BASH/C2/Conditional-execution/English-timed
From Script | Spoken-Tutorial
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 |
| 00:13 | and 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 are shown. |
| 00:32 | For this tutorial I am using |
| 00:35 | * Ubuntu Linux 12.04 OS |
| 00:39 | and * 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 equals 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 whethe 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 i 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 bring 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 & 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 signning off. |
| 11:01 | Thank you for joining. |
