From Docker 1.3 onwards:

docker exec -it <containerIdOrName> bash

Basically, if the Docker container was started using the /bin/bash command you can access it using attach. If not, then you need to execute the command to create a Bash instance inside the container using exec.

Also to exit Bash without leaving Bash running in a rogue process:

exit

Yep, it is that simple.

Although the author of the question specifically said they are interested in a running container, it's also worth noting that if the container is not running, but you'd like to run it to poke around you can run:

docker run -i -t --entrypoint /bin/bash <imageID>

Try this:

sudo docker run -i -t webserver /bin/bash

Source: https://docs.docker.com/articles/basics/#running-an-interactive-shell

Based on @Timur's answer I've created the following handy script

Setup

Put docker-ssh file in your $PATH with the following contents

#!/bin/bash -xe



# docker container id or name might be given as a parameter

CONTAINER=$1



if [[ "$CONTAINER" == "" ]]; then

  # if no id given simply just connect to the first running container

  CONTAINER=$(docker ps | grep -Eo "^[0-9a-z]{8,}b")

fi



# start an interactive bash inside the container

# note some containers don't have bash, then try: ash (alpine), or simply sh

# the -l at the end stands for login shell that reads profile files (read man)

docker exec -i -t $CONTAINER bash -l

Note: Some container do not contain bash, but ash, sh etc. In these cases bash shall be replaced in the above script.

Usage

If you have one running instance simply run

$> docker-ssh 

Otherwise provide it with a docker id parmeter that you get from docker ps (first col)

$> docker-ssh 50m3r4nd0m1d

I've created a containerized SSH server that provides SSH capabilities to any running container. You don't need to change your container. The only requirement is that the container has bash.

If you have a container with name 'web-server1'. The following docker run command would start a second container that would provide SSH for the first container.

docker run -ti --name sshd-web-server1 -e CONTAINER=web-server1 -p 2222:22 

-v /var/run/docker.sock:/var/run/docker.sock -v $(which docker):/usr/bin/docker 

jeroenpeeters/docker-ssh

For more pointers, checkout https://github.com/jeroenpeeters/docker-ssh

If your container doesn't have bash installed you could try sh:

docker exec -it CONTAINER /bin/sh

Or look for shells in /bin first:

docker export CONTAINER|tar -t|egrep ^bin/

@jpetazzo has an awesome post about this subject. The short answer would be to use nsenter:

PID=$(docker inspect --format {{.State.Pid}} <container_name_or_ID>)

nsenter --target $PID --mount --uts --ipc --net --pid

P.S.: Don't forget to check the discussion in the comments of the post...

Cheers

You can also give the Docker container a routeable IP address with Pipework, and after that SSH into the machine with that new IP address.

This will be more "traditional" (ssh), instead of using an application-specific command like docker attach, and will eventually make it more 'portable' across systems and versions.

docker run -it openjdk:8

This works :-)

Sometimes it will be handy to be able to ssh into a Docker container, especially during development. The following Docker image allows to ssh into a container using a private key:

UbuntuWithSSH-Docker

The gist of the Dockerfile is https://gist.github.com/devbkhadka/98792f7bca57f9778793b2db758b3d07.

GOINSIDE

install goinside command line tool with:

sudo npm install -g goinside

and go inside a docker container with a proper terminal size with:

goinside docker_container_name

for more details check this out.

To bash into a running container, type this:

docker exec -t -i container_name /bin/bash

Just for information. If you need to login in a simple container that is not a daemon, you need to use the following commands:

docker start {id}

docker attach {id}

if the container is stopped like for example a data-only container then a good solution is to run a throwaway container every time you want to attach to the data container. In this case the data container itself could be entirely empty, as the temporary container would have the OS tools.

$ docker run --rm --volumes-from mydata -it ubuntu bash

root@645045d3cc87:/# ls /mydata

root@645045d3cc87:/# touch /mydata/foo

root@645045d3cc87:/# exit

exit