BASH/C2/Conditional-execution/English-timed

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