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.
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.
- Taken from a comment on this github issue. You can find more insight in the comments of this issue.
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.
Example
If you would have a Dockerfile in a directory containing:
FROM ubuntu:latest
ADD ./script.sh /timescript.sh
RUN chmod +x /timescript.sh
CMD ["/timescript.sh"]
And have a simple bash script script.sh in the same directory containing:
#!/bin/bash
#trap ctrl-c and exit, couldn't get out
#of the docker container once attached
trap ctrl_c INT
function ctrl_c() {
exit
}
while true; do
time=$(date +%N)
echo $time;
sleep 1;
done
Then build (in this example in the same directory as the Dockerfile and script.sh) and run it with
$ docker build -t nan-xiao/time-test .
..stuff happening...
$ docker run -itd --name time-test nan-xiao/time-test
Finally attach
$ docker attach time-test
You will end up attached to a container printing out the time every second. (CTRL-C to get out)
Example 2
Or if you would have a Dockerfile containing for example the following:
FROM ubuntu:latest
RUN apt-get -y install irssi
ENTRYPOINT ["irssi"]
Then run in the same directory:
$ docker build -t nan-xiao/irssi-test .
Then run it:
$ docker run -itd --name irssi-test nan-xiao/irssi-test
And finally
$ docker attach irssi-test
You would end up in a running irssi window without this particular behaviour. Of course you can substitute irrsi for another program.
