if command -v docker; then how to check if not exists?

When working with Bash scripting, checking for the existence of a command is frequently needed to ensure that scripts run smoothly without errors. The command command -v is used to check if a particular command (like docker) is available in the system's PATH. To determine if a command does not exist, you can negate this condition in various ways. Here’s a comprehensive guide on how to achieve that.

Checking if Docker Exists or Not

1. Using command -v with Negation

To check if Docker does not exist, you can use the conditional ! operator before the command -v check. Here’s a simple script snippet that demonstrates this:

if ! command -v docker &> /dev/null; then
    echo "Docker is not installed, please install Docker."
else
    echo "Docker is installed."
fi

Explanation:

• command -v docker: This checks for the command docker.
• &> /dev/null: Redirects both stdout and stderr to /dev/null, thus silencing any output.
• !: This negates the condition, so the block executes only if docker is not found.

2. Alternative Method Using which

Another way to check for the existence of Docker is to use the which command. Here’s how you can implement it:

if ! which docker &> /dev/null; then
    echo "Docker is not installed, please install Docker."
else
    echo "Docker is installed."
fi

3. Using type Command

The type command can also be used to verify if Docker exists. Here's an example:

if ! type docker &> /dev/null; then
    echo "Docker is not installed, please install Docker."
else
    echo "Docker is installed."
fi

4. Combining with Docker Commands

If you want to check for Docker and also ensure that it is running, you can expand your script as follows:

if ! command -v docker &> /dev/null; then
    echo "Docker is not installed, please install Docker."
elif ! docker info &> /dev/null; then
    echo "Docker is installed but not running."
else
    echo "Docker is successfully installed and running."
fi

Detailed Breakdown:

• The first check ascertains whether Docker is installed.
• The second check (docker info) verifies whether the Docker daemon is running.

Practical Considerations

1. Error Handling: It’s a good practice to handle errors gracefully. The provided examples suppress output for cleaner execution, which is preferable when checking command status.

2. Script Adaptations: This logic can be easily adapted to check for other commands by simply replacing docker with the desired command name.

3. Environment Setup: Ensure your environment is set up correctly if errors occur during checks. Issues often arise in environments where Docker is not configured, such as continuous integration (CI) environments or restricted shells.

Conclusion

Using the command -v, which, or type commands, you can effectively check if Docker (or any other command) exists in a Bash script. This practice helps prevent script failures and improves usability by informing users about the state of dependencies. Protecting scripts with existence checks enhances reliability, especially in automated systems where Docker is a critical component.

Feel free to adjust the conditions further to fit your specific requirements for additional checks, such as confirming whether Docker is running or properly configured.