Search code examples
lxcdocker

Run a complex series of commands in the same Docker container

I'm trying to automate the following loop with Docker: spawn a container, do some work inside of it (more than one single command), get some data out of the container.

Something along the lines of:

for ( i = 0; i < 10; i++ )
    spawn a container
    wget revision-i
    do something with it and store results in results.txt

According to the documentation I should go with:

for ( ... )
    docker run <image> <long; list; of; instructions; separated; by; semicolon>

Unfortunately, this approach is not attractive nor maintanable as the list of instructions grows in complexity.

Wrapping the instructions in a script as in docker run <image> /bin/bash script.sh doesn't work either since I want to spawn a new container for every iteration of the loop.

To sum up:

  1. Is there any sensible way to run a complex series of commands as described above inside the same container?

  2. Once some data are saved inside a container in, say, /home/results.txt, and the container returns, how do I get results.txt? The only way I can think of is to commit the container and tar the file out of the new image. Is there a more efficient way to do it?

Bonus: should I use vanilla LXC instead? I don't have any experience with it though so I'm not sure. Thanks.

Solution

  • I eventually came up with a solution that works for me and greatly improved my Docker experience.

    Long story short: I used a combination of Fabric and a container running sshd.

    Details:

    The idea is to spawn container(s) with sshd running using Fabric's local, and run commands on the containers using Fabric's run.

    To give a (Python) example, you might have a Container class with:

    1) a method to locally spawn a new container with sshd up and running, e.g. 

    local('docker run -d -p 22 your/image /usr/sbin/sshd -D')

    2) set the env parameters needed by Fabric to connect to the running container - check Fabric's tutorial for more on this

    3) write your methods to run everything you want in the container exploiting Fabric's run, e.g.

    run('uname -on')

    Oh, and if you like Ruby better you can achieve the same using Capistrano.

    Thanks to @qkrijger (+1'd) for putting me on the right track :)