How to use environment variable in Dockerfile?

Question

I am just experimenting with configuring Dockerfile-

FROM ubuntu:latest

RUN apt-get update

RUN echo VERSION_TAG="latest" >> /etc/environment

RUN cat /etc/environment

CMD echo $VERSION_TAG

I built the image using (I was in the req directory) -

docker build -t temp/testing:latest .

Ran it using-

docker run temp/testing:latest

Expected output-

latest

Actual Output-

While building the image, the output of cat /etc/environment

Step 4/5 : RUN cat /etc/environment
 ---> Running in 4grdc7b5165a
PATH="/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games:/snap/bin"
VERSION_TAG=latest

It is present inside env variables, still, its value is not being printed, any help will be appreciated.

Note - I want to do it without using
https://docs.docker.com/engine/reference/builder/#env

Can you elaborate on why you do not want to use the ENV command? — Gunder, Mar 04 '22 at 15:35
Learning purposes, still solutions with env variables are welcome. — BeastMaster64, Mar 04 '22 at 15:37
I am able to run this by `ENV VERSION_TAG=latest` but not while explicitly setting it, I need to know the reason. — BeastMaster64, Mar 04 '22 at 15:48

KamilCuk · Accepted Answer · 2022-03-04T15:57:12.780

2

I need to know the reason.

Docker just exec() the command ENTRYPOINT+CMD with nothing in between.

pam is not loaded, so pam_env.so is not loaded, so /etc/environment is not read.

edited Mar 04 '22 at 15:57

answered Mar 04 '22 at 15:49

KamilCuk

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Thanks for the answer, can you point to the resource stating this? – BeastMaster64 Mar 04 '22 at 15:51
? to which part? – KamilCuk Mar 04 '22 at 15:52
CMD does not run a login session – BeastMaster64 Mar 04 '22 at 15:52
There is no resource that something does _not_ happen, usually resources state what happens. It just doesn't run a login session, it's nowhere specified that it does, ergo it doesn't. – KamilCuk Mar 04 '22 at 15:52
How do you know then? (Not doubting you, just out of curiosity) – BeastMaster64 Mar 04 '22 at 15:53
1

because you don't have to input a password, for example. Because it just `exec()`utes `CMD` inside the environment, there is nothing in between. – KamilCuk Mar 04 '22 at 15:53
That makes sense, thanks!! – BeastMaster64 Mar 04 '22 at 15:55

score 1 · Answer 2 · answered Mar 04 '22 at 16:40

The intention is to use ENV for this, but you can set extra parameters on the command line at docker run with -e

docker run -it -e VERSION='latest' -e NAME='abcd' ubuntu:latest /bin/bash

Alternatively, you can provide a file with each variable per-line

docker run -it --env-file ./my_env ubuntu:latest /bin/bash

You can also do a volume mount with -v, which can smuggle further information and entire paths into the container, including information useful to whatever application you run within it (the intent of Docker containers is normally to run a single application within a known environment)

However, if you're trying to determine if the container is running the latest version of its own tag, this will be problematic
Specifically, the tag may change, but your explicit setting of it won't! .. for such a case, you should consider something else, such as having an outside process which replaces the running container with a new one when the tag changes (so you can assume what's running is always latest) or at some frequency (perhaps every day in the morning)

How to use environment variable in Dockerfile?

2 Answers2