I can run an ubuntu container successfully:
# docker run -it -d ubuntu
3aef6e642327ce7d19c7381eb145f3ad10291f1f2393af16a6327ee78d7c60bb
# docker ps
CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES
3aef6e642327 ubuntu "/bin/bash" 3 seconds ago Up 2 seconds condescending_sammet
But executing docker attach hangs:
# docker attach 3aef6e642327
Until I press any key, such as Enter:
# docker attach 3aef6e642327
root@3aef6e642327:/#
root@3aef6e642327:/# ls
bin boot dev etc home lib lib64 media mnt opt proc root run sbin srv sys tmp usr var
Why does docker attach hang?
Update:
After reading the comments, I think I get the answers:
prerequisite:
"docker attach" reuse the same tty, not open new tty.
(1) Executing the docker run without daemon mode:
# docker run -it ubuntu
root@eb3c9d86d7a2:/#
Everything is OK, then run ls command:
root@eb3c9d86d7a2:/# ls
bin boot dev etc home lib lib64 media mnt opt proc root run sbin srv sys tmp usr var
root@eb3c9d86d7a2:/#
(2) Run docker run in daemon mode:
# docker run -it -d ubuntu
91262536f7c9a3060641448120bda7af5ca812b0beb8f3c9fe72811a61db07fc
Actually, the following should have been outputted to stdout from the running container:
root@91262536f7c9:/#
So executing docker attach seems to hang, but actually it is waiting for your input:
# docker attach 91262536f7c9
ls
bin boot dev etc home lib lib64 media mnt opt proc root run sbin srv sys tmp usr var
root@91262536f7c9:/#
It does not really hang. As you can see in the comment below (You are running "/bin/bash" as command) it seems to be expected behaviour when attaching.
If instead of enter you would start typing a command, you would not see the extra empty prompt line. If you were to run
$ docker exec -it ubuntu <container-ID-or-name> bash
where <container-ID-or-name> is the ID or name of the container after you run docker run -it -d ubuntu (so 3aef6e642327 or condescending_sammet in your question) it would run a new command, thus not having this "stdout problem" of attaching to an existing one.
As far as I understand you attach to the running shell and just the stdin/stdout/stderr - depending on the options you pass along with the run command - will just show you whatever goes in/out from that moment. (Someone with a bit more in-depth knowledge hopefuly can explain this on a higher level).
As I wrote in my comment on your question, there are several people who have opened an issue on the docker github repo describing similar behaviour:
Since you mention shell, I assume you have a shell already running. attach doesn't start a new process, so what is the expected behavior of connecting to the in/out/err streams of a running process? I didn't think about this. Of course this is the expected behavior of attaching to a running shell, but is it desirable?
Would it be at all possible to flush stdout/stderr on docker attach thereby forcing the shell prompt to be printed or is it a bit more complex than that? That's what I personally would "expect" when attaching to an already running shell.
Feel free to close this issue if necessary, I just felt the need to document this and get some feedback.