Re: [Xen-devel] [Patch] fix canonicalize-existing vbd file

[Ian Jackson <Ian.Jackson@xxxxxxxxxxxxx>]

Keir Fraser writes ("Re: [Xen-devel] [Patch] fix canonicalize-existing vbd 
file"):
> Do you mean to 'test -f "$file"'? A symlink will always fail 'test -f', and
> $p can be a symlink.

test -f calls stat(2) rather than lstat(2) and only falls back to the
latter if the former fails.  So if the argument is a link which can be
dereferenced, test -f tells you whether the link target is a file.

If you want to know whether $file is a file or a symlink to a file
   test -f "$file"
precisely wrong as it is true in both those cases.

See
    http://www.opengroup.org/onlinepubs/009695399/utilities/test.html
which says
     With the exception of the -h file and -L file primaries, if a
     file argument is a symbolic link, test shall evaluate the
     expression by resolving the symbolic link and using the file
     referenced by the link.

It's actually true, too:

mariner:~/junk> touch a 
mariner:~/junk> ln -s a b
mariner:~/junk> test -f a && echo file
file
mariner:~/junk> test -f b && echo file
file
mariner:~/junk> test -L b && echo link
link
mariner:~/junk> ln -s enoent c
mariner:~/junk> test -f c && echo file
mariner:~/junk> test -L c && echo link
link
mariner:~/junk>

Ian.

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