docker info emits data to both stderr and stdout

[hholst80]

docker info emits information on two channels, where essential information can be lost when piping through less or more.

Steps to reproduce the issue:

1. docker info 1> stdout 2> stderr

Describe the results you received:

stderr contains

 WARNING: Usage of loopback devices is strongly discouraged for production use. Use `--storage-opt dm.thinpooldev` to specify a custom block storage device.

Describe the results you expected:

I think it would be better if all information is kept together in one file. Splitting things up into two files makes it hard to use the pattern docker info | less as warnings would get discarded.

Additional information you deem important (e.g. issue happens only occasionally):

Output of docker version:

Client:
 Version:      1.12.5
 API version:  1.24
 Go version:   go1.6.4
 Git commit:   7392c3b
 Built:        Fri Dec 16 02:23:59 2016
 OS/Arch:      linux/amd64

Server:
 Version:      1.12.5
 API version:  1.24
 Go version:   go1.6.4
 Git commit:   7392c3b
 Built:        Fri Dec 16 02:23:59 2016
 OS/Arch:      linux/amd64

[hholst80]

The warning is not really an error either - I think the intention is to give friendly help to the user not that the command docker info failed in some way.

[cpuguy83]

I'm not sure I agree with this.
The output on stdout is structured data, the output on stderr does not match the structure and is additive.

It also means for things like docker info > blah, the user will still see the warning, which is the intent.
Likewise for the less case, the user will see the warning once less is exited.

[hholst80]

With the same logic: If the user docker info > blah and then just looks at blah the warning will be lost. I think it is unexpected that some information is emitted to another stream. If docker info fails in itself to generate the requested information it should emit to stderr, not otherwise. Like this:

$ docker info > info.txt
warning: could not read /path/to/some/file - some information might be missing!

[hholst80]

This bug is turning 2 years old soon. Should we buy it a present?

[cpuguy83]

I think this is working as expected still.

@thaJeztah ?

[thaJeztah]

Let me close this ticket for now, as it looks like it went stale.