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

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|* '''System variables'''
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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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# '''System variables'''
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'''System variables'''
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# '''User defined variables '''.
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'''User defined variables '''.
  
 
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|* '''BASH_VERSION '''
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|* '''HOSTNAME '''
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|* '''pwd ''' and * '''logname'''.
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|* Write a simple '''Bash''' program
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| Write a simple '''Bash''' program
  
 
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| The Spoken Tutorial Project team:  
 
| The Spoken Tutorial Project team:  
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* Conducts workshops using spoken tutorials.  
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Conducts workshops using spoken tutorials.Gives certificates to those who pass an online test.  
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* Gives certificates to those who pass an online test.  
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Revision as of 16:20, 20 February 2017

Time Narration
00:01 Welcome to the spoken tutorial on Basics of Shell Scripting.
00:05 In this tutorial, we will learn about:
00:09 System variables
00:11 User defined variables and
00:13 Accepting user input via keyboard.
00:16 To follow this tutorial, you should be familiar with Linux Operating System.
00:23 If not, for relevant tutorials, please visit our website which is as shown.
00:29 For this tutorial, I am using:
00:32 Ubuntu Linux 12.04 Operating System and
00:35 GNU Bash version 4.1.10
00:40 Please NoteGNU Bash version 4 or above is recommended for practice.
00:46 Let us start with an introduction to variables.
00:49 Bash variables provide temporary storage for information.
00:55 These variables can be used within the lifespan of the program.
01:01 There are two types of variables:

System variables User defined variables .

01:07 System variables: These are created and maintained by Linux Bash Shell itself.
01:14 They are defined by capital letters.
01:17 Commonly used system variables are-
01:20 BASH_VERSION
01:21 HOSTNAME
01:23 HOME etc.
01:25 Let us open the terminal by pressing Ctrl, Alt and T keys simultaneously on your keyboard.
01:33 Now, type: set and press Enter.
01:38 This will display all the system variables.
01:42 Alternately, you can type env or printenv to view all the system variables.
01:53 Let me clear the prompt.
01:55 Now, type: echo space within double quotes dollar sign HOSTNAME
02:01 now press Enter.
02:04 The hostname of the system will be displayed.
02:07 Now let's find out the full path of homedirectory.
02:11 Type: echo space within double quotes dollar sign HOME(in capital).
02:18 Press Enter.
02:21 The full path of user's home directory will be displayed.
02:26 Now, type:
02:27 echo space within double quotes HOME (in capital).
02:32 Press Enter.
02:34 This will display only the "HOME", not the value of HOME variable.
02:39 So, it is necessary to use dollar sign( $) at the beginning of every variable to display its value.
02:48 Let us switch back to our slides.
02:51 User Defined Variables:
02:53 These variables are created and maintained by users.
02:57 It is always a good idea to avoid uppercase for the names of user defined variables.
03:05 This makes it easy to differentiate between user defined and system variables.
03:12 Switch back to our terminal.
03:14 Type: username equal to sign sunita
03:20 Please note that there should not be any blank space between username, equal to sign and sunita.
03:29 Now, press Enter.
03:30 To display the value of variable username,
03:33 Type: echo space within double quotes dollar sign username
03:40 press Enter.
03:42 This will display sunita on your terminal.
03:46 The value of a variable can be unset.
03:50 Let us switch back to our slides.
03:52 unset- the value of a variable can be unset by using the unset command.
03:59 The syntax for this is- unset variablename
04:03 Let's use the previous example where username is our variable.
04:08 Switch to the Terminal. Now type: unset space username, press Enter.
04:18 Let us check. Type: echo space within double quotes dollar sign username press Enter.
04:28 Nothing will be displayed on the terminal.
04:30 This means that the value of variable username has been removed.
04:36 Now switch back to our slide.
04:39 Global and local variables:
04:42 In Shell script, user defined variables can be declared globally or locally.
04:49 By default, all variables are global.
04:52 This means, their values remain the same inside and outside the function.
04:59 Let us learn how to declare variables globally and locally.
05:04 Switch to the terminal . Type:
05:07 gedit space g_(underscore)variable.sh space & (ampersand sign).
05:16 gedit is the text editor g_(underscore) variable.sh is our file name
05:23 and '&' (ampersand) is used to free up the prompt.
05:28 Press Enter.
05:30 Type the code as shown here, in your "g_(underscore)variable.sh" file.
05:35 Let me explain the code now.
05:38 The first line with the hash and exclamation symbol is a shebang or a bang line.
05:44 username=sunita is the userdefined variable and it is declared globally.
05:51 echo will display the string "outside function:" and
05:55 dollar username will print the value of the variable username.
06:00 This is how we define a function in BASH script.
06:04 We will discuss about functions in detail, in later tutorials.
06:09 This is the body of the function.
06:12 Here, another message "inside function" will be displayed along with the value of username.
06:19 Here, we call the function.
06:21 This is our code. Now let's execute it.
06:23 Come back to our Terminal.
06:26 Let me clear the prompt.
06:28 First we need to make our file executable.
06:31 Type: chmod space plus x space g_(underscore)variable.sh, press Enter.
06:39 Now, type: dot slash g_(Underscore)variable.sh
06:45 Press Enter.
06:47 Observe the output.
06:48 Outside the function, username takes the value sunita.
06:53 Inside the function also, username takes the same value sunita.
06:59 This is because username was declared globally outside the function.
07:04 Next, let us learn how to declare a variable locally.
07:09 Type: gedit space l_(Underscore)variable.sh space & (ampersand sign).
07:18 Press Enter.
07:20 Type the code as shown here, in your "l_(underscore)variable.sh" file.
07:25 Let me explain the code.
07:28 The code is the same as before except for an extra line of code inside the function.
07:36 Inside the function block, we have a line- local space username equals to jack.
07:41 This assigns a new value for the variable username locally.
07:48 Now switch to the Terminal.
07:50 Let's make the file executable
07:52 by typing chmod space plus x space l_variable.sh
08:00 Press Enter.
08:02 Type: dot slash l_variable.sh
08:07 Press Enter.
08:08 The output is displayed.
08:10 Outside the function, username takes the value sunita
08:15 whereas inside the function, username takes the value jack.
08:20 This is because username is assigned this value locally, within the function.
08:26 Now let us quickly see how to get user input via keyboard.
08:31 The read command is used to accept input from the keyboard.
08:36 It can also be used to assign an input value to a user defined variable.
08:41 The syntax of read command is-
08:44 read space hyphen p space within double quotes PROMPT
08:50 Please note that PROMPT is just a string that waits for user input.
08:55 You may replace it with your own string.
08:58 Now, switch to the terminal .
09:00 Type: gedit space read.sh space & (ampersand sign)
09:08 Press Enter.
09:09 Type the code as shown here in your "read.sh" file.
09:14 Let me explain the code.
09:16 In this example, input is given from the keyboard by the user.
09:21 This is the bang line.
09:23 Here -p displays the prompt without a newline and takes input from the keyboard.
09:31 The user input will be stored in the variable username.
09:36 echo command displays the message
09:38 Hello and the name entered by the user via the keyboard.
09:43 So, let us execute the program.
09:45 Come back to our terminal.
09:49 Type: chmod space plus x space read.sh
09:55 press Enter.
09:56 Type: dot slash read.sh press Enter.
10:01 Here it is displayed Enter username:
10:04 I will type ashwini, press Enter.
10:08 The message Hello ashwini is displayed.
10:13 ashwini was assigned as an input value to the user defined variable username.
10:20 Let us go back to our slides and summarize.
10:23 In this tutorial, we learnt:
10:26 System variables
10:27 User defined variables and
10:29 Accepting user input via keyboard.
10:33 As an assignment-
10:34 Write a simple Bash program to get the following system variables.
10:38 pwd and * logname.
10:41 Write a simple Bash program
10:43 to ask username from user
10:46 to exit the program, if user does not enter anything within 10 seconds.
10:51 {Hint: read -(Hyphen)t 10 -(Hyphen)p}
10:56 Watch the video available at the link shown below.
10:59 It summarizes the Spoken-Tutorial project.
11:02 If you do not have good bandwidth, you can download and watch it.
11:07 The Spoken Tutorial Project team:

Conducts workshops using spoken tutorials.Gives certificates to those who pass an online test.

11:16 For more details, please write to contact@spoken-tutorial.org
11:23 Spoken Tutorial Project is a part of the Talk to a Teacher project.
11:27 It is supported by the National Mission on Education through ICT, MHRD, Government of India.
11:34 More information on this mission is available at the link shown below: http://spoken-tutorial.org/NMEICT-Intro
11:40 The script has been contributed by FOSSEE and Spoken Tutorial teams.
11:44 This is Ashwini Patil from IIT Bombay, signing off. Thank you for joining.

Contributors and Content Editors

PoojaMoolya, Pratik kamble, Sandhya.np14

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