BASH/C2/Conditional-execution/English-timed

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Title of script: Conditional Execution in BASH

Author: Lavitha Pereira

Keywords: video tutorial, Bash shell, Simple if, else-if.

Time Narration
00:01 Dear friends, welcome to the spoken tutorial on Conditional execution in Bash.
00:08 In this tutorial, we will learn:
00:10 * Use of test command and
00:13 * Conditional statements.
00:15 We will do this using a few examples.
00:19 To follow this tutorial,
00:21 You should be familiar with GNU/Linux Operating System.
00:26 If not, for relevant tutorials please visit our website which is as shown.
00:32 For this tutorial, I am using:
00:35 * Ubuntu Linux 12.04 OS and
00:39 * GNU Bash version 4.1.10
00:43 GNU Bash version 4 or above is recommended for practice.
00:49 Let us start with an introduction to test.
00:52 test is a built-in command which returns the exit status.
00:57 It returns 0 (zero) for True and 1 (one) for False.
01:02 Return value depends on the evaluation of the expression.
01:07 One can get the return status by typing Dollar and question mark sign ($?).
01:14 An expression can be evaluated in two ways-
01:18 One, by using the keyword test.
01:21 And other, by using the expression enclosed within square brackets.
01:27 Now, open the terminal by pressing Ctrl+Alt and T keys simultaneously.
01:35 Type: test space 4 space hyphen eq space 4 semicolon space echo space dollar sign and a question mark. Press Enter.
01:53 It returns zero which means true.
01:57 4 is equal to 4.
02:00 Next, type:
02:02 opening square bracket space 4 space hyphen eq space 4 space closing square bracket semicolon space echo space dollar sign and a question mark. Press Enter.
02:22 It returns zero which means true.
02:25 i.e.4 is equal to 4.
02:28 Let's take another expression; type:

test space 4 space hyphen eq space 5 semicolon space echo space dollar sign question mark. Press Enter.

02:48 It returns one which means false.
02:52 i.e.4 is not equal to 5.
02:56 Now let's write the same expression within square brackets. Type:
03:01 opening square bracket space 4 space hyphen eq space 5 space closing square bracket semicolon space echo space dollar sign question mark Press Enter.
03:21 It also returns one which means false.
03:25 i.e. 4 is not equal to 5.
03:29 This can be extended for other kind of testing.
03:33 Please type: man space test and explore its usage.
03:40 Now let us go back to our slides.
03:43 Now we will see the syntax for if statement-
03:48 if space opening square bracket space expression space closing square bracket semicolon space then
03:59 On the next line, type: commands or statements that you want to execute.
04:05 Lastly, end the if loop with fi.
04:11 The basic rules of condition are:
04:14 * Always keep spaces between the brackets and the expression.
04:19 * Always terminate the line using semicolon before keyword “then”.
04:25 * Semicolon is used to terminate the statement or an expression.
04:31 It is recommended to quote string variables, if you use them in conditions.
04:38 Don't forget to close the conditional block with “fi”.
04:43 Let us see an example on if statement.
04:46 Come back to the terminal.
04:49 I will open an already existing script file named simpleif.sh.
04:58 This Bash script displays the message “count is 100” when the count equals to 100.
05:06 This is first line of Bash shell script known as shebang line.
05:12 An integer 100 is assigned to a variable count.
05:17 Note that there should not be any space between count, = and 100.
05:24 This expression checks whether count is equal to hundred.
05:30 Here, -eq is comparison operator .
05:35 If the condition is true, it will display the message count is 100,
05:41 fi is to end if block.
05:45 Save the file by pressing “Ctrl + s”
05:49 Go back to the Terminal.
05:51 To make the file executable, type: chmod space plus x space simpleif.sh and press Enter.
06:04 Let me clear the prompt.
06:06 Now type: dot slash simpleif.sh press Enter.
06:14 Here it is displayed:
06:16 Count is 100.
06:18 Try changing the value of variable count and execute the script.
06:24 Switch back to slides.
06:26 We will see if-else condition.
06:30 The general syntax is: if space opening square bracket space condition space closing square bracket space semicolon space then
06:44 On the next line, type commands.
06:47 On the next line, there is else statement.
06:51 And again type 'some other commands'.
06:55 on the next line, type fi to end if block.
07:00 Let us study the usage of if-else with an interesting password program.
07:06 Come back to the terminal.
07:09 I will open the file ifelse.sh
07:14 This is the shebang line.
07:17 Here,abc123 is stored in the variable PASS.
07:23 As abc123 is a string, it should be written within double-quotes.
07:29 The read command reads one line of data from the standard input.
07:35 In this case, standard input is our keyboard.
07:39 Hyphen s is for silent mode.
07:43 Which means the entered password will not be displayed as we type.
07:48 We don't want others to see our password.
07:52 Hyphen p is for prompt.
07:55 It will display a string “'Enter password: ” before it takes input from user.
08:01 mypassword is a variable.
08:04 It stores the string, in this case the password entered by the user.
08:10 This checks that the entered password matches the value of the variable PASS.
08:17 It is stored in a variable mypassword.
08:21 If the password matches, it will display the message:
08:25 “Password accepted”
08:27 else it will display “Access denied”.
08:31 fi is the end of if-else loop.
08:34 Now save the file by pressing “Ctrl s”.
08:38 Come back to our terminal, make the file executable type:

chmod space plus x space ifelse.sh. Press Enter.

08:52 Type dot slash ifelse.sh . Press Enter.


08:57 Here it is displayed :
08:59 Enter password:
09:00 I will type abc press Enter.
09:05 As the password entered is wrong, it displays the message as “Access denied”.
09:11 Let's execute again but this time we will enter password as abc123.
09:21 It is displayed as: Password accepted
09:25 This brings us to the end of this tutorial.
09:28 Come back to our slides and summarize.
09:31 In this tutorial, we have covered usage of test command , simple if statement and if else statement.
09:41 As an assignment-
09:43 # Write a script , take your name as an input.
09:46 # It should check this name with your system's username.
09:51 If the username matches, it should greet you by displaying “Hello”.
09:56 # Else, it should display “Try again”.
10:00 HINT: Your system's username is stored in a variable $USER.
10:06 Watch the video available at the link shown below.
10:09 It summarizes the Spoken Tutorial project.
10:11 If you do not have good bandwidth, you can download and watch it.
10:16 The Spoken Tutorial Project team:
10:18 Conducts workshops using spoken tutorials.
10:22 Gives certificates to those who pass an online test.
10:26 For more details, please write to contact@spoken-tutorial.org
10:33 Spoken Tutorial Project is a part of the "Talk to a Teacher" project.
10:37 It is supported by the National Mission on Education through ICT, MHRD, Government of India.
10:45 More information on this mission is available at the link shown below.
10:51 The script has been contributed by FOSSEE and spoken-tutorial team.
10:56 This is Ashwini from IIT Bombay, signing off.
11:01 Thank you for joining.

Contributors and Content Editors

PoojaMoolya, Pratik kamble, Ranjana, Sandhya.np14