type nul >con suppresses output

[Sponge Belly]

Hello All!

I stumbled across this the other day:

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(cmd /d /u /c type nul >con
echo(first
echo(second
echo(third)

Only the first echo is displayed. The other two are suppressed.

Can anyone explain what’s going on here?

TIA!

- SB

[jeb]

I see this on my Win7 x64 system

first##second##third

Instead of #, an empty box is displayed

If I redirect this to a file, the file contains

first
second
third

jeb

[foxidrive]

I see the same as jeb on Windows 8.1

[Dos_Probie]

On Win7 and 8 It shows

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first♪◙second♪◙third♪◙

[penpen]

The characters "♪◙" are nothing else but the glyphs of "\r\n" (carriage return, newline), so the output is not wrong:
It is only rendered in a wrong way.
one possibility is that the cmd.exe parser may parse the full block and then executes it,
AND the cmd.exe then may 'occupy' all tokens following and executes it as a single command, so the result equals:

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>con cmd /C echo first&echo second&echo third

The wrong rendering of "\r\n" is caused (somehow) by the combination of ">con" and "cmd".
Maybe the inner cmd ("cmd /c") marks these characters somehow as 'consumed' when processing special characters,
so the outer (batch file) renders its glyphs, or ... (multiple possible answers: one as good as another).

penpen

[Sponge Belly]

Hi Penpen,

Thanks for the explanation. I was hoping one of you gurus might find an ingenious use for it, but it didn’t turn out that way.

This next little curiosity, however, might prove to be more fruitful. First, create a test file:

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>a2z.txt echo(the quick brown fox jumps over the lazy dog

And then try this:

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cmd/d /u /c type a2z.txt >con

Outputs only the first character (t) in the file. Next, put it all in a loop along with a couple of pauses:

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for /f delims^=^ eol^= %%c in ('
cmd /d /u /c type a2z.txt ^| (pause^>nul^&pause^>nul^&findstr "^"^)
') do echo(%%c

The output is h, the second character in the file. The snippet above could be easily extended to extract the nth character of a file… so long as the character isn’t CR, LF, or NUL.

- SB