GNU bug report logs - #23832
sed combine d with q

Previous Next

To add a comment to this bug, you must first unarchive it, by sending
a message to control AT debbugs.gnu.org, with unarchive 23832 in the body.
You can then email your comments to 23832 AT debbugs.gnu.org in the normal way.

Toggle the display of automated, internal messages from the tracker.

Message #5 received at submit <at> debbugs.gnu.org (full text, mbox):

From: Xen <xen <at> dds.nl>
To: bug-sed <at> gnu.org
Subject: sed combine d with q
Date: Thu, 23 Jun 2016 10:31:58 +0200 (CEST)

Hey, I am not sure if this is "by design" or not but....

Suppose a text file with empty lines here and there. You want to print up 
to, but not including, the first newline.

The first "^$", I mean.

My idea was to use "/^$/{d;q}. I was under the assumption that both 
commands would get executed.

However q is not executed.

However when we reverse it, and use sed "/^$/{p;q}"; the effect is that 
the first matching newline (empty line) is printed twice, and then the 
program quits. So {p;q} works, but {d;q} doesn't.

You are allowed to double the newline (empty line), but not remove it....

Now when I use:

sed -n "/^$/!p;/^$/q", as a way of not printing the first matching 
newline, and then quitting, which is basically the same as deleting it 
(this feels like if it is sunny weather; cry and moan, but you are not 
allowed to do so; instead, you must cry when it is not cloudy, and moan 
when it is sunny.

Same effect.

Just different logic ;-).

I guess it is intentional. The d command is the only thing that can wipe a 
line, but it will stop command execution and "start a new cycle". The 
following will wipe everything, but it can't wipe that newline:

sed "/^\r$/{:again;N;s/.*\n.*//;t again}".

It's just funkily incredible :p.

Oh well. I had another few lifetimes to waste.... on something ;-).

Regards.

Message #8 received at 23832 <at> debbugs.gnu.org (full text, mbox):

From: Assaf Gordon <assafgordon <at> gmail.com>
To: Xen <xen <at> dds.nl>, 23832 <at> debbugs.gnu.org
Subject: Re: bug#23832: sed combine d with q
Date: Thu, 23 Jun 2016 10:17:54 -0400

tag 23832 notabug
close 23832
stop

Hello,

On 06/23/2016 04:31 AM, Xen wrote:
> Hey, I am not sure if this is "by design" or not but....
[...]
> I guess it is intentional. The d command is the only thing that can wipe a line, but it will stop command execution and "start a new cycle".

This behavior is by design, and mandated by POSIX:
"d - Delete the pattern space and start the next cycle."
http://pubs.opengroup.org/onlinepubs/009604599/utilities/sed.html#tag_04_126_13_03

> Suppose a text file with empty lines here and there. You want to print up to, but not including, the first newline.
>The first "^$", I mean.

The "Q" command (a GNU Sed extension) might be of help:

  $ printf "a\nb\n\nc\nd\n"
  a
  b
  
  c
  d

  $ printf "a\nb\n\nc\nd\n" | sed '/^$/Q'
  a
  b


The Q command quits without printing the pattern space.
To learn more about GNU sed extension command, see here:
  https://www.gnu.org/software/sed/manual/sed.html#Extended-Commands

Alternatively, If you can not use GNU extension, combining two 'sed' might be the simplest work-around:

  $ printf "a\nb\n\nc\nd\n" | sed '/^$/q' | sed '$d'
  a
  b


As such I'm closing this bug, but discussion can continue by replying to this thread.

regards,
 - assaf

Message #11 received at 23832 <at> debbugs.gnu.org (full text, mbox):

From: Xen <xen <at> dds.nl>
To: Assaf Gordon <assafgordon <at> gmail.com>
Cc: 23832 <at> debbugs.gnu.org
Subject: Re: bug#23832: sed combine d with q
Date: Thu, 23 Jun 2016 16:51:10 +0200

Assaf Gordon schreef op 23-06-2016 16:17:

> This behavior is by design, and mandated by POSIX:
> "d - Delete the pattern space and start the next cycle."
> http://pubs.opengroup.org/onlinepubs/009604599/utilities/sed.html#tag_04_126_13_03

> The "Q" command (a GNU Sed extension) might be of help:

That is so wonderful. I had yet not been imaginative enough to try the 
uppercase version of that command. I was using an online tutorial that 
has been there for a long time, but that never mentioned any such thing. 
So thank you. Very much.


> The Q command quits without printing the pattern space.

So simple and so obvious :). (I mean the need for it, also). It appears 
I was not the only person to think of this ;-).

Always happy to know you are not perfectly insane, only readily so ;-) 
:p.

> To learn more about GNU sed extension command, see here:
>   https://www.gnu.org/software/sed/manual/sed.html#Extended-Commands

Thanks.

> Alternatively, If you can not use GNU extension, combining two 'sed'
> might be the simplest work-around:
> 
>   $ printf "a\nb\n\nc\nd\n" | sed '/^$/q' | sed '$d'
>   a
>   b

Yes, I had not considered (or fully realized) that $ matches the last 
line. I was trying to avoid having to use tail just for this single 
thing.

> As such I'm closing this bug, but discussion can continue by replying
> to this thread.

You're welcome, and thanks.

Regards,

Bart.

Message #14 received at submit <at> debbugs.gnu.org (full text, mbox):

From: Davide Brini <dave_br <at> gmx.com>
To: bug-sed <at> gnu.org
Subject: Re: bug#23832: sed combine d with q
Date: Thu, 23 Jun 2016 17:10:40 +0200

On Thu, 23 Jun 2016 10:31:58 +0200 (CEST), Xen <xen <at> dds.nl> wrote:

> Hey, I am not sure if this is "by design" or not but....
> 
> Suppose a text file with empty lines here and there. You want to print up 
> to, but not including, the first newline.
> 
> The first "^$", I mean.
> 
> My idea was to use "/^$/{d;q}. I was under the assumption that both 
> commands would get executed.
> 
> However q is not executed.
> 
> However when we reverse it, and use sed "/^$/{p;q}"; the effect is that 
> the first matching newline (empty line) is printed twice, and then the 
> program quits. So {p;q} works, but {d;q} doesn't.
> 
> You are allowed to double the newline (empty line), but not remove it....
> 
> Now when I use:
> 
> sed -n "/^$/!p;/^$/q", as a way of not printing the first matching 
> newline, and then quitting, which is basically the same as deleting it 
> (this feels like if it is sunny weather; cry and moan, but you are not 
> allowed to do so; instead, you must cry when it is not cloudy, and moan 
> when it is sunny.

You can use

sed -n '/^$/q; p'

-- 
D.

Previous Next

GNU bug tracking system
Copyright (C) 1999 Darren O. Benham, 1997,2003 nCipher Corporation Ltd, 1994-97 Ian Jackson.