable '''username locally'''.

|Let's make file executable

|By Typing '''chmod''' '''space plus x space l_variable.sh'''

|Type '''dot slash l_variable.sh'''

|Press '''Enter.'''

 

|The output is displayed

| Outside the '''function''', '''username''' takes the value '''sunita.'''

|Whereas inside the '''function''', '''username''' takes the value '''jack'''.

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

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

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

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

| The syntax of '''read '''command is

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

|You may replace it with your own string.

| Now Switch to the '''terminal '''  

|Type'''gedit''' '''space read.sh space & ''' (ampersand sign)

|Press '''Enter.'''

 

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

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

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

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

|So, let us  execute the programme.

| Come back to our terminal

|Type '''chmod''' '''space plus x space read.sh'''

| press '''Enter.'''

 

| Type'''dot slash read.sh'''  press '''Enter.'''

|I will type '''ashwini'''  press '''Enter.'''

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

|Let us go back to our slide and summarise.  

| In this tutorial we learnt,

|* '''System variables '''

 

|* '''User defined variables'''

|* Accepting user input via keyboard

|  As an assignment

 

|-

| Write a simple '''Bash''' program to get the following system variables

|* '''pwd ''' and * '''logname'''

|* To ask '''username''' from user''' '''

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

|* {Hint: '''read -(Hyphen)t 10 -(Hyphen)p}'''

|It summarises the Spoken Tutorial project  

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

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

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

|It is supported by the National Mission on Education through ICT, MHRD, Government of India

|More information on this Mission is available at the link shown below  http://spoken-tutorial.org\NMEICT-Intro

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

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