Re: Strange sed behavior

[Micah Cowan]

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Stephane CHAZELAS wrote:
> 2007-10-18, 14:47(+02), David Kastrup:
>> Can anybody explain to me what is happening here?  I am a bit befuddled.
>>
>> $ { echo 1;echo 2;}|sed 'H;${x;P;D;}'
> [...]
> 
> Your code is doing
> 
> $h = $p = ""
> 
> while ($p = readline()) {
> START:
>   $h = $h . "\n" . $p;      # H
>   if (lastline) {
>     ($h, $p) = ($p, $h);    # x
>     print firstlineof($p);  # P
>     removefirstlineof($p); if ($p != "") goto START; # D
>   }
> }

I did _not_ know that D will not result in appending the next input
line, and instead just use the current pattern buffer unless it's empty
(not a "normal cycle"). GNU's docs are great at explaining this;
unfortunately, POSIX seems not at all clear on that point. Maybe I
should issue a defect report on that, if there isn't one...

- --
Micah J. Cowan
Programmer, musician, typesetting enthusiast, gamer...
http://micah.cowan.name/

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