How to mock "redirect to file"?

[Andy-AO]

Wanted to mock > and >>, mock Out-File is useful, but fails to get any arguments.

Is there a better solution other than replacing > with Out-File?

Describe 'root' {
    it 'Symbol' {
        { mock '>' } | Should -Throw '*Could not find Command >*'
    }
    it 'Out-File' {
        function redirect {
            'text' > 'MyFake:\Path\Dir'
        }
        { redirect } | Should -Throw '*Cannot find drive*'
        mock 'Out-File' {
            $InputObject
            $LiteralPath
            $FilePath
            $args
        }
        # Can mock successfully, but cannot get any parameters.
        (redirect) | Should -Be $null
        Should -Invoke 'Out-File' -Exactly 1
    }
}

[fflaten]

Redirect operators aren't commands, but features in the language engine. You can't mock them. Mocking only works for commands that you can see with Get-Command.

Can you say more about why you would need this?

[Andy-AO]

mock > just as mock Out-File does.
It is possible to mock. If you look at the test code carefully, you will find that the mock has been successful, but the parameters cannot be used.

[fflaten]

Sorry, I didn't have time to test it test it. I knew it shared some code, but TIL that it actually invokes out-file. The mock is invoked with text and path set as $InputObject and $FilePath which you could see using Pester's Diagnostics output.

However as I mentioned this operator is a engine feature, so the cmdlet isn't invoked the normal way. It's called in a special pipelineprocessor in which they throw away the output. That's most likely done because a redirection operator is never expected to output anything, but in case it does they stop it from polluting your remaining pipeline (+ better performance I suppose).

I'm not familiar with the PowerShell-codebase, but maybe one of the contributors in https://github.com/PowerShell/PowerShell can explain it if you need more details. It's just how the engine works, so nothing we can fix in Pester. 🙂

[Andy-AO]

@fflaten
(╯▽╰) So it seems that we can only change our usage habits.
Thank you for your reply.