verbose/writeout.md: clarify Windows % symbol escaping

[jay]

• Clarify that in Windows batch files the % must be escaped as %%, and at the command prompt it cannot be escaped which could lead to incorrect expansion.

Prior to this change the doc implied % must be escaped as %% in win32 always.

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Examples:

If curl --write-out "%{http_code}" is executed in a batch file: {http_code}

If curl --write-out "%%{http_code}" is executed in a batch file: %{http_code}

If curl --write-out "%{http_code}" is executed from the command prompt: %{http_code}

If curl --write-out "%%{http_code}" is executed from the command prompt: %%{http_code}

At the command prompt something like "%{speed_download}%{http_code}" would first be parsed by the command interpreter as %{speed_download}% and would be expanded as environment variable {speed_download} if it existed, though that's highly unlikely since Windows environment names don't use braces.

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Reported-by: Muhammad Hussein Ammari

Ref: curl/curl#10337

Fixes curl/curl#10323
Closes #xxxx

[dfandrich]

Is this necessary in PowerShell, too?

[jay]

No, it doesn't need escaping in powershell. Note that in Windows 10 & 11 powershell uses the name 'curl' as an alias for Invoke-WebRequest cmdlet which is confusing especially since Windows comes with actual curl.

edit: I looked into this further and in powershell the stop-parsing token --% can be used but it still expands environment variables which cannot be escaped. It's more correct to say that type of escaping isn't possible, not that it doesn't "need" escaping. It does it's just not possible.

Escaping a % character as %%, the way you can do inside batch files, isn't supported.

So in a ps1 file I'm pretty sure it would look something like this:

&"c:\windows\system32\curl.exe" --% -v --write-out %{http_code} example.com