Chapter 5: Bash Scripting

Question 1

What do bash scripts begin with?

Answer 1

!/bin/bash

Question 2

How do we execute a script called “Hello-World.sh”?

Answer 2

./Hello-World.sh

Question 3

How do we make the script executable?

Answer 3

chmod -x script.sh

Question 4

How do we set a variable?

Answer 4

first_name=good

Question 5

How do we call the variable?

Answer 5

$first_name

Question 6

What is wrong with the follow command? good=Hello World

Answer 6

There is a space between Hello and World which causes the command to declare a variable and process World as a separate command. This can be remedied with single or double quotes.

Question 7

What is the difference between single and double quotes in bash scripting?

Answer 7

Single quotes processes every character literally while double quotes processes every character except for $ ` and . Without quotes, spaces function as command delimiters.

Question 8

What is command substitution?

Answer 8

Command substitution is setting the value of a variable to be the output of a certain program or command.

Question 9

How do we use command substitution? Use an example.

Answer 9

An example would be user=$(whoami)

The command must be surrounded by brackets and preceded outside the brackets with a dollar sign.

Question 10

How do we write the first and second arguments within a bash script?

Answer 10

$1 and $2

Question 11

How many arguments can be put into a bash script?

Answer 11

9. Using $1 … $8 $9

Question 12

Which command allows us to capture user input? Use an example.

Answer 12

The read command. The word proceeding it is turned into a variable later on in the script and can be called upon.

Example:

read answer

echo “Your answer was $answer”

Question 13

Within the read command, how do we specify a prompt?

Answer 13

-p

Question 14

Within the read command, how do we specify a prompt and make the input silent as the user is typing it in?

Answer 14

-sp

Question 15

Explain the if statement. Give an example.

Answer 15

The if statement checks to see if a condition is true. The syntax needs to be precise. For example:

if [ ]
then

fi

Question 16

What are the relevance of the two square brackets?

Answer 16

These allow us to utilise operators within the test command. We do not need to use square brackets.

Question 17

What is the general syntax for the else statement?

Answer 17

if
then
else

Question 18

Explain the elif statement and how it works?

Answer 18

elif serves as a second if. For example, if the condition for if to fire is not met, then a second condition test is undertaken using elif. This can happen repeatedly, in other words you can use multiple elif statements constantly testing for conditions until one fires.

Question 19

What are the two bash boolean operators?

Answer 19

&& is AND and || is OR.

Question 20

Explain the AND operator. When will it execute?

Answer 20

The AND operator will only execute if the previous command was successful.

For example: if a grep command failed to executed, the AND operator won’t execute the command after it. The first command must succeed.

Question 21

Explain the OR operator. When will it execute?

Answer 21

The OR operator is the opposite to the AND operator. It will execute only if the previous command failed.

Question 22

Explain loops. What are they used for?

Answer 22

Loops help us with repetitive tasks that we need to run until a certain criteria is met.

Question 23

Explain for loops. What are they used for?

Answer 23

for loops are used for completing a set of commands on items within a list.

Question 24

What is an example of a for loop?

Answer 24

for ip in $(seq 1 10); do echo 10.11.1.$ip; done

Question 25

Explain while loops. When do the execute?

Answer 25

while loops need some condition to be true in order to execute. they are similar to if in this respect.

Question 26

What is an example of a while loop?

Answer 26

while [ ]
do

done

Question 27

Explain the double parenthesises construct. What does it do?

Answer 27

It performs arithmetic expansion and evaluation at the same time. In the counter=1 example in the course material - it is increasing the variable by 1.

Question 28

What is a function?

Answer 28

In terms of bash scripting, we can think of functions as a script within a script.

You could also think of a function as a subroutine or a blackbox - performing an operation.

Question 29

How do you write a function? Use an example.

Answer 29

function_name () {
echo “Hi”
}

function_name

Question 30

What is the difference between a local variable and a global variable?

Answer 30

A global variable can be accessed throughout the entire script while a local variable is only accessible within the function.