TranslateProject/sources/tech/20230724.0 ⭐️⭐️ Bash Basics Series 7 If Else Statement.md

Bash Basics Series #7: If Else Statement

Bash supports if-else statements so that you can use logical reasoning in your shell scripts.

The generic if-else syntax is like this:

if [ expression ]; then

  ## execute this block if condition is true else go to next

elif [ expression ]; then

  ## execute this block if condition is true else go to next

else 

  ## if none of the above conditions are true, execute this block

fi

As you can notice:

• elif is used for "else if" kind of condition
• The if else conditions always end with fi
• the use of semicolon ; and then keyword

Before I show the examples of if and else-if, let me share common comparison expressions (also called test conditions) first.

Test conditions

Here are the test condition operators you can use for numeric comparison:

Condition	Equivalent to true when
$a -lt $b	$a < $b ($a is less than $b)
$a -gt $b	$a > $b ($a is greater than $b)
$a -le $b	$a <= $b ($a is less or equal than $b)
$a -ge $b	$a >= $b ($a is greater or equal than $b)
$a -eq $b	$a is equal to $b
$a -ne $b	$a is not equal to $b

If you are comparing strings, you can use these test conditions:

Condition	Equivalent to true when
"$a" = "$b"	$a is same as $b
"$a" == "$b"	$a is same as $b
"$a" != "$b"	$a is different from $b
-z "$a"	$a is empty

There are also conditions for file type check:

Condition	Equivalent to true when
-f $a	$a is a file
-d $a	$a is a directory
-L $a	$a is a link

Now that you are aware of the various comparison expressions let's see them in action in various examples.

Use if statement in bash

Let's create a script that tells you if a given number is even or not.

Here's my script named even.sh:

#!/bin/bash

read -p "Enter the number: " num

mod=$(($num%2))

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

The modulus operation (%) returns zero when it is perfectly divided by the given number (2 in this case).

🚧 Pay special attention to space. There must be space between the opening and closing brackets and the conditions. Similarly, space must be before and after the conditional operators (-le, == etc).

Here's what it shows when I run the script:

Did you notice that the script tells you when a number is even but it doesn't display anything when the number is odd? Let's improve this script with the use of else.

Use if else statement

Now I add an else statement in the previous script. This way when you get a non-zero modulus (as odd numbers are not divided by 2), it will enter the else block.

#!/bin/bash

read -p "Enter the number: " num

mod=$(($num%2))

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

Let's run it again with the same numbers:

As you can see, the script is better as it also tells you if the number is odd.

Use elif (else if) statement

Here's a script that checks whether the given number is positive or negative. In mathematics, 0 is neither positive nor negative. This script keeps that fact in check as well.

#!/bin/bash

read -p "Enter the number: " num

if [ $num -lt 0 ]; then
	echo "Number $num is negative"
elif [ $num -gt 0 ]; then
	echo "Number $num is positive"
else
	echo "Number $num is zero"
fi

Let me run it to cover all three cases here:

Combine multiple conditions with logical operators

So far, so good. But do you know that you may have multiple conditions in a single by using logical operators like AND (&&), OR (||) etc? It gives you the ability to write complex conditions.

Let's write a script that tells you whether the given year is a leap year or not.

Do you remember the conditions for being a leap year? It should be divided by 4 but if it is divisible by 100, it's not a leap year. However, if it is divisible by 400, it is a leap year.

Here's my script.

#!/bin/bash

read -p "Enter the year: " year

if [[ ($(($year%4)) -eq 0 && $(($year%100)) != 0) || ($(($year%400)) -eq 0) ]]; then
	echo "Year $year is leap year"
else
	echo "Year $year is normal year"
fi

💡 Notice the use of double brackets above. It is mandatory if you are using logical operators.

Verify the script by running it with different data:

🏋️ Exercise time

Let's do some workout :)

Exercise 1: Write a bash shell script that checks the length of the string provided to it as an argument. If no argument is provided, it prints 'empty string'.

Exercise 2: Write a shell script that checks whether a given file exists or not. You can provide the full file path as the argument or use it directly in the script.

Hint: Use -f for file

Exercise 3: Enhance the previous script by checking if the given file is regular file, a directory or a link or if it doesn't exist.

Hint: Use -f, -d and -L

Exercise 3: Write a script that accepts two string arguments. The script should check if the first string contains the second argument as a substring.

Hint: Refer to the previous chapter on bash strings

You may discuss your solution in the Community:

I hope you are enjoying the Bash Basics Series. In the next chapter, you'll learn about using loops in Bash. Keep on bashing!

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via: https://itsfoss.com/bash-if-else/

作者：Abhishek Prakash 选题：lkxed 译者：译者ID 校对：校对者ID

本文由 LCTT 原创编译，Linux中国 荣誉推出