Difference between revisions of "BASH/C2/Conditional-execution/English"

• Use of test and
• Conditional Statements

We will do this using a few examples.

Prerequisite

spoken-tutorial.org

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

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

System Requirements

• Ubuntu Linux 12.04 OS and
• GNU Bash version 4.1.10

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

Test

• It returns 0 Zero for True and 1 One for False.
• Its return value depends on the evaluation of the expression.

Test

• An expression can be evaluated in two ways-

1. One by using the keyword test
2. Other by using the expression enclosed within square brackets.

Type:

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

Press Enter

i.e. 4 is equal to 4

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

i.e. 4 is equal to 4

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

Press Enter

i.e. 4 is not equal to 5.

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

Press Enter

i.e. 4 is not equal to 5

Please type man test on terminal and explore its usage.

if

Syntax:

if [ condition ]; thencommandsfi

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.

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.

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

Come back to the terminal.

gedit simpleif.sh>> Press Enter

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

#!/bin/bash

count=100

Note that, there should not be any space between count, = and 100.

if [ $count -eq 100 ] ; then

Here -eq is comparison operator .

echo "Count is 100"

fi

fi is to end if block.

To make the file executable, type:

chmod space plus x space simpleif.sh .

Press Enter

Now type dot slash simpleif.sh.

Press Enter

Output

Count is 100

Count is 100.

if-else

synax:

if [ condition ]; thencommands

else

some other commands

fi

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

gedit ifelse.sh>> Press Enter

Come back to the terminal.

I will open the file ifelse.sh

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

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.

It is stored in a variable mypassword

else

echo -e “\nAccess denied”

“Password accepted”

else it will display

“Access denied”

chmod +x ifelse.sh>>press enter

>>./ifelse.sh>>press enter

chmod space plus x space ifelse.sh .

Press Enter.

Now type dot slash ifelse.sh .

Press Enter.

Output

Access denied

Enter password:

I will type abc and press Enter

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

Enter password: abc123

Password accepted

It is displayed as:

Password accepted

<PAUSE>

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.

Assignment

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

1. Else, it should display “Try again”

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

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Acknowledgement

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This is Ashwini from IIT Bombay.

Thank you for joining.