Mastering IF-Else Statements in BASH Scripting: A Complete Tutorial

This tutorial provides a comprehensive overview of IF-Else statements in BASH scripting. From the basics of condition testing to complex nested statements, we will explore a variety of applications using real-world examples.

Beginner30 minutes

Step 1: Introduction to IF-Else Statements

IF-Else statements are used in BASH for conditional execution of code. The structure is if [condition]; then commands; else commands; fi.

if [ "$a" -lt 20 ]; then
   echo "a is less than 20"
else
   echo "a is not less than 20"
fi

Practice Exercise

Write a BASH script that prints 'Even' or 'Odd' for a given number.

Show Solution

if [ $(($number%2)) -eq 0 ]; then
   echo "Even"
else
   echo "Odd"
fi

Step 2: Understanding the Condition Bracket

The condition in an IF-Else statement is enclosed in square brackets. The left bracket [ is actually a command that returns a status. A space must be included after [ and before ].

if [ "$a" -lt 20 ]; then
   # commands
fi

Practice Exercise

Fix the following incorrect BASH IF-Else statement: if [$a -lt 20]; then echo "Correct"; fi

Show Solution

if [ $a -lt 20 ]; then
   echo "Correct"
fi

Step 3: Elif Statements

Elif (else if) is used for multiple conditions. It allows for more complex conditional branches.

if [ "$a" -lt 20 ]; then
   # commands
elif [ "$a" -eq 20 ]; then
   # commands
else
   # commands
fi

Practice Exercise

Write a BASH script that categorizes a person's age as 'Child', 'Teen', 'Adult', or 'Senior'.

Show Solution

if [ $age -lt 13 ]; then
   echo "Child"
elif [ $age -lt 20 ]; then
   echo "Teen"
elif [ $age -lt 65 ]; then
   echo "Adult"
else
   echo "Senior"
fi

Step 4: Nested IF-Else Statements

Nested IF-Else statements involve placing an IF-Else statement within another. This allows for even more complex decision-making.

if [ "$a" -lt 20 ]; then
   if [ "$a" -gt 10 ]; then
      # commands
   fi
fi

Practice Exercise

Create a BASH script that checks if a number is within the range 10-20, inclusive. Print a specific message for each condition.

Show Solution

if [ $number -le 20 ]; then
   if [ $number -ge 10 ]; then
      echo "Number is within range."
   else
      echo "Number is less than range."
   fi
else
   echo "Number is greater than range."
fi

Step 5: Combining Conditions

Conditions can be combined using AND (-a or &&) and OR (-o or ||) operators.

if [ "$a" -gt 10 -a "$a" -lt 20 ]; then
   # commands
fi

Practice Exercise

Write a BASH script that checks if a number is between 10-20 or 30-40.

Show Solution

if [ $number -ge 10 -a $number -le 20 ] || [ $number -ge 30 -a $number -le 40 ]; then
   echo "Number is within range."
else
   echo "Number is out of range."
fi