From: Rob Biedenharn <Rob@...>
Date: 2008-08-25T22:46:22+09:00
Subject: Re: Open file, get first line, delete first line close file


On Aug 25, 2008, at 9:21 AM, Erik Hollensbe wrote:

> Rob Biedenharn wrote:
>> On Aug 25, 2008, at 4:07 AM, Erik Hollensbe wrote:
>>
>>> Dave Bass wrote:
>>>> And... it's a lot easier to delete the *last* line of a file than  
>>>> the
>>>> first.
>>>
>>> I don't think this is actually true, can you explain further?
>>>
>>> -Erik
>>
>>
>> You can just truncate the file size.  You don't have any subsequent
>> lines (bytes) to move into a new position within the file.
>
> How do you know which line is the last line?
>
> Unless there's something I don't know, that involves reading the whole
> file, or a combination of seek/read from the end until you find the  
> last
> newline, which is essentially what tail +2 does, but starts at the
> beginning of the file.
>
> "Moving" data in a file is the worst possible scenario for I/O at all.
> You can do both of these operations in a single pass read of the file
> without shoving the whole thing into memory at once. It just involves
> writing to one file and reading from another, is all.
> -- 
> Posted via http://www.ruby-forum.com/.

Well, if you want/need the last line(s) of a file (presumably text or  
how would you define a "line"), you can take a look at the File::Tail  
gem.

   gem install file-tail

I had some Perl code (lifted from some forum or article) that would  
cut initial lines out of a log file using sysread/syswrite with a  
truncate to reset the end-of-file.  I don't recall if it used a single  
file descriptor or two separate ones, but the idea is the same -- move  
bytes "backward" across the gap that you want to eliminate.  I agree  
with your "worst possible scenario for I/O" assessment.

-Rob

Rob Biedenharn		http://agileconsultingllc.com
Rob@AgileConsultingLLC.com