How can I run bash in a new container of a docker image?

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).