BASH/C2/Conditional-execution/English-timed

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 and a 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 condition.
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 our 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 the 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	Now 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	Now switch back to our 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 displays 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: 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.
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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