GNU bug report logs - #21250
sed: empty label for :/b/t commands

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

From: Stephane Chazelas <stephane.chazelas <at> gmail.com>
To: bug-sed <at> gnu.org
Subject: sed: empty label for :/b/t commands
Date: Thu, 13 Aug 2015 15:37:54 +0100

Hello.

Not really a bug as such, just a note of how a corner case of
GNU sed syntax parsing is being misused:

Recently, on unix.stackexchange.com, I've seen several times
people (might have been the same person every time) doing things
like:

sed ':;s/../../;t'

That is, define an empty label, and branch on it later on.

There's an example at
https://unix.stackexchange.com/revisions/223003/1
https://unix.stackexchange.com/a/223003

as per the documentation, it's not meant to work that way. The
label for ":" is not optional and "t" or "b" without label are
meant to jump to the end (start a new label).

I've warned that person that relying on an unspecified behaviour
was not a good idea as it may no longer work in the future, but
now if several people follow his example, that may mean it will
become a de-facto feature of GNU sed, and then the sed
maintainers would have to make sure it's not removed in future
versions (and document it).

So maybe it would be a good idea to either remove that
"feature" or make it an official one before its usage becomes
widespread.

In any case the:

sed ': label;s/../../;t label'

syntax is not portable anyway. The POSIX syntax being:

sed -e ': label' -e 's/../../;t label'

or

sed ':label
s/../../;t label'

Note that at the moment, GNU sed is not POSIX in that sed ':a;b'
is meant to define a label called "a;b", but that's going to change
in the next version of the spec where the behaviour will be
unspecified instead (so GNU sed doesn't need to be changed).
http://austingroupbugs.net/view.php?id=945

-- 
Stephane

Message #8 received at 21250 <at> debbugs.gnu.org (full text, mbox):

From: Jim Meyering <jim <at> meyering.net>
To: Stephane Chazelas <stephane.chazelas <at> gmail.com>
Cc: 21250 <at> debbugs.gnu.org
Subject: Re: bug#21250: sed: empty label for :/b/t commands
Date: Sat, 22 Aug 2015 22:02:42 -0700

On Thu, Aug 13, 2015 at 7:37 AM, Stephane Chazelas
<stephane.chazelas <at> gmail.com> wrote:
> Hello.
>
> Not really a bug as such, just a note of how a corner case of
> GNU sed syntax parsing is being misused:
>
> Recently, on unix.stackexchange.com, I've seen several times
> people (might have been the same person every time) doing things
> like:
>
> sed ':;s/../../;t'
>
> That is, define an empty label, and branch on it later on.
>
> There's an example at
> https://unix.stackexchange.com/revisions/223003/1
> https://unix.stackexchange.com/a/223003
>
> as per the documentation, it's not meant to work that way. The
> label for ":" is not optional and "t" or "b" without label are
> meant to jump to the end (start a new label).

Thank you for pointing that out.
I am preparing a patch to remove that functionality, making sed do
this, instead:

  $ sed :
  sed: -e expression #1, char 1: ":" lacks a label

Does anyone feel differently?
However, note that I have not surveyed other sed implementations.

> I've warned that person that relying on an unspecified behaviour
> was not a good idea as it may no longer work in the future, but
> now if several people follow his example, that may mean it will
> become a de-facto feature of GNU sed, and then the sed
> maintainers would have to make sure it's not removed in future
> versions (and document it).
>
> So maybe it would be a good idea to either remove that
> "feature" or make it an official one before its usage becomes
> widespread.

Message #11 received at 21250 <at> debbugs.gnu.org (full text, mbox):

From: Stephane Chazelas <stephane.chazelas <at> gmail.com>
To: Jim Meyering <jim <at> meyering.net>
Cc: 21250 <at> debbugs.gnu.org
Subject: Re: bug#21250: sed: empty label for :/b/t commands
Date: Sun, 23 Aug 2015 13:08:30 +0100

2015-08-22 22:02:42 -0700, Jim Meyering:
[...]
> I am preparing a patch to remove that functionality, making sed do
> this, instead:
> 
>   $ sed :
>   sed: -e expression #1, char 1: ":" lacks a label
> 
> Does anyone feel differently?
> However, note that I have not surveyed other sed implementations.
[...]

Thanks.

Solaris 11 (/bin/sed or /usr/xpg4/bin/sed):

$ echo aa | sed -e : -e s/a/b/ -e t
ba

(it doesn't complain on :, but t branches to the end, not to
that empty label).

Same with busybox sed.

With FreeBSD sed:

$ echo aa | sed -e : -e s/a/b/ -e t
sed: 1: ":
": empty label

(exit status: 1)

-- 
Stephane

Message #14 received at 21250 <at> debbugs.gnu.org (full text, mbox):

From: Jim Meyering <jim <at> meyering.net>
To: Stephane Chazelas <stephane.chazelas <at> gmail.com>
Cc: 21250 <at> debbugs.gnu.org
Subject: Re: bug#21250: sed: empty label for :/b/t commands
Date: Sun, 23 Aug 2015 21:54:30 -0700

[Message part 1 (text/plain, inline)]

On Sun, Aug 23, 2015 at 5:08 AM, Stephane Chazelas
<stephane.chazelas <at> gmail.com> wrote:
> 2015-08-22 22:02:42 -0700, Jim Meyering:
> [...]
>> I am preparing a patch to remove that functionality, making sed do
>> this, instead:
>>
>>   $ sed :
>>   sed: -e expression #1, char 1: ":" lacks a label
>>
>> Does anyone feel differently?
>> However, note that I have not surveyed other sed implementations.
> [...]
>
> Thanks.
>
> Solaris 11 (/bin/sed or /usr/xpg4/bin/sed):
>
> $ echo aa | sed -e : -e s/a/b/ -e t
> ba
>
> (it doesn't complain on :, but t branches to the end, not to
> that empty label).
>
> Same with busybox sed.
>
> With FreeBSD sed:
>
> $ echo aa | sed -e : -e s/a/b/ -e t
> sed: 1: ":
> ": empty label
>
> (exit status: 1)

Thanks for checking those.
Here's the patch I expect to push tomorrow:

[0001-sed-reject-a-command-without-a-label.patch (text/x-patch, attachment)]

Message #21 received at 21250-done <at> debbugs.gnu.org (full text, mbox):

From: Assaf Gordon <assafgordon <at> gmail.com>
To: 21250-done <at> debbugs.gnu.org
Subject: Re: bug#21250: sed: empty label for :/b/t commands
Date: Wed, 25 Jan 2017 02:28:34 +0000

tags 21250 fixed
close 21250
stop

Hello,

This fix has been commited here:
http://git.savannah.gnu.org/cgit/sed.git/commit/?id=31c84cbcfd2516e2

And was included in the recently released sed-4.3:

 $ echo aa | sed -e : -e s/a/b/ -e t
 sed: -e expression #1, char 1: ":" lacks a label


I'm marking this as 'done'.

regards,
- assaf

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