If Not Condition in Bash

In Bash, figuring out how you interpret the term if not condition before digging into the solutions is mandatory. For example, are you taking as negating the condition/expression, which means make the condition/expression False if it is True and vice versa? Or, you want to make some comparisons to assess inequality; this comparison can be of numbers or strings. We covered all the use cases below, and you can jump around to find your desired solution.

Comparing Numbers/Strings for Inequality

Use the -ne operator to check if two numbers are not equal to each other in Bash.

Use != operator to check if strings are not equal to each other in Bash.

We can also assign string values to variables without wrapping them around with single or double quotes. See the following example.

Don’t add white space before and/or after the = sign while assigning value to the variables, such as string1= 'Mehmood' or string1 = Mehmood; otherwise, you will get an error illustrating the command not found.

Negating the Condition/Expression

Use the ! operator to negate the value of the specified condition meaning turn the resulting value to True if it is False and vice versa.

This example is similar to the one where we compared two numbers using the -ne operator. In the above code, we negated the condition, which means the output of the [ $number1 -ne $number2 ] condition was True because 8 is not equal to 3, but we turned this result into False using ! operator; that’s why if block was not executed but else block.

Similarly, use the ! operator to negate the value of the specified condition for string values by turning the resulting value to True if it is False and vice versa.

For this code, the [ $string1 != $string2 ] resulted in True, but the ! operator made it False, so the echo command within the if block was not executed and we didn’t get any output. So let’s practice the above example with an expression as follows.

Use the ! operator to negate the value of the specified expression for string values by turning the resulting value to True if it is False and vice versa.

Here, we used -z to check if the specified string is empty, so the [ -z "$string"] expression returned False because $string is not empty, but it was turned to True due to using ! operator and resulted in if block execution; otherwise, the else block would be executed.