View source for BASH/C2/Conditional-execution/English

'''Title of script: Conditional Execution in BASH'''

'''Author:''' Lavitha Pereira

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



{| style="border-spacing:0;"
| style="border:1pt solid #000000;padding:0.097cm;"| <center>'''Visual Cue'''</center>
| style="border:1pt solid #000000;padding:0.097cm;"| <center>'''Narration'''</center>

|-
| style="border:1pt solid #000000;padding:0.097cm;"| Display Slide 1
| style="border:1pt solid #000000;padding:0.097cm;"| Dear friends, Welcome to the spoken tutorial on '''Conditional execution in Bash.'''

|-
| style="border:1pt solid #000000;padding:0.097cm;"| Display Slide 2
| style="border:1pt solid #000000;padding:0.097cm;"| In this tutorial, we will learn

* Use of''' test '''and
* '''Conditional''' Statements 

We will do this using a few examples.

|-
| style="border:1pt solid #000000;padding:0.097cm;"| Display Slide 3

'''Prerequisite'''


spoken-tutorial.org 
| style="border:1pt solid #000000;padding:0.097cm;"| To follow this tutorial, 

You should be familiar with the '''GNU/Linux Operating System.'''


If not, for relevant tutorials please visit''' spoken hyphen tutorial dot org.'''

|-
| style="border-top:none;border-bottom:1pt solid #000000;border-left:1pt solid #000000;border-right:1pt solid #000000;padding:0.097cm;"| Display Slide 4

'''System Requirements'''
| style="border-top:none;border-bottom:1pt solid #000000;border-left:1pt solid #000000;border-right:1pt solid #000000;padding:0.097cm;"| For this tutorial I am using 

* '''Ubuntu Linux 12.04''' OS and 
* '''GNU Bash''' version '''4.1.10'''

'''GNU Bash '''version '''4''' or above is recommended to practice this tutorial.

|-
| style="border-top:none;border-bottom:1pt solid #000000;border-left:1pt solid #000000;border-right:1pt solid #000000;padding:0.097cm;"| 
| style="border-top:none;border-bottom:1pt solid #000000;border-left:1pt solid #000000;border-right:1pt solid #000000;padding:0.097cm;"| Let us start with an introduction to''' test.'''

|-
| style="border:1pt solid #000000;padding:0.097cm;"| Display Slide 5

'''Test'''



| style="border:1pt solid #000000;padding:0.097cm;"| * '''test''' is a '''built-in''' command, which returns the '''exit status.'''
* It returns''' 0''' Zero for '''True''' and''' 1''' One for False.
* Its '''return '''value''' '''depends on the evaluation of the expression.



|-
| style="border-top:none;border-bottom:1pt solid #000000;border-left:1pt solid #000000;border-right:1pt solid #000000;padding:0.097cm;"| Display Slide 6

'''Test'''
| style="border-top:none;border-bottom:1pt solid #000000;border-left:1pt solid #000000;border-right:1pt solid #000000;padding:0.097cm;"| * One can get the '''return''' status by typing''' Dollar question mark ($?)'''

* An expression can be evaluated in two ways-

# One by using the keyword''' test'''
# Other by using'' ''the expression enclosed within square brackets.



|-
| style="border:1pt solid #000000;padding:0.097cm;"| 
| style="border:1pt solid #000000;padding:0.097cm;"| Let us see an example.

|-
| style="border:1pt solid #000000;padding:0.097cm;"| Press''' Ctrl+Alt+T'''>> Type '''test 4 -eq 4; echo $?''' >>press '''Enter'''



| style="border:1pt solid #000000;padding:0.097cm;"| Open the '''terminal''' by pressing '''Ctrl+Alt+T''' keys simultaneously.


Type:

'''test space 4 space hyphen eq space 4 semicolon space echo space dollar sign and a question mark.'''

Press''' Enter'''

|-
| style="border-top:none;border-bottom:1pt solid #000000;border-left:1pt solid #000000;border-right:1pt solid #000000;padding:0.097cm;"| 
| style="border-top:none;border-bottom:1pt solid #000000;border-left:1pt solid #000000;border-right:1pt solid #000000;padding:0.097cm;"| It returns''' zero '''which means''' true''' 


i.e.''' 4''' is equal to''' 4'''

|-
| style="border:1pt solid #000000;padding:0.097cm;"| Type '''<nowiki>[ 4 -eq 4 ]; echo $?</nowiki>''' >>press '''Enter'''
| style="border:1pt solid #000000;padding:0.097cm;"| Next type:

'''opening square bracket space 4 space hyphen eq space 4 space closing square bracket semicolon space echo space dollar and a question mark.'''

Press''' Enter'''




|-
| style="border-top:none;border-bottom:1pt solid #000000;border-left:1pt solid #000000;border-right:1pt solid #000000;padding:0.097cm;"| 
| style="border-top:none;border-bottom:1pt solid #000000;border-left:1pt solid #000000;border-right:1pt solid #000000;padding:0.097cm;"| It returns''' zero '''which means''' true''' 


i.e.''' 4''' is equal to''' 4'''

|-
| style="border:1pt solid #000000;padding:0.097cm;"| Type''' test 4 -eq 5; echo $?''' >>press '''Enter'''
| style="border:1pt solid #000000;padding:0.097cm;"| Let's take another expression; type:

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

Press''' Enter'''




|-
| style="border-top:none;border-bottom:1pt solid #000000;border-left:1pt solid #000000;border-right:1pt solid #000000;padding:0.097cm;"| 
| style="border-top:none;border-bottom:1pt solid #000000;border-left:1pt solid #000000;border-right:1pt solid #000000;padding:0.097cm;"| It returns''' one '''which means''' false''' 

i.e.''' 4''' is not equal to''' 5.'''

|-
| style="border:1pt solid #000000;padding:0.097cm;"| Type'''<nowiki> [ 4 -eq 5 ]; echo $?</nowiki>''' >>press '''Enter'''
| style="border:1pt solid #000000;padding:0.097cm;"| Now let's write the same expression within square brackets, type:

'''opening square bracket''' space''' 4''' space''' hyphen eq''' space''' 5''' space''' closing square bracket semicolon''' space''' echo''' space''' dollar question mark'''

Press''' Enter'''

|-
| style="border-top:none;border-bottom:1pt solid #000000;border-left:1pt solid #000000;border-right:1pt solid #000000;padding:0.097cm;"| 
| style="border-top:none;border-bottom:1pt solid #000000;border-left:1pt solid #000000;border-right:1pt solid #000000;padding:0.097cm;"| It also returns''' one '''which means '''false''' 


i.e.''' 4''' is not equal to''' 5'''

|-
| style="border:1pt solid #000000;padding:0.097cm;"| 
| style="border:1pt solid #000000;padding:0.097cm;"| This can be extended for other kind of testing.

Please type''' man test''' on terminal and explore its usage.

|-
| style="border-top:none;border-bottom:1pt solid #000000;border-left:1pt solid #000000;border-right:1pt solid #000000;padding:0.097cm;"| 
| style="border-top:none;border-bottom:1pt solid #000000;border-left:1pt solid #000000;border-right:1pt solid #000000;padding:0.097cm;"| Let us go back to our slides.

|-
| style="border:1pt solid #000000;padding:0.097cm;"| Display Slide 7


'''if'''

'''Syntax:'''

'''if<nowiki> [ </nowiki>condition ]; thencommandsfi'''
| style="border:1pt solid #000000;padding:0.097cm;"| Now we will see the syntax for''' if''' statement-


'''if space opening square bracket space expression space closing square bracket semicolon space then'''


On the next line,''' '''type '''commands '''or '''statements '''that you want to execute.


Lastly, end the''' if loop''' with '''fi.'''

|-
| style="border:1pt solid #000000;padding:0.097cm;"| Display Slide 8



| style="border:1pt solid #000000;padding:0.097cm;"| The Basic rules of condition are:

1. Always keep spaces between the brackets and the expression.

2. Always terminate the line using '''semicolon''' before keyword “'''then”'''.

3. '''Semicolon '''is used to terminate the statement or an expression.

|-
| style="border-top:none;border-bottom:1pt solid #000000;border-left:1pt solid #000000;border-right:1pt solid #000000;padding:0.097cm;"| Display Slide 9
| style="border-top:none;border-bottom:1pt solid #000000;border-left:1pt solid #000000;border-right:1pt solid #000000;padding:0.097cm;"| 4. It is recommended'' ''to quote string variables, if you use them in conditions.

5. Don't forget to close the conditional block with “'''fi”.'''

|-
| style="border-top:none;border-bottom:1pt solid #000000;border-left:1pt solid #000000;border-right:1pt solid #000000;padding:0.097cm;"| 
| style="border-top:none;border-bottom:1pt solid #000000;border-left:1pt solid #000000;border-right:1pt solid #000000;padding:0.097cm;"| Let us see an example on''' if statement.'''

Come back to the '''terminal'''.

|-
| style="border:1pt solid #000000;padding:0.097cm;"| On '''terminal>> '''Type

'''gedit simpleif.sh>> '''Press''' Enter'''



| style="border:1pt solid #000000;padding:0.097cm;"| I will open an already existing script file named '''simpleif.sh'''

This Bash script displays the message “count is 100” when the '''count''' equals to 100

|-
| style="border:1pt solid #000000;padding:0.097cm;"| '''Highlight'''

'''<nowiki>#!/bin/bash</nowiki>'''
| style="border:1pt solid #000000;padding:0.097cm;"| This is first line of''' Bash shell''' script known as''' shebang line.'''

|-
| style="border:1pt solid #000000;padding:0.097cm;"| '''Highlight'''

'''count<nowiki>=100</nowiki>'''



| style="border:1pt solid #000000;padding:0.097cm;"| '''An integer 100''' is assigned to a''' variable count.'''

Note that, there should not be any space between''' count, <nowiki>=</nowiki>''' and''' 100.'''

|-
| style="border:1pt solid #000000;padding:0.097cm;"| '''Highlight'''

'''<nowiki>if [ $count -eq 100 ] ; then</nowiki>'''



| style="border:1pt solid #000000;padding:0.097cm;"| This expression checks'' ''whether''' count''' is''' equal to hundred.'''

Here''' -eq '''is '''comparison operator''' .

|-
| style="border:1pt solid #000000;padding:0.097cm;"| '''Highlight'''

'''echo "Count is 100"'''

'''fi'''
| style="border:1pt solid #000000;padding:0.097cm;"| If the condition is true, it will display the message''' count is 100'''

'''fi''' is to end '''if '''block.

|-
| style="border-top:none;border-bottom:1pt solid #000000;border-left:1pt solid #000000;border-right:1pt solid #000000;padding:0.097cm;"| 
| style="border-top:none;border-bottom:1pt solid #000000;border-left:1pt solid #000000;border-right:1pt solid #000000;padding:0.097cm;"| Save the file by pressing''' “Ctrl + s”'''

|-
| style="border:1pt solid #000000;padding:0.097cm;"| '''On Terminal>>'''Type''' chmod +x simpleif.sh>>'''Press''' Enter>>'''Type''' ./simpleif.sh >>'''press '''Enter'''



| style="border:1pt solid #000000;padding:0.097cm;"| Now go to''' Terminal.''' 

To make the file executable, type:

'''chmod space plus x space simpleif.sh''' .

Press''' Enter'''


Now type dot slash''' simpleif.sh.'''

Press''' Enter'''

|-
| style="border:1pt solid #000000;padding:0.097cm;"| Highlight

'''Output'''

'''Count is 100'''
| style="border:1pt solid #000000;padding:0.097cm;"| Here it is displayed:

'''Count is 100.'''




|-
| style="border-top:none;border-bottom:1pt solid #000000;border-left:1pt solid #000000;border-right:1pt solid #000000;padding:0.097cm;"| 
| style="border-top:none;border-bottom:1pt solid #000000;border-left:1pt solid #000000;border-right:1pt solid #000000;padding:0.097cm;"| Try changing the value of''' '''variable '''count '''and execute the script.

|-
| style="border-top:none;border-bottom:1pt solid #000000;border-left:1pt solid #000000;border-right:1pt solid #000000;padding:0.097cm;"| 
| style="border-top:none;border-bottom:1pt solid #000000;border-left:1pt solid #000000;border-right:1pt solid #000000;padding:0.097cm;"| Switch back to slides. Now, let us study the''' if-else'''.

|-
| style="border:1pt solid #000000;padding:0.097cm;"| Display Slide 10

'''if-else'''

'''synax:'''

'''<nowiki>if [ condition ]; then</nowiki>commands'''

'''else'''

'''some other commands'''

'''fi'''
| style="border:1pt solid #000000;padding:0.097cm;"| The general syntax is:

'''if space opening square bracket space condition space close square bracket semicolon space then'''

On the next line, type '''commands'''

On the next line, there is the '''else '''statement

and again type other''' some other commands'''

on the next line, type''' fi''' to end '''if '''block

|-
| style="border:1pt solid #000000;padding:0.097cm;"| On '''Terminal>> '''Type

'''gedit ifelse.sh>> '''Press''' Enter'''



| style="border:1pt solid #000000;padding:0.097cm;"| Let us study the usage of''' if-else''' with an interesting password program.

Come back to the terminal.

I will open the file''' ifelse.sh'''

|-
| style="border:1pt solid #000000;padding:0.097cm;"| '''<nowiki>#!/</nowiki>bin/bash'''



| style="border:1pt solid #000000;padding:0.097cm;"| This is the''' shebang line'''

|-
| style="border:1pt solid #000000;padding:0.097cm;"| '''PASS<nowiki>="</nowiki>abc123"'''
| style="border:1pt solid #000000;padding:0.097cm;"| Here,''' abc123''' is stored in the variable''' PASS'''

As''' abc123''' is a string, it should be written within''' double-quotes.'''

|-
| style="border:1pt solid #000000;padding:0.097cm;"| '''read -s -p "Enter password:" mypassword'''
| style="border:1pt solid #000000;padding:0.097cm;"| The''' read command''' reads''' one line''' of data from the''' standard input.'''

In this case, standard input is our keyboard'''.'''

'''Hyphen s''' is''' '''for '''silent '''mode.

Which means the entered password will not be displayed as we type.

We don't want others to see our password.

'''Hyphen p''' is''' for prompt.'''

It will display a string “'Enter password: ” before it takes input from user.

'''mypassword''' is a''' variable.'''

It stores the''' string,''' in this case the''' '''password entered by the user.

|-
| style="border:1pt solid #000000;padding:0.097cm;"| '''if<nowiki> [ "$</nowiki>mypassword" = "$PASS" ]; then'''
| style="border:1pt solid #000000;padding:0.097cm;"| This checks that the entered password matches the value of the variable '''PASS.'''

It is stored in a variable''' mypassword'''

|-
| style="border:1pt solid #000000;padding:0.097cm;"| '''echo -e “\nPassword accepted”'''

'''else'''

'''echo -e “\nAccess denied”'''
| style="border:1pt solid #000000;padding:0.097cm;"| If the password matches, it will display the'' ''message

'''“Password accepted”'''

else it will display

'''“Access denied”'''

|-
| style="border:1pt solid #000000;padding:0.097cm;"| '''fi'''
| style="border:1pt solid #000000;padding:0.097cm;"| '''fi '''is the end of '''if-else loop'''

|-
| style="border:1pt solid #000000;padding:0.097cm;"| 
| style="border:1pt solid #000000;padding:0.097cm;"| Now save the file by pressing “'''Ctrl s”''' and close it.

|-
| style="border-top:none;border-bottom:1pt solid #000000;border-left:1pt solid #000000;border-right:1pt solid #000000;padding:0.097cm;"| On '''Terminal>> '''Type

'''chmod +x ifelse.sh'''>>press '''enter'''

>>'''./ifelse.sh>>'''press '''enter'''



| style="border-top:none;border-bottom:1pt solid #000000;border-left:1pt solid #000000;border-right:1pt solid #000000;padding:0.097cm;"| Go to the '''terminal''', to make it'' ''an'' ''executable type:

'''chmod''' space''' plus x''' space''' ifelse.sh''' .

Press''' Enter.'''

Now type''' dot slash ifelse.sh .'''

Press''' Enter.'''

|-
| style="border:1pt solid #000000;padding:0.097cm;"| On''' Terminal>>'''Type '''abc>> '''Press''' Enter password: '''


'''Output'''

'''Access denied'''
| style="border:1pt solid #000000;padding:0.097cm;"| When we run this program on '''terminal''', it is displayed:

Enter''' password''':

I will type '''abc '''and press '''Enter'''

As the password entered is wrong, it displays the message as''' “Access denied”'''

|-
| style="border-top:none;border-bottom:1pt solid #000000;border-left:1pt solid #000000;border-right:1pt solid #000000;padding:0.097cm;"| '''./ifelse.sh'''

'''Enter password: abc123'''


'''Password accepted'''
| style="border-top:none;border-bottom:1pt solid #000000;border-left:1pt solid #000000;border-right:1pt solid #000000;padding:0.097cm;"| Let's execute again, but this time we will enter password as '''abc123'''

It is displayed as:

'''Password accepted'''


'''<nowiki><PAUSE></nowiki>'''

|-
| style="border:1pt solid #000000;padding:0.097cm;"| Display Slide 11

'''Summary'''
| style="border:1pt solid #000000;padding:0.097cm;"| '''Summary'''

Lets summarize what we have covered in this tutorial,


* We saw how to use '''test '''command with examples'''.'''

and

* Simple''' if'''

&

* '''if else '''statement in a BASH script.



|-
| style="border:1pt solid #000000;padding:0.097cm;"| Display Slide 12

Assignment
| style="border:1pt solid #000000;padding:0.097cm;"| As an assignment


# Write a script which will take your name as an input
# It should check this name with your system's username.<br/> 3. If the username matches, it should greet you by displaying “'''Hello'''”

# Else, it should display “'''Try again'''”

HINT: Your system's username is stored in a variable '''$USER'''

|-
| style="border-top:none;border-bottom:1pt solid #000000;border-left:1pt solid #000000;border-right:1pt solid #000000;padding:0.097cm;"| Display Slide 13
| style="border-top:none;border-bottom:1pt solid #000000;border-left:1pt solid #000000;border-right:1pt solid #000000;padding:0.097cm;"| Watch the video available at the link shown below

It summarizes the Spoken Tutorial project 

If you do not have good bandwidth, you can download and watch it

|-
| style="border:1pt solid #000000;padding:0.097cm;"| Display Slide 14


Spoken Tutorial Workshops
| style="border:1pt solid #000000;padding:0.097cm;"| The Spoken Tutorial Project Team 

Conducts workshops using spoken tutorials 

Gives certificates to those who pass an online test 

For more details, please write to

contact@spoken-tutorial.org

|-
| style="border:1pt solid #000000;padding:0.097cm;"| Display Slide 15

Acknowledgement
| style="border:1pt solid #000000;padding:0.097cm;"| Spoken Tutorial Project is a part of the Talk to a Teacher project

It is supported by the National Mission on Education through ICT, MHRD, Government of India

More information on this Mission is available at: http://spoken-tutorial.org/NMEICT-Intro




|-
| style="border:1pt solid #000000;padding:0.097cm;"| 
| style="border:1pt solid #000000;padding:0.097cm;"| The script has been contributed by FOSSEE and spoken-tutorial team.


This is Ashwini from IIT Bombay.

Thank you for joining.

|}