CODEWITHSUNDEEP
windowspowershellbatch-filecmdcommand-prompt
James Ko
James Ko

Reputation: 34499

Batch scripting: Why doesn't redirecting stdout work in this scenario?

If you open up a command prompt and type this:

echo foobar > nul

it will print nothing, since nul swallows all of its input. But if you run the command with PowerShell:

powershell "echo foobar" > nul

it will output foobar to the console. Why is this, and what can I do to fix it?

edit: Here is the output of $PSVersionTable. It looks like I'm using PowerShell v5.0.

Upvotes: 4

Views: 170

Answers (1)

mklement0
mklement0

Reputation: 437618

Note: I'm assuming you're invoking your command from cmd.exe, not from within PowerShell, which is consistent with the symptoms I'm seeing.

Methinks you've stumbled upon a bug in PS (PowerShell) v5 (not present in v3; comments on the question suggest it's also not in v4), though I don't fully understand why PS is to blame, because I'd expect cmd.exe to handle the redirection.
I may be missing something, however, so do let me know.

PowerShell should send its so-called success stream - things output by default, including with echo, which is an alias of Write-Output - to the outside world's stdout.

In older PS versions >NUL does effectively suppresses PowerShell's output.
Curiously, the bug in v5 only affects NUL, whereas redirecting to an actual file works.

As for workarounds:

If your code is v2-compatible, try this:

powershell -version 2 "echo foobar" > NUL

Otherwise, redirect to an actual file and delete that file afterward:

powershell "echo foobar" > "%TEMP%\NUL-bug-workaround" & del "%TEMP%\NUL-bug-workaround"

Upvotes: 5

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