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From: Douglas McIlroy <douglas.mcilroy@dartmouth.edu>
To: The Eunuchs Hysterical Society <tuhs@tuhs.org>
Subject: Re: [TUHS] head/sed/tail (was The Unix shell: a 50-year view)
Date: Fri, 16 Jul 2021 08:09:59 -0400	[thread overview]
Message-ID: <CAKH6PiWVc03DtJ0ujAauFZNJn7cuH7-QTCRYi83_KESsZ2AKOQ@mail.gmail.com> (raw)

>> -r is weird because it enables backwards reading, but only as
>> limited by count. Better would be a program, say revfile, that simply
>> reads backwards by lines. Then tail p  has an elegant implementation:
>>      revfile p | head | revfile

> tail -n can be smarter in that it can simply read the last K bytes
> and see if there are n lines. If not, it can read back further.
> revfile would have to read the whole file, which could be a lot
> more than n lines! tail -n < /dev/tty may never terminate but it
> will use a small finite amount of memory.

Revfile would work the same way. When head has seen enough
and terminates, revfile will get SIGPIPE and stop. I agree that,
depending on scheduling and buffer management, revfile might
read more than tail -n,  but  it wouldn't read the whole of a
humongous file.

Doug

             reply	other threads:[~2021-07-16 12:10 UTC|newest]

Thread overview: 34+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-07-16 12:09 Douglas McIlroy [this message]
2021-07-16 14:32 ` Bakul Shah
  -- strict thread matches above, loose matches on Subject: below --
2021-07-18 20:07 Douglas McIlroy
     [not found] <CAKH6PiW58PDPb5HRi12aKE+mT+O8AjETr9R51Db6U3KcEp_KkA@mail.gmail.com>
2021-07-16 14:17 ` Nelson H. F. Beebe
2021-07-16 16:13   ` Theodore Y. Ts'o
2021-07-15 22:26 Nelson H. F. Beebe
2021-07-15 23:18 ` Jim Davis
2021-07-16  0:02   ` John Floren
2021-07-16  1:02     ` Nelson H. F. Beebe
2021-07-16  8:27     ` Lars Brinkhoff
2021-07-16 15:28       ` John Floren
2021-07-16  0:02 ` Clem Cole
2021-07-16  0:25   ` Nelson H. F. Beebe
2021-07-16  8:50     ` Lars Brinkhoff
2021-07-15 22:00 Douglas McIlroy
2021-07-15 22:12 ` John Cowan
2021-07-15 21:26 Paul Ruizendaal
2021-07-15 19:01 Norman Wilson
2021-07-15 19:27 ` Clem Cole
2021-07-15 19:28   ` Clem Cole
2021-07-15 19:34   ` Warner Losh
2021-07-16  7:38     ` arnold
2021-07-16 16:09       ` Warner Losh
2021-07-16  8:05   ` Lars Brinkhoff
2021-07-16 14:19     ` Clem Cole
2021-07-17  0:34       ` Charles Anthony
2021-07-15 16:54 Nelson H. F. Beebe
2021-07-15 15:44 Norman Wilson
2021-07-15  2:38 Douglas McIlroy
2021-07-15  4:19 ` arnold
2021-07-15  4:25   ` Adam Thornton
2021-07-15  7:20   ` Thomas Paulsen
2021-07-15 14:28 ` Theodore Y. Ts'o
2021-07-15 22:29 ` Bakul Shah

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