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
next 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
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=CAKH6PiWVc03DtJ0ujAauFZNJn7cuH7-QTCRYi83_KESsZ2AKOQ@mail.gmail.com \
--to=douglas.mcilroy@dartmouth.edu \
--cc=tuhs@tuhs.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).