Docker TTY

What is “tty” in Docker

Many docker-compose.yml files will contain a line tty:true. A lot of engineers only copy this line when they create a new one but few of them actually know what it is for. Why do we need to write it down in our service? Do we really need it? In this post, I’ll explain why we use it and how can we use it.

What is tty?

First thing first. tty is a linux command which will retrun the file name of the terminal. I got the following in my terminal:

$ tty
/dev/ttys000

Remember that linux operating system considers everything as a file, like the output devices that we attach are also represented as a file. So the terminal is also a file, that’s why we can have a file name for it.

Why do we use it in docker?

The main reason we use tty:true in compose file is to keep the container running. When we bring up a container, it will quit automatically if we don’t pass a command to keep it running. Consequently, we use tty:true to keep it running. Here I will use two examples to show you how it works. I will bring up a ubuntu service in detach mode with docker-compose up -d without passing any command to it. Then I use docker-compose ps to inspect the state of the container.

without tty: true

docker-compose.yml:

version: '3'
services:
  test-local:
    image: ubuntu

$ docker-compose up -d
$ docker-compose ps
             Name                 Command    State    Ports
-----------------------------------------------------------
docker-playground_test-local_1   /bin/bash   Exit 0        

with tty: true

docker-compose.yml:

version: '3'
services:
  test-local:
    image: ubuntu
    tty: true

$ docker-compose up -d
$ docker-compose ps
             Name                 Command    State   Ports
----------------------------------------------------------
docker-playground_test-local_1   /bin/bash   Up           

Analysis

Compare the output and you can see how it works. With tty: true, the container’s state is Up.

Conclusion

In a word, tty: true is docker compose file help us keep the container running. However we can also do it in different ways, like pass commands like: sleep infity or tail -f /dev/null. But at least as an engineer, it’s helpful to know what it means when we write down the code!