Bash Fundamentals Sequence #7: If Else Assertion

Bash helps if-else statements so to use logical reasoning in your shell scripts.

The generic if-else syntax is like this:

if [ expression ]; then

  ## execute this block if situation is true else go to subsequent

elif [ expression ]; then

  ## execute this block if situation is true else go to subsequent

else 

  ## if not one of the above situations are true, execute this block

fi

As you possibly can discover:

• elif is used for “else if” type of situation
• The if else situations at all times finish with fi
• using semicolon ; and then key phrase

Earlier than I present the examples of if and else-if, let me share widespread comparability expressions (additionally known as take a look at situations) first.

Check situations

Listed below are the take a look at situation operators you need to use for numeric comparability:

In case you are evaluating strings, you need to use these take a look at situations:

There are additionally situations for file kind examine:

Now that you’re conscious of the varied comparability expressions let’s have a look at them in motion in varied examples.

Use if assertion in bash

Let’s create a script that tells you if a given quantity is even or not.

Here is my script named even.sh:

#!/bin/bash

learn -p "Enter the quantity: " num

mod=$(($numpercent2))

if [ $mod -eq 0 ]; then
	echo "Quantity $num is even"
fi

The modulus operation (%) returns zero when it’s completely divided by the given quantity (2 on this case).

Here is what it reveals once I run the script:

Did you discover that the script tells you when a quantity is even but it surely does not show something when the quantity is odd? Let’s enhance this script with using else.

Use if else assertion

Now I add an else assertion within the earlier script. This manner whenever you get a non-zero modulus (as odd numbers will not be divided by 2), it’s going to enter the else block.

#!/bin/bash

learn -p "Enter the quantity: " num

mod=$(($numpercent2))

if [ $mod -eq 0 ]; then
	echo "Quantity $num is even"
else
	echo "Quantity $num is odd"
fi

Let’s run it once more with the identical numbers:

As you possibly can see, the script is best because it additionally tells you if the quantity is odd.

Use elif (else if) assertion

Here is a script that checks whether or not the given quantity is constructive or destructive. In arithmetic, 0 is neither constructive nor destructive. This script retains that truth in examine as nicely.

#!/bin/bash

learn -p "Enter the quantity: " num

if [ $num -lt 0 ]; then
	echo "Quantity $num is destructive"
elif [ $num -gt 0 ]; then
	echo "Quantity $num is constructive"
else
	echo "Quantity $num is zero"
fi

Let me run it to cowl all three circumstances right here:

Mix a number of situations with logical operators

Thus far, so good. However are you aware that you could have a number of situations in a single through the use of logical operators like AND (&&), OR (||) and so on? It offers you the power to write down advanced situations.

Let’s write a script that tells you whether or not the given yr is a intercalary year or not.

Do you bear in mind the situations for being a intercalary year? It ought to be divided by 4 however whether it is divisible by 100, it isn’t a intercalary year. Nonetheless, whether it is divisible by 400, it’s a intercalary year.

Here is my script.

#!/bin/bash

learn -p "Enter the yr: " yr

if [[ ($(($year%4)) -eq 0 && $(($year%100)) != 0) || ($(($year%400)) -eq 0) ]]; then
	echo "12 months $yr is intercalary year"
else
	echo "12 months $yr is regular yr"
fi

Confirm the script by working it with totally different knowledge:

🏋️ Train time

Let’s do some exercise 🙂

Train 1: Write a bash shell script that checks the size of the string supplied to it as an argument. If no argument is supplied, it prints ’empty string’.

Train 2: Write a shell script that checks whether or not a given file exists or not. You may present the total file path because the argument or use it immediately within the script.

Trace: Use -f for file

Train 3: Improve the earlier script by checking if the given file is common file, a listing or a hyperlink or if it does not exist.

Trace: Use -f, -d and -L

Train 3: Write a script that accepts two string arguments. The script ought to examine if the primary string comprises the second argument as a substring.

Trace: Confer with the earlier chapter on bash strings

You could focus on your answer within the Group:

Follow Train in Bash Fundamentals Sequence #7: If Else Statements

In case you are following the Bash Fundamentals collection on It’s FOSS, you possibly can submit and focus on the solutions to the train on the finish of the chapter: Fellow skilled members are inspired to offer their suggestions to new members. Do observe that there might be a couple of reply to a given drawback.

I hope you might be having fun with the Bash Fundamentals Sequence. Within the subsequent chapter, you will study utilizing loops in Bash. Carry on bashing!