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?
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).