docker ENV vs RUN export

Question

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.