How to determine if a process runs inside lxc/Docker?

Question

Is there any way to determine if a process (script) runs inside an lxc container (~ Docker runtime)? I know that some programs are able to detect whether they run inside a v

Answer 1

We use the proc's sched (/proc/$PID/sched) to extract the PID of the process. The process's PID inside the container will differ then it's PID on the host (a non-container system).

For example, the output of /proc/1/sched on a container will return:

root@33044d65037c:~# cat /proc/1/sched | head -n 1
bash (5276, #threads: 1)

While on a non-container host:

$ cat /proc/1/sched  | head -n 1
init (1, #threads: 1)

This helps to differentiate if you are in a container or not.

Answer 2

On a new ubuntu 16.04 system, new systemd & lxc 2.0

sudo grep -qa container=lxc /proc/1/environ

Answer 3

Here's a solution in Ruby,

# Usage: DockerHelper.running_in_docker?
module DockerHelper
  extend self

  def running_in_docker?
    !!(File.read("/proc/1/cgroup") =~ %r[^\d+:\w+:/docker/]) # !! => true/false
  rescue Errno::ENOENT
    false
  end
end

If you like tests with your code, here's a spec in the gist.

Answer 4

This is an old question, but a REALLY good one. :)

I've written some automation scripts that we run on baremetal, VM and in a docker container, with logic branching based on which platform the script is executing on. In my case I have the privilege of creating both the container and the docker image, so this solution will only work if you are in control of the entire stack:

Snippet of Dockerfile:

FROM ubuntu:18.04

ENV PLATFORM="docker"

RUN apt update; \
...

The script can then just check the value of $PLATFORM for desired outcomes on each platform:

#!/bin/bash

# Check for executor specification in environment
case $PLATFORM in
  docker)
    # If running in Docker, do this stuff
    echo "Running containerized, proceeding..."
    ;;
  virtual)
    # If running in a VM, do different stuff
    echo "Running on a VM, loading VM stuff..."
    modprobe some-kernel-module
    ;;
  *)
    echo "Unknown executor specified! Exiting..."
    exit 1
    ;;
esac

I've omitted baremetal in the above code to keep it concise.