determine OS distribution of a docker image

Question

I need to determine the OS distribution name for any docker image. I can tag ubuntu:latest as image1:latest, but I should be able to get the distribution information of image1:latest when it is launched.

For achieving this, I used the below mentioned command to determine the OS version:

$ docker tag ubuntu image1 $ $ docker run -it image1 /bin/sh -c "echo import platform > test.py; echo print\(platform.dist\(\)\) >> test.py; python3 test.py" ('Ubuntu', '14.04', 'trusty') $ 

However, this has a dependency on whether the image has python2 or python3 in it. It fails for ubuntu:12.04 and I need to use python2 there.

$ docker run -it ubuntu /bin/sh -c "echo import platform > test.py; echo print\(platform.dist\(\)\) >> test.py; python3 test.py" ('Ubuntu', '14.04', 'trusty') $ $ docker run -it ubuntu:12.04 /bin/sh -c "echo import platform > test.py; echo print\(platform.dist\(\)\) >> test.py; python3 test.py" /bin/sh: 1: python3: not found $ $ docker run -it ubuntu:12.04 /bin/sh -c "echo import platform > test.py; echo print\(platform.dist\(\)\) >> test.py; python2 test.py" ('Ubuntu', '12.04', 'precise') $  

Q1. Is there a way I can achieve the same without knowing which version of python is there in a particular image?

NOTE: The goal is to determine which was the base image used to build this image. I don't have access to the Dockerfile used to build this image.

Q2. There is another approach of using entrypoint. I can build a separate image from the current image using Dockerfile. Or, I can specify entrypoint in cmdline when creating container but I need the script to be accessible within the container. I am guessing that I might need shared storage when using cmdline, is there a better way to achieve this? Any pointers would be really helpful.

Thanks.

Answer 1

The Filesystem Hierarchy Standard has a standard definition for /etc/os-release, which should be available on most distributions:

The /etc/os-release and /usr/lib/os-release files contain operating system identification data.

The basic file format of os-release is a newline-separated list of environment-like shell-compatible variable assignments. It is possible to source the configuration from shell scripts.

This means you can just source /etc/os-release and use $NAME or $ID to identify the distribution. As an example, on Fedora it looks like this:

% source /etc/os-release % echo $NAME Fedora % echo $ID fedora 

On Debian:

% source /etc/os-release % echo $NAME Debian GNU/Linux % echo $ID debian