docker ENV vs RUN export

Question

Let's say I want combine these commands

RUN command_1
ENV FOO bar
RUN command_2

into

RUN command_1 && export FOO=bar && command_2

and was wondering if setting the variable with RUN export vs ENV was equivalent.

In other words, is there a difference between these commands in a Dockerfile?

ENV FOO bar

vs

RUN export FOO=bar

Answer 1

As illustrated by issue 684, export won't persist across images. (Don't forget that each Dockerfile directive will generate an intermediate container, committed into an intermediate image: that image won't preserve the exported value)
ENV will:

The environment variables set using ENV will persist when a container is run from the resulting image.
You can view the values using docker inspect, and change them using docker run --env <key>=<value>.

The issue was illustrating that with:

RUN export PATH=$PATH:/foo/bar # from directly in builder

When I do docker run [img] bash -c 'echo $PATH' it never includes /foo/bar.

Try it

Create a new dockerfile containing:

FROM centos:6
ENV FOO=foofoo
RUN export BAR=barbar
RUN export BAZ=bazbaz && echo "$FOO $BAR $BAZ"

Then build it. The output of the last step is:

Step 4/4 : RUN export BAZ=bazbaz && echo "$FOO $BAR $BAZ"
 ---> Running in eb66196b238d
foofoo  bazbaz

You can see:

• FOO persists through intermediate containers, thanks to the ENV keyword;
• BAR doesn't persist on the next step, because of the export command;
• BAZ is correctly displayed because the variable is used on the same container.