Pronoy Chopra

Pronoy Chopra


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Docker Notes - Part 1

I am not a big fan of Docker because I think it creates more problems than it solves. However I’ve needed to use it for work because well - nevermind. Anyway, here are some Docker notes I am keeping for myself so that I can refer them later.

Docker quick intro

Docker image

Docker image contains a snapshot of the filesystem required to run a particular command and the command itself. e.g. a busybox image will include dev bin sbin etc etc. so that we can use the busybox utils. So it comes with this stuff and a command you can use to run things.

Docker container

Docker uses existing resources on the system to create “containers”. A container is a logical space/way for the kernel to organize and route system calls to particular resources depending on the processes that called it. So in the case of a busybox image, it copies the fs snapshot to a sanctioned space on system/host storage and routes all system calls from say ls -alh run within the busybox container to this space.

The stupid thing about all of this is that it doesn’t actually have native implementations. You’d think that you would have a native logical machine for major OS since that’s why docker is so popular in the first place. But nope, docker requires you to download your own abstract linux VM running a linux kernel. Good thing is, it does that for you but goodby bare metal performance. Anyway, enough bitching.

Docker commands

docker run

docker run <container-name> e.g. docker run hello-world

Basically same as doing the following

  1. Creates a container docker create hello-world spits out an ID
  2. Starts the container docker start -a <ID> Note that without -a it won’t actually attach the container to stdout

docker ps

  1. Get truncated results of all containers running docker ps
  2. Get detailed results of all containers running/exited docker ps --all
  3. Re run container even if exited by grabbing container ID and hitting docker run <ID>

docker system prune

  1. Clears all exited containers
  2. Removes build cache meaning images will have to be re-downloaded from docker hub
  3. Clears unused networks

docker logs

Usage: docker logs <ID>

Show the logs of an already running or exited container

docker stop

SIGTERM the process in the container and stop it within 10 seconds otherwise it’s going to be killed

docker kill

SIGKILL the process - don’t wait for anything just kill it

Multiple commands within a container

usage: docker exec -it <ID> <sub command>

The -it flag is required to get an interactive terminal if you are using something like a CLI. It’s actually two flags -i and -t. The first attaches the standard input and the latter makes stuff pretty (I know)

e.g. docker exec -it busybox sh

Oh and BTW the containers are isolated, but that was obvious.