How does `docker run` conceptually if no `-d` or `-it` flags passed? - Stack Overflow

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I am trying to conceptually understand how docker run behaves if no flags -it or -d are provided.

• If the -it flags are provided, docker runs in "interactive mode". The -i flag causes stdin to be attached to the container. I think that -t both allocates a tty environment and causes stdout and stderr to be attached to the container.
• If the -d flag is provided, docker runs in "detached mode". This doesn't attach stdin or stdout.

However, the behavior without either flag is similar in some causes. No stdin or stdout.

So what is a good way of conceptually understanding how a container runs if neither -d nor -it flags are provided?

Some examples:

The default CMD for the image ubuntu:latest is /bin/bash.

$ docker pull ubuntu:latest
$ docker history ubuntu:latest
CMD ["/bin/bash"]

The container will exit immediately when we run it. This is because the default command is /bin/bash.

$ docker run ubuntu:latest
$ docker ps -a
4f1d3f7ebccf ubuntu:latest "/bin/bash" 1 second ago Exited (0) Less than a second ago                                                                                                boring_kowalevski

If we run in interactive mode, the container will remain open until CTRL+D is pressed. If we press CTRL+D, it exits.

$ docker run -it ubuntu:latest
root@b9cdc0aba0e1:/#
exit # (press CTRL+D)
b9cdc0aba0e1 ubuntu:latest "/bin/bash" 38 seconds ago Exited (127) 20 seconds ago relaxed_hugle

If we run in detached mode, the container also exits. It prints a hash before exiting.

$ docker run -d ubuntu:latest
97e6bef6750399782127e4ce9fe1400bebf20e209ba661ee637e69c758ea31fb
$ docker ps -a
97e6bef67503 ubuntu:latest "/bin/bash" 27 seconds ago Exited (0) 27 seconds ago zealous_bhaskara

If we run with -t only it behaves in a strange way. We can't exit using CTRL+D. We don't appear to be able to run anything, such as echo hello.

$ docker run -t ubuntu:latest
root@7c79b9c28eca:/# echo hello
^C^C
root@7c79b9c28eca:/#

The only way to stop it appears to be to issue a docker stop command.

$ docker ps -a
7c79b9c28eca ubuntu:latest "/bin/bash" About a minute ago Up About a minute kind_franklin
$ docker stop kind_franklin

After about 10 seconds, the container will stop.

root@7c79b9c28eca:/# user@host:~$

$ docker ps -a
7c79b9c28eca ubuntu:latest "/bin/bash" 3 minutes ago Exited (137) 33 seconds ago kind_franklin

If we run with -i then the output looks a bit weird. But the container does remain running. Here's a side-by-side comparison of -i and -it.

$ docker run -i ubuntu:latest
echo hello
hello # (CTRL+D)
$ docker run -it ubuntu:latest
root@7e3c2d21989a:/# echo hello
hello
root@7e3c2d21989a:/# # (CTRL+D)
exit
$

Running with all three flags is interesting. In this case, the container continues to run in the same way as it does with -it, however the session is not attached yet. We can then attach to the container with docker attach and then it continues to function as if we had run with -it and without -d directly.

$ docker run -dit ubuntu:latest
f84c37823d83a26258e8ab372176e177b685e911f6668e648f7a3990850cd8e0
$ docker ps
f84c37823d83 ubuntu:latest "/bin/bash" 4 seconds ago Up 3 seconds charming_varahamihira
$ docker attach charming_varahamihira
root@f84c37823d83:/# echo hello
hello
root@f84c37823d83:/#
exit

Now let's try something different. Change the default behavior so that the container runs forever. Now, rather than exiting when no flags are supplied, it continues to run but does not receive input or send output. There is no way to stop it other than to run docker stop.

docker run ubuntu:latest /bin/bash -c 'sleep infinity'
echo hello # input
# (CTRL+D)

If we run with the -d flag, the container behaves in a sensible way. It continues to run indefinitely.

$ docker run -d ubuntu:latest /bin/bash -c 'sleep infinity'
6523960d7740afb3282bb7edaeb9c90f0a2ef12efa1cdd7481c5d2c428c72546
$ docker ps
6523960d7740 ubuntu:latest "/bin/bash -c 'sleep…" 8 seconds ago Up 7 seconds agitated_payne

We can create a new bash session with docker exec -it agitated_payne bash. In this case CTRL+D will exit this bash session, but not the original bash session, so the container will continue to run.

If we attach to the container with docker attach, the container again behaves strangely with no input or output. We cannot stop it with CTRL+D. The only way to get our prompt back is to stop the container with docker stop.

We can also try all three flags.

$ docker run -dit ubuntu:latest /bin/bash -c 'sleep infinity'

But this appears to behave the same way as just having the -d flag. docker attach does not attach i/o and the container cannot be stopped with CTRL+D.

With just the flags -it, the container behaves similarly.

I am trying to conceptually understand how docker run behaves if no flags -it or -d are provided.

• If the -it flags are provided, docker runs in "interactive mode". The -i flag causes stdin to be attached to the container. I think that -t both allocates a tty environment and causes stdout and stderr to be attached to the container.
• If the -d flag is provided, docker runs in "detached mode". This doesn't attach stdin or stdout.

However, the behavior without either flag is similar in some causes. No stdin or stdout.

So what is a good way of conceptually understanding how a container runs if neither -d nor -it flags are provided?

Some examples:

The default CMD for the image ubuntu:latest is /bin/bash.

$ docker pull ubuntu:latest
$ docker history ubuntu:latest
CMD ["/bin/bash"]

The container will exit immediately when we run it. This is because the default command is /bin/bash.

$ docker run ubuntu:latest
$ docker ps -a
4f1d3f7ebccf ubuntu:latest "/bin/bash" 1 second ago Exited (0) Less than a second ago                                                                                                boring_kowalevski

If we run in interactive mode, the container will remain open until CTRL+D is pressed. If we press CTRL+D, it exits.

$ docker run -it ubuntu:latest
root@b9cdc0aba0e1:/#
exit # (press CTRL+D)
b9cdc0aba0e1 ubuntu:latest "/bin/bash" 38 seconds ago Exited (127) 20 seconds ago relaxed_hugle

If we run in detached mode, the container also exits. It prints a hash before exiting.

$ docker run -d ubuntu:latest
97e6bef6750399782127e4ce9fe1400bebf20e209ba661ee637e69c758ea31fb
$ docker ps -a
97e6bef67503 ubuntu:latest "/bin/bash" 27 seconds ago Exited (0) 27 seconds ago zealous_bhaskara

If we run with -t only it behaves in a strange way. We can't exit using CTRL+D. We don't appear to be able to run anything, such as echo hello.

$ docker run -t ubuntu:latest
root@7c79b9c28eca:/# echo hello
^C^C
root@7c79b9c28eca:/#

The only way to stop it appears to be to issue a docker stop command.

$ docker ps -a
7c79b9c28eca ubuntu:latest "/bin/bash" About a minute ago Up About a minute kind_franklin
$ docker stop kind_franklin

After about 10 seconds, the container will stop.

root@7c79b9c28eca:/# user@host:~$

$ docker ps -a
7c79b9c28eca ubuntu:latest "/bin/bash" 3 minutes ago Exited (137) 33 seconds ago kind_franklin

If we run with -i then the output looks a bit weird. But the container does remain running. Here's a side-by-side comparison of -i and -it.

$ docker run -i ubuntu:latest
echo hello
hello # (CTRL+D)
$ docker run -it ubuntu:latest
root@7e3c2d21989a:/# echo hello
hello
root@7e3c2d21989a:/# # (CTRL+D)
exit
$

Running with all three flags is interesting. In this case, the container continues to run in the same way as it does with -it, however the session is not attached yet. We can then attach to the container with docker attach and then it continues to function as if we had run with -it and without -d directly.

$ docker run -dit ubuntu:latest
f84c37823d83a26258e8ab372176e177b685e911f6668e648f7a3990850cd8e0
$ docker ps
f84c37823d83 ubuntu:latest "/bin/bash" 4 seconds ago Up 3 seconds charming_varahamihira
$ docker attach charming_varahamihira
root@f84c37823d83:/# echo hello
hello
root@f84c37823d83:/#
exit

Now let's try something different. Change the default behavior so that the container runs forever. Now, rather than exiting when no flags are supplied, it continues to run but does not receive input or send output. There is no way to stop it other than to run docker stop.

docker run ubuntu:latest /bin/bash -c 'sleep infinity'
echo hello # input
# (CTRL+D)

If we run with the -d flag, the container behaves in a sensible way. It continues to run indefinitely.

$ docker run -d ubuntu:latest /bin/bash -c 'sleep infinity'
6523960d7740afb3282bb7edaeb9c90f0a2ef12efa1cdd7481c5d2c428c72546
$ docker ps
6523960d7740 ubuntu:latest "/bin/bash -c 'sleep…" 8 seconds ago Up 7 seconds agitated_payne

We can create a new bash session with docker exec -it agitated_payne bash. In this case CTRL+D will exit this bash session, but not the original bash session, so the container will continue to run.

If we attach to the container with docker attach, the container again behaves strangely with no input or output. We cannot stop it with CTRL+D. The only way to get our prompt back is to stop the container with docker stop.

We can also try all three flags.

$ docker run -dit ubuntu:latest /bin/bash -c 'sleep infinity'

But this appears to behave the same way as just having the -d flag. docker attach does not attach i/o and the container cannot be stopped with CTRL+D.

With just the flags -it, the container behaves similarly.

• docker

Share Improve this question Follow edited Nov 23, 2024 at 12:05 mkrieger1 22.9k77 gold badges6363 silver badges7979 bronze badges asked Nov 22, 2024 at 9:21 user2138149user2138149 16.2k3030 gold badges140140 silver badges283283 bronze badges 1

• Is your question about what -d, -i, and -t do, or why running bash as the main container process has specific behavior? In Unix more generally, are "background process", "stdin", and "tty" meaningful terms to you? Also see for example Docker container will automatically stop after "docker run -d" and Confused about Docker -t option to Allocate a pseudo-TTY. – David Maze Commented Nov 22, 2024 at 11:26

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1 Answer 1

Sorted by: Reset to default 0

If the -d flag is provided, docker runs in "detached mode". This doesn't attach stdin or stdout.

Nope. Input and output to the process are separate. The thing that is detached is the docker CLI client from the running container on the docker server.

So what is a good way of conceptually understanding how a container runs if neither -d nor -it flags are provided?

It is conceptually the same as if you ran:

$some_command </dev/null

Output is sent to the attached docker client, but no input is attached to the process.

If you were to detach, the logs would still be accessible by docker logs, making it conceptually the same as:

$some_command </dev/null &>output.log &

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