GNU bug report logs -
#12115
Please add tip about using tac to reverse a file byte-by-byte
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Reported by: Reuben Thomas <rrt <at> sc3d.org>
Date: Wed, 1 Aug 2012 21:46:01 UTC
Severity: normal
Done: Paul Eggert <eggert <at> cs.ucla.edu>
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Message #5 received at submit <at> debbugs.gnu.org (full text, mbox):
Patch:
From 3c1b7d442dc6161e3c88703c9acc3f768f9ba689 Mon Sep 17 00:00:00 2001
From: Reuben Thomas <rrt <at> sc3d.org>
Date: Wed, 1 Aug 2012 22:35:38 +0100
Subject: [PATCH] doc: Add tip on reversing a file byte-by-byte
* doc/coreutils.texi: Note how to use tac to reverse a file
byte-by-byte.
---
doc/coreutils.texi | 6 ++++++
1 file changed, 6 insertions(+)
diff --git a/doc/coreutils.texi b/doc/coreutils.texi
index 516ec73..66b0838 100644
--- a/doc/coreutils.texi
+++ b/doc/coreutils.texi
@@ -1625,6 +1625,12 @@ Use @var{separator} as the record separator,
instead of newline.
@end table
+To reverse a file byte by byte, use:
+
+@example
+tac -r -s '.\| ' @var{file}
+@end example
+
@exitstatus
--
1.7.9.5
--
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Message #8 received at 12115 <at> debbugs.gnu.org (full text, mbox):
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On 08/01/2012 03:37 PM, Reuben Thomas wrote:
> Patch:
>
>>From 3c1b7d442dc6161e3c88703c9acc3f768f9ba689 Mon Sep 17 00:00:00 2001
> From: Reuben Thomas <rrt <at> sc3d.org>
> Date: Wed, 1 Aug 2012 22:35:38 +0100
> Subject: [PATCH] doc: Add tip on reversing a file byte-by-byte
>
> * doc/coreutils.texi: Note how to use tac to reverse a file
> byte-by-byte.
> ---
> doc/coreutils.texi | 6 ++++++
> 1 file changed, 6 insertions(+)
>
> diff --git a/doc/coreutils.texi b/doc/coreutils.texi
> index 516ec73..66b0838 100644
> --- a/doc/coreutils.texi
> +++ b/doc/coreutils.texi
> @@ -1625,6 +1625,12 @@ Use @var{separator} as the record separator,
> instead of newline.
>
> @end table
>
> +To reverse a file byte by byte, use:
> +
> +@example
> +tac -r -s '.\| ' @var{file}
Cool tip. But not quite accurate - you need a literal newline instead
of a space on the other side of that regex alternation.
--
Eric Blake eblake <at> redhat.com +1-919-301-3266
Libvirt virtualization library http://libvirt.org
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Message #11 received at 12115 <at> debbugs.gnu.org (full text, mbox):
On 1 August 2012 22:50, Eric Blake <eblake <at> redhat.com> wrote:
>
> Cool tip. But not quite accurate - you need a literal newline instead
> of a space on the other side of that regex alternation.
Sorry, I just copied what I found at
http://stackoverflow.com/questions/6438099/linux-command-line-tool-to-reverse-byte-order-change-endianess
without thinking. I see your point.
--
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Message #14 received at 12115 <at> debbugs.gnu.org (full text, mbox):
Reuben Thomas wrote:
...
> +To reverse a file byte by byte, use:
> +
> +@example
> +tac -r -s '.\| ' @var{file}
> +@end example
> +
Thanks!
That's a good demonstration.
Please note that it's not efficient and include the perl one-liner
for reference.
Here's a portable way to include the newline correction Eric mentioned:
nl='
'
tac -r -s ".\\|$nl" @var{file}
I would add that with a shell like bash or zsh you can do it more cleanly,
using $'\n' in place of the shell variable:
(note that below there's the added advantage of not having to
double-quote to include $nl and hence not having to double-backslash)
tac -r -s '.\|'$'\n' @var{file}
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Message #17 received at 12115 <at> debbugs.gnu.org (full text, mbox):
Jim Meyering <jim <at> meyering.net> writes:
> (note that below there's the added advantage of not having to
> double-quote to include $nl and hence not having to double-backslash)
You can have that with the variable as well.
Andreas.
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Message #20 received at 12115 <at> debbugs.gnu.org (full text, mbox):
Andreas Schwab wrote:
> Jim Meyering <jim <at> meyering.net> writes:
>
>> (note that below there's the added advantage of not having to
>> double-quote to include $nl and hence not having to double-backslash)
>
> You can have that with the variable as well.
You're right. We could write it like this:
printf '\na\nb'|tac -rs '.\|'$nl
or even drop the quotes altogether:
printf '\na\nb'|tac -rs .\\\|$nl
Golf is fun, but those seem less well suited as documentation examples.
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Message #23 received at 12115 <at> debbugs.gnu.org (full text, mbox):
Jim Meyering <jim <at> meyering.net> writes:
> Andreas Schwab wrote:
>> Jim Meyering <jim <at> meyering.net> writes:
>>
>>> (note that below there's the added advantage of not having to
>>> double-quote to include $nl and hence not having to double-backslash)
>>
>> You can have that with the variable as well.
>
> You're right. We could write it like this:
>
> printf '\na\nb'|tac -rs '.\|'$nl
No, you need the quotes around $nl.
Andreas.
--
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Message #26 received at 12115 <at> debbugs.gnu.org (full text, mbox):
Andreas Schwab wrote:
> Jim Meyering <jim <at> meyering.net> writes:
>
>> Andreas Schwab wrote:
>>> Jim Meyering <jim <at> meyering.net> writes:
>>>
>>>> (note that below there's the added advantage of not having to
>>>> double-quote to include $nl and hence not having to double-backslash)
>>>
>>> You can have that with the variable as well.
>>
>> You're right. We could write it like this:
>>
>> printf '\na\nb'|tac -rs '.\|'$nl
>
> No, you need the quotes around $nl.
You need the double quotes around $nl with bash,
but not with zsh:
zsh$ nl='
quote> '
zsh$ printf %s $nl
zsh$
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Message #29 received at 12115 <at> debbugs.gnu.org (full text, mbox):
Jim Meyering wrote (Thursday, August 02, 2012 2:08 PM):
> You need the double quotes around $nl with bash,
> but not with zsh:
>
> zsh$ nl='
> quote> '
> zsh$ printf %s $nl
>
> zsh$
For bash, one solution is:
$ printf '\na\nb'|tac -rs '.\|'$(printf "\n")
b
a
$
;-)
Have a nice day,
Berny
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Message #32 received at 12115 <at> debbugs.gnu.org (full text, mbox):
"Voelker, Bernhard" <bernhard.voelker <at> siemens-enterprise.com> writes:
> Jim Meyering wrote (Thursday, August 02, 2012 2:08 PM):
>
>> You need the double quotes around $nl with bash,
Or any other POSIX shell.
> For bash, one solution is:
>
> $ printf '\na\nb'|tac -rs '.\|'$(printf "\n")
No, command substitution strips trailing newlines.
Andreas.
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Message #35 received at 12115 <at> debbugs.gnu.org (full text, mbox):
[Message part 1 (text/plain, inline)]
On 08/02/2012 07:18 AM, Andreas Schwab wrote:
>> For bash, one solution is:
>>
>> $ printf '\na\nb'|tac -rs '.\|'$(printf "\n")
>
> No, command substitution strips trailing newlines.
So don't make it trailing:
printf '\na\nb'|tac -rs "$(printf '\n\\|.')"
--
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Libvirt virtualization library http://libvirt.org
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Message #38 received at 12115 <at> debbugs.gnu.org (full text, mbox):
Eric Blake <eblake <at> redhat.com> writes:
> On 08/02/2012 07:18 AM, Andreas Schwab wrote:
>>> For bash, one solution is:
>>>
>>> $ printf '\na\nb'|tac -rs '.\|'$(printf "\n")
>>
>> No, command substitution strips trailing newlines.
>
> So don't make it trailing:
>
> printf '\na\nb'|tac -rs "$(printf '\n\\|.')"
It's still a useless command substitution.
Andreas.
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Message #41 received at 12115 <at> debbugs.gnu.org (full text, mbox):
On 2 August 2012 15:00, Andreas Schwab <schwab <at> linux-m68k.org> wrote:
> Eric Blake <eblake <at> redhat.com> writes:
>
>> On 08/02/2012 07:18 AM, Andreas Schwab wrote:
[blah blah blah]
All this just because there's no standard interface in coreutils (or
indeed other GNU programs) to control the regexp engine, so you can
make "." match any character. LOL.
--
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Message #44 received at 12115 <at> debbugs.gnu.org (full text, mbox):
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On 2 August 2012 15:03, Reuben Thomas <rrt <at> sc3d.org> wrote:
> On 2 August 2012 15:00, Andreas Schwab <schwab <at> linux-m68k.org> wrote:
> > Eric Blake <eblake <at> redhat.com> writes:
> >
> >> On 08/02/2012 07:18 AM, Andreas Schwab wrote:
>
> [blah blah blah]
>
> All this just because there's no standard interface in coreutils (or
> indeed other GNU programs) to control the regexp engine, so you can
> make "." match any character. LOL.
>
> --
> http://rrt.sc3d.org
>
Given how long it took how many experts to come up with a valid
incantation, it seems to me it *is* a useful documentation example. So can
we have an agreed version substituted into my patch and committed, please?
--
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You have taken responsibility.
(Fri, 01 Feb 2013 21:37:03 GMT)
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Message #49 received at 12115-done <at> debbugs.gnu.org (full text, mbox):
On 02/01/13 11:14, Reuben Thomas wrote:
> Given how long it took how many experts to come up with a valid
> incantation, it seems to me it *is* a useful documentation example.
Sure, but the valid incantation is a bit confusing, and it's not
documented correctly (it reverses characters, not bytes).
As far as I know there's no portable way to use 'tac' to
reverse a file byte-by-byte.
I installed this:
From dad0a3c743464664ed6a7ae8445cb58b8326db6d Mon Sep 17 00:00:00 2001
From: Paul Eggert <eggert <at> cs.ucla.edu>
Date: Fri, 1 Feb 2013 13:32:48 -0800
Subject: [PATCH] doc: say how to tac char-by-char
This fixes Bug#12115, reported by Reuben Thomas.
* doc/coreutils.texi (tac invocation): Document how to reverse a
file character by character. Break out MS-DOS into a separate
section, like 'cat' does.
---
doc/coreutils.texi | 15 +++++++++++----
1 file changed, 11 insertions(+), 4 deletions(-)
diff --git a/doc/coreutils.texi b/doc/coreutils.texi
index eac8d52..e29af8b 100644
--- a/doc/coreutils.texi
+++ b/doc/coreutils.texi
@@ -1613,10 +1613,7 @@ precedes in the file.
@itemx --regex
@opindex -r
@opindex --regex
-Treat the separator string as a regular expression. Users of @command{tac}
-on MS-DOS/MS-Windows should note that, since @command{tac} reads files in
-binary mode, each line of a text file might end with a CR/LF pair
-instead of the Unix-style LF.
+Treat the separator string as a regular expression.
@item -s @var{separator}
@itemx --separator=@var{separator}
@@ -1626,8 +1623,18 @@ Use @var{separator} as the record separator, instead of newline.
@end table
+On systems like MS-DOS that distinguish between text and binary files,
+@command{tac} reads and writes in binary mode.
+
@exitstatus
+Example:
+
+@example
+# Reverse a file character by character.
+tac -r -s 'x\|[^x]'
+@end example
+
@node nl invocation
@section @command{nl}: Number lines and write files
--
1.7.11.7
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Message #52 received at 12115-done <at> debbugs.gnu.org (full text, mbox):
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On 1 February 2013 21:36, Paul Eggert <eggert <at> cs.ucla.edu> wrote:
> On 02/01/13 11:14, Reuben Thomas wrote:
>
> > Given how long it took how many experts to come up with a valid
> > incantation, it seems to me it *is* a useful documentation example.
>
> Sure, but the valid incantation is a bit confusing, and it's not
> documented correctly (it reverses characters, not bytes).
> As far as I know there's no portable way to use 'tac' to
> reverse a file byte-by-byte.
>
Thanks!
What about a way to do it on a GNU system? It seems reasonable to document
GNU-specific hacks in GNU documentation when a portable solution is not
forthcoming.
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Message #55 received at 12115-done <at> debbugs.gnu.org (full text, mbox):
On 02/01/13 13:43, Reuben Thomas wrote:
> What about a way to do it on a GNU system?
I think it works if you set LC_ALL='C', as the
C locale is unibyte on a GNU system, but someone
should double-check this.
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Message #58 received at 12115-done <at> debbugs.gnu.org (full text, mbox):
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On 1 February 2013 21:46, Paul Eggert <eggert <at> cs.ucla.edu> wrote:
> On 02/01/13 13:43, Reuben Thomas wrote:
> > What about a way to do it on a GNU system?
>
> I think it works if you set LC_ALL='C', as the
> C locale is unibyte on a GNU system, but someone
> should double-check this.
>
A bit of searching does not reveal to me how to verify or disprove this,
but how about using a known unibyte-locale such as an ISO8859 one? Or is
the problem then that there's none that can be guaranteed to exist?
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Message #61 received at 12115-done <at> debbugs.gnu.org (full text, mbox):
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On 02/01/2013 02:43 PM, Reuben Thomas wrote:
> On 1 February 2013 21:36, Paul Eggert <eggert <at> cs.ucla.edu> wrote:
>
>> On 02/01/13 11:14, Reuben Thomas wrote:
>>
>>> Given how long it took how many experts to come up with a valid
>>> incantation, it seems to me it *is* a useful documentation example.
>>
>> Sure, but the valid incantation is a bit confusing, and it's not
>> documented correctly (it reverses characters, not bytes).
>> As far as I know there's no portable way to use 'tac' to
>> reverse a file byte-by-byte.
Use LC_ALL=C to force single-byte characters.
>>
>
> Thanks!
>
> What about a way to do it on a GNU system? It seems reasonable to document
> GNU-specific hacks in GNU documentation when a portable solution is not
> forthcoming.
tac is a GNU program not standardized elsewhere, so if you have tac, it
is likely GNU tac to begin with.
The manual already contains:
@example
# Reverse a file character by character.
tac -r -s 'x\|[^x]'
@end example
Do we need a further example, or just modify that existing example to
call out the use of LC_ALL=C for reversing by bytes instead of characters?
--
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Message #64 received at 12115-done <at> debbugs.gnu.org (full text, mbox):
On 02/01/13 13:52, Reuben Thomas wrote:
> how about using a known unibyte-locale such as an ISO8859 one? Or is
> the problem then that there's none that can be guaranteed to exist?
Yes, that's the problem. POSIX does not require the existence
of any unibyte locale. Even the C locale might be multibyte,
and is multibyte, at least on OS X systems.
bug archived.
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