Negate an “If” Condition in Bash [4 Examples]

The negation of an if statement implies that the output of the satisfied condition has been altered. To understand the context of negating an ‘if’ condition in Bash, you can exercise several conditional checks such as string comparison, integer comparison and file comparison etc. Additionally, you can combine multiple conditions together and perform a negation operation on them.

In this article, I will show you 4 practical examples to negate an ‘if’ condition in Bash.

What is Negation in Bash?

Negation is the process of inverting or changing the true value of an expression. The logical NOT (!) operator is used to accomplish the task. This negation (!) operator reverses the logical state of a conditional expression. It allows one to execute a code block when the given condition evaluates to False instead of True and vice versa.

To negate an ‘if’ condition, you can append the NOT (!) operator outside [ ] or [[ ]] and alter the entire logic executed inside like the following syntax:

if ! [ condition ]; then
#code to execute if the condition is false
fi

The negate (!) operator can also be used to invert individual conditional expressions inside [ ] or [[ ]] using the syntax if [ ! condition ];.

4 Examples of Negating an ‘if’ Condition in Bash

Bash avails different forms of negating operational tests on strings, integers, files and directories. Let’s have a clear discussion about them in the following section:

Example 1: Negating String Comparison

The negation (!) operator in string comparison involves reversing the outcome of the string comparison result. In Bash, there are various string comparison operators such as = (equal to), != (not equal to), > (greater than), < (less than), >= (greater than or equal to), <= (less than or equal to), -z (string is empty), and -n (string is not empty).

To negate the string comparison, use the NOT (!) operator along with these string operators. For example:

#!/bin/bash

#Declaring two variables
str1="Linux"
str2="World"

#Checking if two strings are not equal
if ! [ "$str1" = "$str2" ]; then
echo "These strings are not equal."
fi

From the above image, you can see that the given strings are not equal according to the condition.

Example 2: Negating Integer Comparison

In addition to string comparison operators, Bash also provides some integer comparison operators such as -eq (equal to), -ne (not equal to), -gt (greater than), -ge (greater than or equal to), -lt (less than), -le (less than or equal to). The negation (!) operator can also be used to invert the result of the integer comparison expression.

Let’s see an illustration of reversing the integer comparison condition:

#!/bin/bash

#Declaring two variables
num1=2
num2=6

#Checking if two variables are not equal
if ! [ "$num1" -gt "$num2" ]; then
echo "num1 is not less than num2."
fi

The above image shows that the given num1 is not greater than num2 according to the condition.

Example 3: Negating Conditions on Files & Directories

Bash includes a number of file test operators to perform checks on files and directories. It’s very common to rewrite a conditional expression using the negation (!) operator in conjunction with different file test operators.

Below is an example explaining how to negate file-related conditions in Bash:

#!/bin/bash

#Checking if the file exists
if ! [ -f "/home/nadiba/Desktop/to/file1.txt" ]; then #Placing the actual path of the desired file in the ‘/path/to/file.txt’ format.
echo "The file does not exist."
fi

As you can see from the image, in my system, the file1.txt exists in the location ‘/home/nadiba/Desktop’. But as I used the negation (!) operator, it outputs file1.txt as a non-existent file.

Example 4: Negating Multiple Conditions

To negate multiple conditions, use the negation (!) operator with the logical operators such as logical AND (&&), logical OR (||). Negating multiple conditions can be of two types i.e. 1) negating individual expression, 2) negating entire expression simultaneously.

Negating Individual Expression in Combined Conditions

To negate individual conditions in multiple conditional expressions, use the negation (!) operator individually for each test condition. You can either use the syntax if ! [ "$VARIABLE" OPERATOR VALUE ] or if [ ! "$VARIABLE" OPERATOR VALUE ] to accomplish the task.

Following is an example demonstrating the negation of individual expression in multiple conditions:

#!/bin/bash

#Defining two variables
num1=35
num2=23

#Checking multiple conditions
if ! [ "$num1" -lt 17 ] || ! [ "$num2" != 9 ]; then
echo "At least one condition is true."
fi

From the image, you can see that one of these two conditions has been transformed into a true condition because of the inversion by the negation (!) operator.

Negating Multiple Conditions Simultaneously

To negate multiple conditions simultaneously, apply the negation (!) operator only once to a group of combined conditions enclosed within curly braces { }. In this effort, you can use the syntax if ! { [ "$VARIABLE" OPERATOR VALUE ] <logical operator> [ "$VARIABLE" OPERATOR VALUE ] <logical operator> [ "$VARIABLE" OPERATOR VALUE ]; };.

Here’s an example demonstrating how to negate multiple conditions at once:

#!/bin/bash

#Defining variables
var1="X"
var2=false
var3=8

#Checking multiple conditions
if ! { [ "$var1" = "Y" ] && [ "$var2" = "true" ] && [ "$var3" -eq 8 ]; }; then
echo "At least two conditions are true."
fi

In the above image, at least two conditions out of three are true because of the inversion by the negation (!) operator.

Conclusion

Throughout the article, you have learned several schemes of how to negate an ‘if’ condition in Bash. However, among all, select the correct one that is suitable according to your requirements and script readability.

People Also Ask

When should I use ‘negate if’ in Bash scripting?

You should use ‘negate if’ in Bash scripting when you need to run a specific block of code if a certain condition is not satisfied.

Does ‘negate if’ apply only to file and directory checks in Bash?

No, the concept ‘negate if’ is not only confined to file and directory checks in Bash but also it can be used in various schemes such as string comparison, integer comparison etc.

What happens if the condition within ‘negate if’ returns an error in Bash?

If the condition within ‘negate if’ returns an error in Bash, it can affect the performance of the script execution. Generally, the square brackets [ ] are used for performing test operations in Bash. But when the condition inside [ ] returns an error, it leads to undesired consequences. For example:

#!/bin/bash

#Checking if file exists
if ! [ -f "/path/to/nonexistent_file" ]; then
echo "File does not exist."
else
echo "File exists."
fi

Can ‘negate if’ be nested within other conditional statements for complex logic in Bash?

Yes, ‘negate if’ can be nested within other conditional statements for complex logic in Bash. For example:

#!/bin/bash

#Defining variables
num1=7
num2=40
num3=2

#Nested negated conditions
if ! [ "$num1" -eq 12 ]; then
echo "num1 is not equal to 12."
if ! [ "$num2" -lt 39 ]; then
echo "num2 is not less than 39."
if ! [ "$num3" -gt 5 ]; then
echo "num3 is not greater than 5."
fi
fi
fi

Are there any common mistakes when using ‘negate if’ in Bash?

Yes, one of the most common mistakes when using ‘negate if’ in Bash is not setting group conditions properly, as well as not knowing the correct syntax and the logical result of a negated condition.

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