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Subject: |
"cat x >> x" error even when x is empty |
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Date: |
Thu, 11 Sep 2014 15:00:44 +0200 |
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User-agent: |
Mutt/1.5.23-6361-vl-r59709 (2014-07-25) |
With coreutils 8.23 under Debian/unstable:
ypig% : > x
ypig% cat x >> x
cat: x: input file is output file
ypig% POSIXLY_CORRECT=1 cat x >> x
cat: x: input file is output file
while there's no reason to return an error in this case: the file
should just remain empty. Using the same file for input and output
isn't disallowed by POSIX, AFAIK.
This may not seem really useful here, but this can potentially break
scripts with things like:
cat "$foo" >> "$bar"
where "$foo" may be the same file as "$bar" only if it is empty.
BTW, when x isn't empty, I wonder whether an error is correct if
POSIXLY_CORRECT is set. The result will typically depend on the
implementation and possibly be non-deterministic, but POSIX doesn't
seem to allow an error (except FS errors, such as disk full).
--
Vincent Lefèvre <address@hidden> - Web: <https://www.vinc17.net/>
100% accessible validated (X)HTML - Blog: <https://www.vinc17.net/blog/>
Work: CR INRIA - computer arithmetic / AriC project (LIP, ENS-Lyon)
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--- Begin Message ---
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Subject: |
Re: bug#18449: "cat x >> x" error even when x is empty |
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Date: |
Thu, 11 Sep 2014 08:52:17 -0700 |
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Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.0 |
Eric Blake wrote:
Testing for an empty file is enough of an additional special case over
the existing check for same files that I don't think it is worth it.
It's a really cheap check, as a system call is needed only when the
input and output files are the same, and even then it's only an lseek
with SEEK_CUR so it's all in-memory. Plus, the POSIX 'cat' spec gives
an example of using 'cat' to copy an empty regular file to itself, and
(not unreasonably) says that should work.
As far as POSIX conformance goes, I don't think we need to worry about
POSIXLY_CORRECT here. We'll get POSIX fixed instead. Eric has already
started the ball rolling on that (thanks). Though the POSIX fix should
involve updating its example -- and quite possibly POSIX should continue
to allow catting an empty file to itself.
I installed the attached patch and am marking this as done. The 'cat'
source code is simpler now, so that's a win anyway....
0001-cat-allow-copying-empty-files-to-themselves.patch
Description: Text document
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