I am able to run arbitrary shell commands in a container created from docker/whalesay image.
$ docker run docker/whalesay ls -l total 56 -rw-r--r-- 1 root root 931 May 25 2015 ChangeLog -rw-r--r-- 1 root root 385 May 25 2015 INSTALL -rw-r--r-- 1 root root 1116 May 25 2015 LICENSE -rw-r--r-- 1 root root 445 May 25 2015 MANIFEST -rw-r--r-- 1 root root 1610 May 25 2015 README -rw-r--r-- 1 root root 879 May 25 2015 Wrap.pm.diff drwxr-xr-x 2 root root 4096 May 25 2015 cows -rwxr-xr-x 1 root root 4129 May 25 2015 cowsay -rw-r--r-- 1 root root 4690 May 25 2015 cowsay.1 -rw-r--r-- 1 root root 54 May 25 2015 install.pl -rwxr-xr-x 1 root root 2046 May 25 2015 install.sh -rw-r--r-- 1 root root 631 May 25 2015 pgp_public_key.txt $ docker run docker/whalesay lsb_release -a No LSB modules are available. Distributor ID: Ubuntu Description: Ubuntu 14.04.2 LTS Release: 14.04 Codename: trusty However, I am unable to run a shell in a container created from this image.
$ docker run docker/whalesay bash $ docker ps CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES $ docker ps -a CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES 7ce600cc9904 docker/whalesay "bash" 5 seconds ago Exited (0) 3 seconds ago loving_mayer Why did it not work? How can I make it work?
21 Answer
If you docker run without attaching a tty, and only call bash, then bash finds nothing to do, and it exits. That's because by default, a container is non-interactive, and a shell that runs in non-interactive mode expects a script to run. Absent that, it will exit.
To run a disposable new container, you can simply attach a tty and standard input:
docker run --rm -it --entrypoint bash <image-name-or-id> Or to prevent the above container from being disposed, run it without --rm.
Or to enter a running container, use exec instead:
docker exec -it <container-name-or-id> bash In comments you asked
Do you know what is the difference between this and
docker run -it --entrypoint bash docker/whalesay?
In the two commands above, you are specifying bash as the CMD. In this command, you are specifying bash as the ENTRYPOINT.
Every container is run using a combination of ENTRYPOINT and CMD. If you (or the image) does not specify ENTRYPOINT, the default entrypoint is /bin/sh -c.
So in the earlier two commands, if you run bash as the CMD, and the default ENTRYPOINT is used, then the container will be run using
/bin/sh -c bash If you specify --entrypoint bash, then instead it runs
bash <command> Where <command> is the CMD specified in the image (if any is specified).
