[sf-lug] question about cp -al

[Alex Kleider]

On 2016-09-18 22:41, Michael Paoli wrote:

Thanks, Michael, for the explanations and elucidations!

>> From: "Alex Kleider" <akleider at sonic.net>
>> Subject: [sf-lug] question about cp -al
>> Date: Sun, 18 Sep 2016 16:31:19 -0700
> 
>> I'm curious as to what exactly the cp -al command does with regard to
>> directories.
>> Rather than make another copy of each regular file, which it would do
>> if not for the -l option, hard links are created instead.  What
>> puzzles me is how are directories 'copied?'  I assume that they are
>> copied in the same way that would happen without the -l option.  Is
>> that correct?
> 
> Let's see ...
> http://pubs.opengroup.org/onlinepubs/9699919799/utilities/cp.html
> ... those options don't look standard, well, let's see ... guessing 
> here
> we're talkin' Linux, probably GNU cp ...
> https://www.gnu.org/software/coreutils/manual/html_node/cp-invocation.html#cp-invocation
> 
> ...
> `-a'
> `--archive'
> Preserve as much as possible of the structure and attributes of the
> original files in the copy (but do not attempt to preserve internal
> directory structure
> ...
> `-l'
> `--link'
> Make hard links instead of copies of non-directories.
> ...
> 
> Directories are "just" another type of file - of type directory.
> But hardlinking directories is generally a no no (Unix allows it, but
> it's strongly discouraged, Linux doesn't let it happen - the kernel
> won't let one do it).  Anyway, that way, madness lies ;-), hence it's
> discouraged.  And yes, I've hard linked directories before ... but not
> in a long time - really no need or reason to do that anymore, and it 
> was
> pretty much always a bad idea to begin with (but made for some cool
> tests/demos, to see how well one *really* understood Unix filesystems).
> 
> So, yeah, hard link wouldn't apply to directory ... certainly not from
> GNU cp -l option in the land of Linux.
> 
>> In other words: could the cp -al command be replaced by a routine
>> that follows the source directory structure and with in a destination
>> directory creates (mkdir) directories and populates them using the ln
>> command?  Would the result be the same?
> 
> Probably same or similar, yes.  Let's see ...
> $ find s -exec ls -dino \{\} \;
> 213496530 drwxr-xr-x 3 1003 140 Sep 18 20:00 s
> 213498082 -rw-r--r-- 2 1003 0 Sep 18 20:00 s/hard_link
> 213498082 -rw-r--r-- 2 1003 0 Sep 18 20:00 s/l
> 213496560 lrwxrwxrwx 1 1003 1 Sep 18 19:59 s/ln2d -> d
> 213497320 -rw-r--r-- 1 1003 0 Sep 18 19:59 s/f
> 213497304 drwxr-xr-x 2 1003 40 Sep 18 19:59 s/d
> $ cp -al s t
> $ find [st] -exec ls -dino \{\} \;
> 213496530 drwxr-xr-x 3 1003 140 Sep 18 20:00 s
> 213498082 -rw-r--r-- 4 1003 0 Sep 18 20:00 s/hard_link
> 213498082 -rw-r--r-- 4 1003 0 Sep 18 20:00 s/l
> 213496560 lrwxrwxrwx 2 1003 1 Sep 18 19:59 s/ln2d -> d
> 213497320 -rw-r--r-- 2 1003 0 Sep 18 19:59 s/f
> 213497304 drwxr-xr-x 2 1003 40 Sep 18 19:59 s/d
> 213498964 drwxr-xr-x 3 1003 140 Sep 18 20:00 t
> 213498082 -rw-r--r-- 4 1003 0 Sep 18 20:00 t/l
> 213498082 -rw-r--r-- 4 1003 0 Sep 18 20:00 t/hard_link
> 213497320 -rw-r--r-- 2 1003 0 Sep 18 19:59 t/f
> 213498965 drwxr-xr-x 2 1003 40 Sep 18 19:59 t/d
> 213496560 lrwxrwxrwx 2 1003 1 Sep 18 19:59 t/ln2d -> d
> $ rm -rf t
> $ ed a_routine
> a_routine: No such file or directory
> 0a
> mkdir t && cd s && find . -depth -print0 | pax -rw -0dl ../t
> .
> w
> 61
> q
> $ chmod u+x a_routine
> $ ./a_routine
> $ find [st] -exec ls -dino \{\} \;
> 213496530 drwxr-xr-x 3 1003 140 Sep 18 20:00 s
> 213498082 -rw-r--r-- 4 1003 0 Sep 18 20:00 s/hard_link
> 213498082 -rw-r--r-- 4 1003 0 Sep 18 20:00 s/l
> 213496560 lrwxrwxrwx 2 1003 1 Sep 18 19:59 s/ln2d -> d
> 213497320 -rw-r--r-- 2 1003 0 Sep 18 19:59 s/f
> 213497304 drwxr-xr-x 2 1003 40 Sep 18 19:59 s/d
> 213496785 drwxr-xr-x 3 1003 140 Sep 18 20:00 t
> 213496786 drwxr-xr-x 2 1003 40 Sep 18 19:59 t/d
> 213497320 -rw-r--r-- 2 1003 0 Sep 18 19:59 t/f
> 213496560 lrwxrwxrwx 2 1003 1 Sep 18 19:59 t/ln2d -> d
> 213498082 -rw-r--r-- 4 1003 0 Sep 18 20:00 t/l
> 213498082 -rw-r--r-- 4 1003 0 Sep 18 20:00 t/hard_link
> $
> 
> Yup, sure looks to me like we could have a_routine that does that.
> 
> And yes, the -0 option to find and pax is also a GNU extension.
> 
>> ps by way of explanation: I've written a little script for doing
>> backups- it relies on the cp -la command but the -l option is not
>> available on the MacOS so my goal is to create a "work around."
> 
> MacOS isn't Linux, it's Unix.  So in many cases GNU stuff commonly
> found on Linux that's not specified by POSIX, etc., won't be found on
> Unix - e.g. MacOS X.
> 
> Of course if you're doing hard links for "backup", you're not really
> backing up the file data, but merely creating additional hard links -
> so, if they become unlinked, you still have another link, but if the
> contents is overwritten, there is no backup, there's just one file with
> multiple hard links.