ENV documentation suggests misleading alternative

[rhcarvalho]

Is this a docs issue?

• My issue is about the documentation content or website

Type of issue

Information is incorrect

Description

In the documentation for the ENV instruction, there is an example that reads:

If an environment variable is only needed during build, and not in the final image, consider setting a value for a single command instead:

RUN DEBIAN_FRONTEND=noninteractive apt-get update && apt-get install -y ...

I think that suggestion is misleading, as IIUC DEBIAN_FRONTEND is only defined for the first command apt-get update and not for the apt-get install step. It is therefore not equivalent to the examples that use ENV or ARG.

---

Example demonstrating the problem:

$ FOO=bar printenv FOO PATH && echo 123 && printenv FOO PATH
bar
/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
123
/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin

In the above example, FOO is defined only in the first call to printenv.

Location

https://docs.docker.com/reference/dockerfile/

Suggestion

Perhaps the most obvious solution is to repeat DEBIAN_FRONTEND:

RUN DEBIAN_FRONTEND=noninteractive apt-get update && DEBIAN_FRONTEND=noninteractive apt-get install -y ...

For this particular use case in which the environment variable needs to be available to multiple commands but not be persistent in the final image, I'd emphasize the use of ARG instead, as it is less error-prone.

[docker-robot]

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[rhcarvalho]

/remove-lifecycle stale

[rhcarvalho]

This hasn't been updated and is still wrong in the live documentation.

[NickeZ]

Another solution is to export it for that shell

RUN export DEBIAN_FRONTEND=noninteractive; apt-get update && apt-get install -y ...