[sf-lug] question about cp -al

[Michael Paoli]

> 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.