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Small documentation updates regarding environment variables

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Hongli Lai (Phusion) 2014-02-25 13:37:55 +01:00
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@ -212,11 +212,11 @@ The following example runs `ls` without running the startup files and with less
<a name="environment_variables"></a>
### Environment variables
If you use `/sbin/my_init` as the main container command, then any environment variables set with `docker run --env` will be picked up by `my_init`, and passed to all child processes, including `/etc/my_init.d` startup scripts, Runit and Runit-managed services. There are however a few caveats you should be aware of:
If you use `/sbin/my_init` as the main container command, then any environment variables set with `docker run --env` or with the `ENV` command in the Dockerfile, will be picked up by `my_init`. These variables will also be passed to all child processes, including `/etc/my_init.d` startup scripts, Runit and Runit-managed services. There are however a few caveats you should be aware of:
* Environment variables on Unix are inherited on a per-process basis. This means that it is generally not possible for a child process to change the environment variables of other processes.
* Because of the aforementioned point, there is no good central place for defining environment variables for all applications and services. Debian has the `/etc/environment` file but it only works in some situations.
* Some services change environment variables for child processes. Nginx is one such example: it removes all environment variables unless you explicitly instruct it to retain them through the `env` configuration option. If you host any applications on Nginx (e.g. using the [passenger-docker](https://github.com/phusion/passenger-docker) image, or using Phusion Passenger in your own image) then they will not see the environment variables that were originally passed to `docker run --env`.
* Some services change environment variables for child processes. Nginx is one such example: it removes all environment variables unless you explicitly instruct it to retain them through the `env` configuration option. If you host any applications on Nginx (e.g. using the [passenger-docker](https://github.com/phusion/passenger-docker) image, or using Phusion Passenger in your own image) then they will not see the environment variables that were originally passed by Docker.
`my_init` provides a solution for all these caveats.