GNU bug report logs - #18179
stat doesn't dereference symlinked file name

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

From: Tomasz Pala <gotar <at> polanet.pl>
To: bug-coreutils <at> gnu.org
Subject: stat doesn't dereference symlinked file name
Date: Sun, 3 Aug 2014 11:20:01 +0200

~: touch a
~: ln -s a b
~: stat -c "%n" a
a
~: stat -c "%n" b
b
~: stat -Lc "%n" b
b

the last command should return 'a'. To be more verbose:

~: stat b
  File: `b' -> `a'
  Size: 1               Blocks: 0          IO Block: 4096   symbolic link
Device: 10h/16d Inode: 60112068    Links: 1

~: stat -L b
  File: `b'
  Size: 0               Blocks: 0          IO Block: 4096   regular empty file
Device: 10h/16d Inode: 60112022    Links: 1
[...]

one cannot get the righthand part of dereference only (the same applies
to %N).

-- 
Tomasz Pala <gotar <at> pld-linux.org>

Message #8 received at 18179 <at> debbugs.gnu.org (full text, mbox):

From: Bob Proulx <bob <at> proulx.com>
To: Tomasz Pala <gotar <at> polanet.pl>
Cc: 18179 <at> debbugs.gnu.org
Subject: Re: bug#18179: stat doesn't dereference symlinked file name
Date: Sun, 3 Aug 2014 15:39:33 -0600

tag 18179 + notabug
close 18179
thanks

Thanks for the bug report.  However this seems to be a
misunderstanding of the available print formats.

Tomasz Pala wrote:
> ~: touch a
> ~: ln -s a b
> ~: stat -c "%n" a
> a
> ~: stat -c "%n" b
> b
> ~: stat -Lc "%n" b
> b

You need %N to show what you are wanting to show.

  $ stat -c %N b
  ‘b’ -> ‘a’
  $ stat -Lc %N b
  ‘b’

> ~: stat -Lc "%n" b
> b
> the last command should return 'a'.

Not quite.  The %n returns the file name as handed to stat on the
command line.  To get the quoted name dereference if a symbolic link
then you need the %N format.

         %N     quoted file name with dereference if symbolic link

> To be more verbose:
> 
> ~: stat b
>   File: `b' -> `a'
>   Size: 1               Blocks: 0          IO Block: 4096   symbolic link
> Device: 10h/16d Inode: 60112068    Links: 1
> 
> ~: stat -L b
>   File: `b'
>   Size: 0               Blocks: 0          IO Block: 4096   regular empty file
> Device: 10h/16d Inode: 60112022    Links: 1
> [...]
> 
> one cannot get the righthand part of dereference only (the same applies
> to %N).

I read the above part several times but your meaning was not clear.
In any case I see no bug in stat.

Perhaps you really want readlink?

  $ readlink b
  a

  $ readlink -f b
  /tmp/a

For what I believe you are doing the typical idiom is to use
"readlink" to print the value of the symbolic link.

Bob

Message #15 received at 18179 <at> debbugs.gnu.org (full text, mbox):

From: Tomasz Pala <gotar <at> polanet.pl>
To: Bob Proulx <bob <at> proulx.com>
Cc: 18179 <at> debbugs.gnu.org
Subject: Re: bug#18179: stat doesn't dereference symlinked file name
Date: Mon, 4 Aug 2014 20:33:34 +0200

On Sun, Aug 03, 2014 at 15:39:33 -0600, Bob Proulx wrote:

> misunderstanding of the available print formats.
> 
>> ~: stat -Lc "%n" b
>> b
> 
> You need %N to show what you are wanting to show.

No, I've mentioned %N later and it's just a shortcut for general state:
while dereferencing symlink, all the information come from dereferenced
file _except_ it's name.

~: ln -s a b
~: stat -L b

  File: `b'
  Size: 0               Blocks: 0          IO Block: 4096   regular empty file
Device: ah/10d  Inode: 26596       Links: 1
Access: (0600/-rw-------)  Uid: (  500/   gotar)   Gid: (  500/   gotar)
Access: 2014-08-04 20:12:40.008000499 +0200
Modify: 2014-08-04 20:12:40.008000499 +0200
Change: 2014-08-04 20:12:40.008000499 +0200

Inode 26596 contains file 'a', stat doesn't dereference file name.

>> ~: stat -Lc "%n" b
>> b
>> the last command should return 'a'.
> 
> Not quite.  The %n returns the file name as handed to stat on the
> command line.

But I've ordered to dereference that symlink (-L) first. What makes %n (and only
%n) to behave differently than %s or all the other sequences? In
different words: what makes %n to evaluate _before_ dereference (-L)
unike all the other sequences, which are evaluated _after_?

>  To get the quoted name dereference if a symbolic link
> then you need the %N format.
> 
>          %N     quoted file name with dereference if symbolic link

The problem with %N is: when dereferencing with -L switch, it doesn't
work as described:

~: stat -c "%N" b
`b' -> `a'

~:  stat -Lc "%N" b
`b'

The later one should print righthand part of the symlink. Anyway, I
don't want the quoted filename (especially useless `like this'), I need
simply the name. %N is all about quoting and printing reference.

> Perhaps you really want readlink?

No, as it operates on symlinks only, and I need to get real file.

Just take a look at this:

~: stat -c "%n" b
b			<- this is expected
~: stat -Lc "%n" b
b			<- this is not, -L is not effective

What's the point of returning the same data twice? None.

~: stat -c "%N" b
`b' -> `a'		<- like described in manual
~: stat -Lc "%N" b
`b'			<- definitely wrong, where's the dereference value?

Notice, I've used 4 different options and got 4 times 'b' and just one
'a' - this itself indicates a problem.

best regards,
-- 
Tomasz Pala <gotar <at> pld-linux.org>

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