How do I run a command on an already existing Docker container?

Question

I created a container with -d so it's not interactive.

docker run -d shykes/pybuilder bin/bash 

I see that the container has exited:

CONTAINER ID        IMAGE                     COMMAND             CREATED             STATUS                      PORTS               NAMES d6c45e8cc5f0        shykes/pybuilder:latest   "bin/bash"          41 minutes ago      Exited (0) 2 seconds ago                        clever_bardeen 

Now I would like to run occasional commands on the machine and exit. Just to get the response.

I tried to start the machine. I tried attaching. I thought I could call run with a container, but that does not seem to be allowed. Using start just seems to run and then exist quickly.

I'd like to get back into interactive mode after exiting.

I tried:

docker attach d6c45e8cc5f0 

But I get:

2014/10/01 22:33:34 You cannot attach to a stopped container, start it first 

But if I start it, it exits anyway. Catch 22. I can't win.

Answer 1

In October 2014 the Docker team introduced docker exec command: https://docs.docker.com/engine/reference/commandline/exec/

So now you can run any command in a running container just knowing its ID (or name):

docker exec -it <container_id_or_name> echo "Hello from container!" 

Note that exec command works only on already running container. If the container is currently stopped, you need to first run it with the following command:

docker run -it -d shykes/pybuilder /bin/bash 

The most important thing here is the -d option, which stands for detached. It means that the command you initially provided to the container (/bin/bash) will be run in the background and the container will not stop immediately.

Answer 2

Your container will exit as the command you gave it will end. Use the following options to keep it live:

• -i Keep STDIN open even if not attached.
• -t Allocate a pseudo-TTY.

So your new run command is:

docker run -it -d shykes/pybuilder bin/bash 

If you would like to attach to an already running container:

docker exec -it CONTAINER_ID /bin/bash 

In these examples /bin/bash is used as the command.