The reason is simple: log messages are not part of the command output, but of extra information provided to humans running it.

Your issue comes from a misunderstanding of what stdout and stderr are about (naming them stdout and stderr creates this confusion btw. They are just the first and the second pipes). stderr is not only about reporting errors (reporting errors is the job of the exit code)

Hi@stof,

Thanks for getting back to me so quickly.

I don't fully understand your point here, although I certainly acknowledge the difference between detecting command success or failure through the exit code.

However in our use-case we would like to distinguish not only between a command that failed entirely, but also between commands that had some kind of non-fatal error indicated by output sent to stderr (as opposed to stdout). It doesn't mean that the command failed as such, but the underlying application events had something negative that we would like to treat differently.

Part of the guidelines we have been using here is from Symfony documentation onhttp://symfony.com/doc/current/components/process.html, the section 'Getting real-time Process Output' where it's checking forProcess::ERR === $type.

If we run a standard Symfony console command it sends errors (uncaught exceptions) to stderr and general output from theOutputInterface ($output->writeln) to stdout.

This seems at odds to the Monolog ConsoleHandler which is dumping everything to stderr. Although maybe I don't understand the true nature of underlying pipes, surely there is an inconsistency here?

See discussion in#15728. You can reconfigure it to use stdout by callingsetOutput manually.

As Stof explained, stderr doesn't mean error output, it means not-important output. Anything that shouldn't be pipped through should be put to stderr. Logging shouldn't be piped and thus has to be send to stderr. Detecting "non-fatal failure" through checking if output was sent to stderr is wrong, stderr hasn't anything to do with failure or success.

If anything, either stderr or stdout should be used for all log levels imo (only directing log messages to stderr or stdout based on the log level seems to be incorrect, as it is based on the assumption that stderr means error).

Just have a look at Git output for instance, it does use stderr for non-error output.

Actuallygit status is outputted tostdout,git -k (throws error) is outputted tosdterr.