Usages of NOT (!) Operator in Bash Scripting, the logical operator “!“ (NOT) is used to flip a command’s exit state. It is useful error checking and conditions. I’ll go over three distinct uses of the NOT operator in bash scripting in this article. Now let’s get going.

Key Takeaways

• Understanding the role of the NOT operator in bash script.
• Checking the existence of the variable value using the NOT operator in a bash script.

NOT Operator in Shell Script

Usages of NOT (!) Operator In shell scripting, the NOT operator is represented by the exclamation mark (!). It is used to negate a condition or logical expression. When you place the NOT operator before a condition, it flips the condition’s truth value. For example, ! true becomes false, and ! false becomes true. The  NOT operator is commonly used in conditional statements to reverse the logic and perform actions when a condition is not met. It’s a fundamental tool for making decisions and creating complex logic in shell scripts.

Syntax of the NOT Operator in Bash Script

In Bash, the basic syntax of NOT Operator is as follows:

if [ ! condition ]; then
# Code to execute if the condition is not true
fi

Or,

if [[ ! condition ]]; then
# Code to execute if the condition is not true
fi

Or,

if ! test condition; then
command1
command2
fi

The exclamation mark (!) in each syntax flips the conditions’ output value, returning false in the case that the condition returns false and true in the case that the conditional exit status is true.

4 Practical Examples of Using the NOT Operator in Bash Scripts

Here are four real-world instances that I have explained for your review. The example will demonstrate how to compare strings and integers effectively. Additionally, you will discover how to use the NOT operator to determine if a variable is empty or not. Finally, you will discover how to eliminate the IF conditional. Remember to execute those scripts from your command line.

Example 01: Using NOT Operator for String Comparison in Bash

The aim of my initial code is to enable the user to enter two strings and thereafter perform a comparison to ascertain their equality. Verify, adhere to the instructions provided below, and repeat the process in your terminal.

Steps to Follow >

1. At first, launch an Ubuntu Terminal.
2. Write the following command to open a file in Nano:

   nano string_comparison.sh
   EXPLANATION

   • nano: Opens a file in the Nano text editor.
   • string_comparison.sh: Name of the file.
3. Copy the script mentioned below:

   #!/bin/bash
   
   #taking the input value from the user
   read -p "Enter the first string: " str1
   read -p "Enter the second string: " str2
   
   #checking whether the given strings are equal or not
   if [ "$str1" != "$str2" ]; then
   echo "Strings are not equal."
   else
   echo "Strings are equal."
   fi
   
   EXPLANATION

   Two inputted strings can be compared by the user with the help of the supplied Bash script. It starts by asking the user to enter the first string, which is then saved in the str1 variable. It then gives the user instructions to enter the second string and stores it in str2.

4. Press CTRL+O and ENTER to save the file; CTRL+X to exit.
5. Use the following command to make the file executable:

   chmod u+x string_comparison.sh

   EXPLANATION

   • chmod: Changes the permissions of files and directories.
   • u+x: Here, u refers to the “user” or the owner of the file and +x specifies the permission being added, in this case, the “execute” permission. When u+x is added to the file permissions, it grants the user (owner) permission to execute (run) the file.
   • string_comparison.sh: File name.
6. Run the script by the following command:

   ./string_comparison.sh

   Upon giving Hello and World strings to the command line, the bash script declares “Strings are not equal” to the command line. However, when I put LinuxSimply twice as the variable value, the code returns “Strings are equal”, indicating the string match based on the provided string.

Example 02: Inserting NOT Operator for Integer Comparison in Bash

I want to allow users to enter two numerical numbers and compare them to see if they are equal or not, using the previous code as a guide. I’ll use a bash script with the NOT operator to complete the operation. Adhere to the script provided here.

To save and make the script executable, simply follow the instructions in Example 01.

Script (integer_comparison.sh) >

#!/bin/bash

read -p "Enter the first number: " number1
read -p "Enter the second number: " number2

if [ ! "$number1" -eq "$number2" ]; then
echo "Numbers are not equal."
else
echo "Numbers are equal."
fi

Use the following command to run the code

./integer_comparison.sh

As you can understand from the image given above, the code can successfully identify the equality of two given integers and return the corresponding response to the command line.

Example 03: Checking the Variable Whether Empty Using NOT Operator in Bash

The goal of the code in my previous example is to let the user enter a value and then assess if it is empty or not. Like previously, I’ll use a NOT operator to complete the operation.

To save and make the script executable, simply follow the instructions in Example 01.

Script (variable_check.sh) >

#!/bin/bash

read -p "Enter a value: " userVar

if [ ! -z "$userVar" ]; then
echo "The variable is not empty."
else
echo "The variable is empty."
fi

Run the code by using the following command

./variable_check.sh

The accompanying graphic illustrates how the bash code prompts the user to enter a value. In the first instance, the code returns “The variable is empty” to the command line since I neglected to set the assigned variable to a null value. But in the second scenario, the script states that “The variable is not empty” after the user enters a text representing the “Random Value” into the variable.

Example 04: Negate the IF condition in a Bash Script Using the ! (NOT) Operator

You will learn how to negate an if condition in a bash script using two different methods by examining the script provided below, which uses the! NOT operator. The difference between the two is that one places the! sign inside the double square bracket while the other leaves it outside.

The expression(s) enclosed in the brackets will be run and the result will be inverted when the NOT operator is located outside the double square brackets [[. This implies that if you use more than one expression, the outcome will be canceled out. However, the operator will only be applied to the specific phrase if you place the NOT Operator inside the double square.

Script (negate_if.sh) >

#!/bin/bash

# Assign a value to the 'num' variable
read -p "Give me the first number to be evaluated: " num1

# Check if 'num1' is not equal to 0
if ! [[ $num1 -eq 0 ]]
then
# If the condition is true, print a message
echo "Value of first number is not 0"
fi

read -p "Insert the second number: " num2

# Check if 'num2' is not equal to 0
if [[ ! $num2 -eq 0 ]]
then
# If the condition is true, print a message
echo "Value of the second number is not 0 either"
fi

Run the code by using the following command

./negate_if.sh

As you can see from the image above, both methods return the same output successfully.

Conclusion

The NOT operator in Bash scripting effectively flips an expression or condition’s truth value. The NOT operator can be used to convert true statements into false statements and false statements into true statements. I’ve covered the basics of the Bash NOT operator in this article. However, please feel free to leave a comment below if you have any questions or concerns about this article. I’m grateful.