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Subject: |
t incorrectly branching |
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Date: |
Tue, 21 Aug 2018 12:36:25 +0200 |
printf 'Hello\n' | sed '
s/foobar//
t end
s/Hello/Goodbye/
:end'
This works as expected, it prints Goodbye.
printf 'Hello\n' | sed '
s/Hello/Hello to you/
s/foobar//
t end
s/Hello/Goodbye/
:end'
Since t should only look at whether the *last* substitution changed the pattern
space, it is my understanding that this should print:
Goodbye to you
But sed prints instead:
Hello to you
If I got this right, that means there's a bug in sed – maybe resetting the
"last substitution was successful" flag isn't done properly? Or am I
misunderstanding something here after all?
Tried on these versions, both did exactly the same:
$ sed --version | head -n1
sed (GNU sed) 4.2.2
sed (GNU sed) 4.5
pgpswBlJUDFn7.pgp
Description: OpenPGP digital signature
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Subject: |
Re: bug#32494: t incorrectly branching |
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Date: |
Tue, 21 Aug 2018 10:52:52 -0500 |
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User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.9.1 |
tag 32494 notabug
thanks
On 08/21/2018 05:36 AM, Ruben Maes wrote:
printf 'Hello\n' | sed '
s/Hello/Hello to you/
s/foobar//
t end
s/Hello/Goodbye/
:end'
Since t should only look at whether the *last* substitution changed the pattern
space,
That's not how POSIX describes it:
http://pubs.opengroup.org/onlinepubs/9699919799/utilities/sed.html
"[2addr]t [label]
Test. Branch to the : command verb bearing the label if any
substitutions have been made since the most recent reading of an input
line or execution of a t. If label is not specified, branch to the end
of the script."
'info sed' words it a bit differently:
't LABEL'
Branch to LABEL only if there has been a successful 's'ubstitution
since the last input line was read or conditional branch was taken.
The LABEL may be omitted, in which case the next cycle is started.
which seems to emphasize that the previous 't' must have been
successfully taken before the condition gets reset (but if the last
conditional 't' was not taken, then there has not been a successful
match, so I don't know if the difference can be observed in practice).
it is my understanding that this should print:
Goodbye to you
But sed prints instead:
Hello to you
sed is behaving correctly; it is your understanding that was off. It is
not "branch if last substitution succeeded", but "branch if ANY
substitution has succeeded since the last input or 't'".
One possible fix to your script, then, is to bound any substitution that
you want to test in isolation with an earlier 't', perhaps looking
something like:
t reset
: reset
s/...//
t end
such that whether or not 't reset' fires, execution resumes at s/// in
question with the condition cleared.
As such, I'm closing this as not a bug, but feel free to add more
comments on the topic.
--
Eric Blake, Principal Software Engineer
Red Hat, Inc. +1-919-301-3266
Virtualization: qemu.org | libvirt.org
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