How Do You Attach and Detach from Docker's Process?

TLDR

You can "attach" to a running Docker container to view its output or interact with its main process using docker attach <container>. To detach without stopping the container, use the keyboard shortcut Ctrl-p then Ctrl-q. This lets the container keep running in the background.

Why Attach to a Container?

Attaching is useful when you want to:

• See real-time logs or output from the main process
• Interact with a shell or foreground process
• Debug or monitor a running container

How to Attach to a Running Container

To attach to a running container, use:

docker attach <container-name-or-id>

This connects your terminal to the container's main process (usually PID 1). You'll see its standard output and can interact if it's a shell or interactive app.

Example:

docker run -it --name demo ubuntu bash
# In another terminal:
docker attach demo

Now, anything typed in the attached terminal is sent to the container's shell.

How to Detach Without Stopping the Container

To safely detach and leave the container running, use this keyboard sequence:

Ctrl-p Ctrl-q

• Hold Ctrl, press p, then press q (release Ctrl after).
• Your terminal returns to the host shell, and the container keeps running.

What Happens If You Use Ctrl-c?

Pressing Ctrl-c sends an interrupt signal (SIGINT) to the container's main process. This usually stops the process and the container exits. Use Ctrl-p Ctrl-q to detach instead if you want the container to keep running.

Reattaching and Multiple Attachments

• You can attach again later with docker attach <container>.
• Multiple terminals can attach to the same container, but input/output may get mixed.
• For a new shell session, use docker exec -it <container> bash instead of attach.

Troubleshooting

• If you can't detach, check if your terminal is capturing the key sequence (try a different terminal or SSH session).
• If the container stops when you detach, you may have pressed Ctrl-c or the main process exited.
• For containers started with -d (detached mode), you can still attach later.

Conclusion

Attaching and detaching from Docker containers is a handy way to interact with running processes. Remember to use Ctrl-p Ctrl-q to detach safely, and use docker exec for new shell sessions without interfering with the main process.