Docker exec: the input device is not TTY

Recently, we encountered an issue where certain docker exec commands would not run when executed from a CI/CD pipeline (GitHub Actions in our case). While these commands worked perfectly on our local development machines, migrating them into a script for automation led to unexpected errors. The typical command looked like this:

docker exec -it <container_name> bin/console importmap:install

While this worked on our dev machines, the pipeline failed with the following error:

the input device is not a TTY

Why does this happen?

The culprit is the -it switch, which we often use by habit on local machines. This flag is actually a combination of two options:

• -i / --interactive: Keep STDIN open even if not attached
• -t / --tty: Allocate a pseudo-TTY (teletypewriter)

When you run a command interactively in your terminal, Docker can allocate a TTY. However, in CI/CD pipelines, scripts, or non-interactive shells, there is no TTY device available, so Docker cannot fulfill this request and fails with the error above.

TL;DR: The Solution

Remove the -it flags when running in non-interactive environments!

Use this instead:

docker exec <container_name> bin/console importmap:install

This runs the command non-interactively, which is suitable for CI/CD pipelines and scripts.

When should you use -it?

• Use -it when you want to interact with the container (e.g., open a shell, run a REPL, or debug interactively).
• Omit -it in scripts, CI/CD pipelines, or any non-interactive environment.

You can also use -i or -t separately if you only need one of the features. For example, -i is useful for commands that need STDIN but not a TTY.

What's a TTY?

TTY stands for Teletypewriter. Originally, it referred to physical devices used for textual input and output. Today, it's a software layer that emulates this behavior, allowing programs to interact with users via text.

In Docker, the -t flag allocates a pseudo-TTY, which is necessary for interactive programs. In non-interactive environments (like CI/CD), there is no TTY available, so Docker cannot allocate one, resulting in the error.

Troubleshooting Checklist

If you encounter the "the input device is not a TTY" error:

1. Check your command: Are you using -it unnecessarily?
2. Determine your environment: Is it interactive (local terminal) or non-interactive (script, CI/CD)?
3. Try removing -it: Most commands in automation do not need a TTY.
4. Consult documentation: See Docker's docs for more details on exec and TTY options.

References

• Docker exec documentation
• What is a TTY? (Wikipedia)

By understanding the difference between interactive and non-interactive environments, and how Docker handles TTY allocation, you can avoid this common pitfall in your automation scripts and pipelines.