Bash ! (NOT) Operator

Bash `!` (NOT) Operator

The ! operator in Bash is used to negate a condition. It is commonly referred to as the “NOT” operator.

This operator inverts the exit status of a command or a conditional expression. In simple terms, if a command or expression returns true (exit status 0), the ! operator changes it to false (non-zero exit status), and if it returns false (non-zero exit status), it changes it to true (exit status 0).

The ! operator is commonly used in conditional statements to check the opposite of a condition or to negate a condition. It can be used with if statements, loops, and other conditional expressions.

Syntax of NOT Operator

</>

Copy

if ! command; then
  # commands to run if the command fails
fi

Here, the ! is placed before a command or condition. If the command or condition returns a failure (non-zero exit status), the ! negates it and makes the if condition true.

The ! operator is typically used with commands or test expressions to perform a logical negation. If the command returns false, the negation makes it true.

The ! operator can be used in various places where Bash expects a condition, such as within if statements, while loops, and with the test command.

Examples of `!` (NOT) Operator

Example 1: Using `!` to Negate a Command

In this example, we will use the ! operator to negate the success of a command. If the command fails (non-zero exit status), we can use ! to perform an action when the command fails.

Program

check_directory.sh

</>

Copy

#!/bin/bash

# Try to create a directory
mkdir /tmp/test_directory

# Check if the command fails using the NOT operator
if ! mkdir /tmp/test_directory; then
  echo "Directory could not be created, it may already exist."
else
  echo "Directory created successfully."
fi

Steps Explanation:

The script uses the ! operator to negate the success of the mkdir command.
If the mkdir command fails (for example, if the directory already exists), the if condition will be true and the script will output that the directory could not be created.

Output

Directory could not be created, it may already exist.

Example 2: Using `!` with `test` Command

The test command can be used to evaluate conditions in Bash scripts. We can use the ! operator with test to negate conditions. In this example, we will check if a file exists using the test command and negate the result to check if the file does not exist.

Program

check_file.sh

</>

Copy

#!/bin/bash

# Check if the file does not exist
if ! test -f /tmp/test_file.txt; then
  echo "File does not exist."
else
  echo "File exists."
fi

Steps Explanation:

The script checks if the file /tmp/test_file.txt does not exist using the ! operator with test.
If the file does not exist, the script will output that the file is missing.

Output

File does not exist.

TutorialKart

Bash ! (NOT) Operator