The Console\Application class is doing a side effect on environment variables for usage in one implementation of the Logger.

Reading and writing global variables inside classes looks like a bad practice. This should be delegated to a config/context layer.

@GromNaN it's not exactly for that, but to create an environment context that mirrors the -vvv verbosity flags. Then this context is leveraged by the logger yes, but this is independent (you could set the env var in a webserver context to control the logger's verbosity also.)
I don't have much better idea for now that wouldn't imply a pile of code we don't need nor want.

Thank you @nicolas-grekas.

TLDR: the value in $output->getVerbosity() is overwritten in the switch ($shellVerbosity = (int) getenv('SHELL_VERBOSITY')) {…} block added in this PR, is it an unexpected site effect?

It looks like this PR overwrites the verbosity level set by calling setVerbosity(). Maybe this wasn't a good idea though.

Anyway, here is how we used it in LiipFunctionalTestBundle:

1. we created an Application: https://github.com/liip/LiipFunctionalTestBundle/blob/bf909887fbd3aac10215efeb6f6287c72afc568d/Test/WebTestCase.php#L130
2. we instantiated a StreamOutput with the verbosity level: https://github.com/liip/LiipFunctionalTestBundle/blob/bf909887fbd3aac10215efeb6f6287c72afc568d/Test/WebTestCase.php#L146 (so we expect StreamOutput->verbosity to contain the expected verbosity value)
3. we called Application::run(): https://github.com/liip/LiipFunctionalTestBundle/blob/bf909887fbd3aac10215efeb6f6287c72afc568d/Test/WebTestCase.php#L148
4. this last method calls configureIO that has been changed in this PR: https://github.com/symfony/symfony/pull/24425/files#diff-38eebdda7d6ebfcb7405b91167f05c9c

During my own tests, getenv('SHELL_VERBOSITY') is equal to 3 (I'm still wondering why). So $output->setVerbosity(OutputInterface::VERBOSITY_DEBUG); is called and the value in $output->getVerbosity(); is overwritten.

During my own tests, getenv('SHELL_VERBOSITY') is equal to 3 (I'm still wondering why).

That's probably because of the Kernel class setting it explicitly if it wasn't set before: https://github.com/symfony/symfony/blob/master/src/Symfony/Component/HttpKernel/Kernel.php#L115-L119

For the rest, I suggest to open a new issue if you feel something is not working as expected.

Unexpected result of this PR that CLI commands in debug mode now do not write debug and info logs, because of default value LogLevel::WARNING. I think that for debug mode verbosity should be debug to keep logging all debug lines.