How to Use the If else Statement in Bash

Introduction

The if-else statement in Bash is a conditional statement that executes specific commands depending on the results of an evaluation. The statement evaluates expressions to check whether a condition returns true or false. The if statement runs when a condition returns true and the else statement runs when the condition returns false.

This article explains how to use the if-else statement in Bash to test conditions and execute specific commands when the result is true or false.

If Statement

The if statement evaluates the result of a condition and executes the respective commands when the condition evaluates to true. The statement has the following components:

if condition
then
   # command
fi

In the above code:

• if: Defines the condition that you want to evaluate.
• then: Defines the commands to execute if the condition returns true.
• fi: Defines the end of the statement.

Example:

#!/bin/bash

if [ 1 -eq 1 ]; 
then
    echo "1 is equal to 1"
fi

Output:

1 is equal to 1

The above script evaluates the equality of numeric values and returns the result 1 is equal to 1 if the condition returns true.

If-else Syntax in Bash

The If-else statement evaluates a condition as true or false and executes the if part when true and the else part when false. The statement executes alternative commands if a condition does not match the if block. The following is the if-else statement syntax.

if condition
then
   # command
else
   # command
fi

For example:

#!/bin/bash

if [ 1 -eq 1 ]; 
then
    echo "1 is equal to 1"

else
    echo "The numbers are not equal".
fi

Output:

1 is equal to 1

In the above script, the if-else statement executes different commands depending on whether the condition is true or false. The else block executes alternative commands if the condition is false, while the if block executes commands if the condition is true.

Create If-else Statements in Bash Shell

If-else statements use conditional expressions to evaluate the result of a condition. You can replace the test operator with [ or [[ to evaluate a condition and execute specific commands. Follow the steps below to create a bash shell If else statement to test a condition and execute specific commands.

1. Create an If-else statement to test whether a number is greater than or equal to another number.

   bash Copy

   #!/bin/bash
   
   if [ 10 -gt 5 ]; 
   then
       echo "The number is greater than 5"
   
   else
       echo "The number is less than 5".
   fi
   

   Explain Code

   Output:

   The number is greater than 5

2. Create an If-else statement that checks if a data.txt file exists and creates it if it does not exist.

   bash Copy

   #!/bin/bash
   
   if [ -f data.txt ]; 
   then
   
       echo "File exists"
   
   else
       touch data.txt
       echo "New file created"
   fi
   

   Explain Code

   Output:

   New file created

3. Create another If-else statement that compares the values of two strings.

   bash Copy

   #!/bin/bash
   
   user="admin"
   
   administrator="admin"
   
   if [[ "$user" == "$administrator" ]]; 
   then
       echo "The strings match"
   else
       echo "The strings do not match"
   fi
   

   Explain Code

   Output:

   The strings match

Use If-else Statements with Variables

Variables store data and values in Bash for flexible operations. You can combine If-else statements with variables to evaluate conditions and manipulate variable data based on different results.

1. Create a new auth_users variable and set the value to admin. Then, add a new read statement to capture a user's input to a new user variable. Declare an If-else statement that compares the user input variable to auth_users to evaluate whether the condition is true or false and run specific commands.

   bash Copy

   #!/bin/bash
   
   auth_users="admin"
   
   read -p "What's your username? " user
   
   if [[ "${user}" == "${auth_users}" ]]; 
   then
       echo "You are an administrative user authorized to manage this system"
   else
       echo "You are a standard user, you can run basic commands on the system"
   fi
   

   Explain Code

   Output:

   What's your username? admin
   You are an administrative user authorized to manage this system

   The above script runs the If-else statement and displays the You are an administrative user authorized to manage this system message if the condition returns true. If the condition is false, the script displays You are a standard user, you can run basic commands on the system message.

Use If-elif-else Statements

If-else-if (If-elif-else) is a conditional structure that compares multiple conditions to filter and execute matching commands. The following is the If-elif-else command syntax.

if condition 
then
    # Commands
elif condition
then
    # Commands
else
    echo # Commands
fi

Change the previous if-else statement to include a new elif statement and output another result if the user input is empty.

#!/bin/bash

auth_users="admin"

read -p "What's your username? " user

if [[ "${user}" == "${auth_users}" ]]; 
then
  echo "You are an administrative user authorized to manage this system"
elif [[ -z "$user" ]]
then
  echo "Please enter a username"
  read user
  echo "Hello $user, Greetings from Vultr"
else
  echo "You are a standard user, you can run basic commands on the system"
fi

Output:

What's your username? 
Please enter a username
example
Hello example, Greetings from Vultr

The above script runs the if block when the condition is true. If the user input is empty, the elif block queries for a new variable value while else displays a You are a standard user, you can run basic commands on the system message when the condition is false.

Use Multiple Conditions in If-else Statements in Bash

You can use multiple conditions in bash If-else statements using logical operators such as AND and OR to evaluate two or more commands in one condition. This approach is important when the second command depends on the success or failure of the first command.

For example:

#!/bin/bash

if [ 10 -gt 5 ] && [[ 5 -lt 8 ]]; 
then
    echo "The number is greater than 5"

else
    echo "The number is less than 5"
fi

Output:

The number is greater than 5

In the above script, the AND operator (&&) tests if the first command is successful before executing the second command.

Nested If-else Statements in Bash

Nested if-else statements allow you to place one if-else statement inside another. This structure is useful when evaluating multiple conditions to meet a specific result to allow further decision-making.

#!/bin/bash

if condition
then
  # Commands
else
  # Commands
if condition
then
# Commands
else
# Commands
fi
fi

For example:

#!/bin/bash

if [ 10 -gt 5 ] && [[ 5 -lt 8 ]]; 
then
    echo "The number is greater than 5"
else
    echo "The number is less than 5"
if [[1 -eq 1]];
then
    echo "The second number is equal to 1"
else
    echo "The second number is not equal to 1"
fi
fi

Output:

The number is greater than 5

The above script uses a nested If-else statement in which the inner condition executes only if the outer condition triggers the else block.

Conclusion

You have used the If-else statement in Bash to create and test conditions. If-else is an important conditional statement that evaluates conditions as true or false before executing a set of instructions. You can also use If-else with other Bash components, such as loops to create advanced scripts.