Quinn's Blog

Docker Tips and Tricks

September 07, 2014

Enter a container

To do this you’ll need a tool called nsenter. The easiest way to get it is to use the docker based installer: https://github.com/jpetazzo/nsenter One you have this you can enter a running instance or run a command on a running instance:

sudo nsenter \
  --target `docker inspect --format '{{.State.Pid}}' postgres` \
  --mount --uts --ipc --net --pid /bin/bash

If you used the above installer you’ll also have a shortcut for this:

docker-enter postgres

Inspect API traffic

you can use socat to monitor traffic to the docker socket to introspect on API usage:

sudo socat -t100 -v \
  UNIX-LISTEN:/tmp/proxysocket.sock,mode=777,reuseaddr,fork \

Leave this open and open a new terminal. Specify the socket created by socat as the docker host:

DOCKER_HOST=unix:///tmp/proxysocket.sock docker ps

As the command runs observe the output from the previous terminal. This can be very helpful when developing against the docker API to see how the terminal commands operate.

Batch commands

Sometimes it can be quicker to operate on multiple docker containers at once, especially while testing a new container to get the parameters correct.

Remove all containers of a given image:

docker rm `docker ps -a | grep 0fa4de | cut -f1 -d" "`
# also this format uses xargs instead of backticks
docker ps -a | grep 0fa4de | cut -d ' ' -f 1 | xargs docker rm
# or if you need to stop them before removing replace rm with kill:
docker ps -a | grep 0fa4de | cut -d ' ' -f 1 | xargs docker kill
docker ps -a | grep 0fa4de | cut -d ' ' -f 1 | xargs docker rm

Remove stopped containers:

docker ps -a | grep Exit | cut -d ' ' -f 1 | xargs docker rm

You can use similar techniques for deleting images (though removing multiple images is less common):

docker ps -a | grep string-of-text | cut -d ' ' -f 1 | xargs docker rmi

Hopefully this helps! Let me know if you have any other common techniques while using docker.