Understanding Docker Run Arguments: -I, -T, and --Attach

When working with Docker, you often use various command-line arguments to customize the behavior of containers when they are launched. Three commonly used arguments are -i, -t, and --attach. These arguments are often used together, and they serve different purposes in controlling how your container interacts with the terminal and user input.

-i - Interactive Mode

The -i flag stands for “interactive.” When you include this flag in your docker run command, it tells Docker to keep STDIN (standard input) open, allowing you to interact with the container’s command or application. Here’s what it means in more detail:

• Interactive Mode: Docker starts the container in interactive mode, which means that it listens for input from your terminal. You can send input to the container and receive output from it in real-time.

• Termination Behavior: If you quit or close the terminal session where the container is running (e.g., by pressing Ctrl+C or typing exit), Docker will terminate the container. This behavior is because the container is tied to the terminal’s lifecycle. When the terminal ends, so does the container.

Example:

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docker run -i ubuntu

In this example, you start an interactive Ubuntu container, and you can run commands and interact with it in the terminal. If you exit the terminal, the container will be terminated.

-t - Allocate a Pseudo-Terminal (TTY)

The -t flag stands for “allocate a pseudo-TTY.” It is used to allocate a terminal session for the container. When you include this flag, Docker provides a terminal-like interface for your container’s command or application. Here’s what it means:

• Pseudo-Terminal (TTY): The -t flag allocates a pseudo-terminal (TTY) for the container. This makes the container’s output more readable and allows you to see the formatting correctly.

• Control Characters: Without the -t flag, control characters like Ctrl+C might not work as expected inside the container. The -t flag ensures that these control characters are captured correctly.

Example:

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docker run -it ubuntu

In this example, you start an interactive Ubuntu container with a pseudo-TTY, making it easier to work with the container’s command line.

--attach - Attach to STDIN, STDOUT, and STDERR

The --attach flag is used to attach to the standard input (STDIN), standard output (STDOUT), and standard error (STDERR) streams of a running container. When you include this flag, you can interact with the container’s input and output streams.

• Attach Mode: --attach attaches your terminal to the container’s input and output streams, allowing you to send input to the container and receive its output.

• Use Cases: This flag is handy when you want to connect to a running container and interact with its shell or running process.

Example:

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docker attach <container_id>

In this example, you use docker attach to attach to a running container by specifying its container ID.

To summarize, when using Docker, the -i, -t, and --attach flags are often used together to create an interactive and TTY-enabled container with the ability to attach to its terminal. This combination makes it easier to work with containers and run interactive applications within them.