docker exec vs attach

Topic: Containers core

docker exec runs a new command in a running container. docker attach attaches to the main process stdin/stdout. Use exec for debugging or one-off commands; avoid attach for long-running or interactive processes. Use this when you need to run a command inside a container.

Intent: How-to

Quick answer

  • docker exec -it container_id /bin/sh runs a new shell. Safe; does not affect main process. Use for debugging and one-off commands.
  • docker attach container_id connects to main process. Ctrl+C can stop the container. Use only when you intend to interact with main process.
  • Prefer exec. Use exec for cron, health checks, and ad-hoc commands. Attach only when you need to send input to main process.

Prerequisites

Steps

  1. Use exec

    docker exec -it mycontainer /bin/sh. Run any command. Exit with exit or Ctrl+D; container keeps running.

  2. When to use attach

    attach only when you need to interact with the main process. Beware Ctrl+C stopping container.

  3. Best practice

    Use exec for shells and one-off commands. Use attach only for intentional main-process interaction.

Summary

Use docker exec for new commands in a container; use attach only when interacting with the main process.

Prerequisites

Steps

Step 1: Use exec

Run docker exec -it container /bin/sh or any command; exit without stopping container.

Step 2: When to use attach

Use attach only when you need to talk to the main process; avoid Ctrl+C if you do not want to stop it.

Step 3: Best practice

Prefer exec for debugging and one-off tasks.

Verification

  • exec runs command and exits; container still running. attach shows main process I/O.

Troubleshooting

No TTY — Use -it for interactive. Container exits on attach exit — That is normal for attach; use exec for shells.

Next steps

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