docker run --tty 和 --interactive 开关之间的区别

Question

I feel there is a subtle difference between the --tty and --interactive switches of the docker run command, that I don't grasp:

• --interactive , -i : Keep STDIN open even if not attached
• --tty , -t : Allocate a pseudo-TTY

So I decided to run some tests.
First I created a basic Python script, which continuously prints a string.
Then I created a basic docker image, which will run this script when a container is started.

my_script.py

import time

while True:
    time.sleep(1)
    print('still running...')

Dockerfile

FROM python:3.8.1-buster
COPY my_script.py /
CMD [ "python3", "/my_script.py"]

Built using command:

docker build --tag pytest .

Test 1
I run docker run --name pytest1 -i pytest , to test the interactive behaviour of the container. Nothing is printed to the console, but when I press Control+C the python script is interrupted and the container stops running.

This confirms my thinking that stdin was open on the container and my keyboard input entered the container.

Test 2
I run docker run --name pytest1 -t pytest , to test the pseudo-tty behaviour of the container. It repeatedly prints still running... to the console, ánd when I press Control+C the python script is interrupted and the container stops running.

Test 3
I run docker run --name pytest1 -it pytest , to test the combined behaviour. The behaviour is the same as in Test 2.

Questions

• What are the nuances I'm missing here?
• Why would one use the combined -it switches, as you often see, if there is no benefit to the -t switch?
• Does the --tty switch just keeps bóth stdin and stdout open?

Answer 1

-t option is needed if you want to interact with a shell like /bin/sh for instance. The shell works by controlling tty. No tty available, no shell.

we use -i in combination with -t to be able to write commands to the shell we opened

few tests you could reproduce to understand:

docker run alpine /bin/sh : the container exits. shell needs to wait for stdin

docker run -i alpine /bin/sh : the container stays, but the shell won't start. we cannot type commands

docker run -t alpine /bin/sh : shell starts, but we are stuck, the keys we press are not interpreted

docker run -it alpine /bin/sh : yeah our shell is working