50

How to move directories that have files in common from one to another partition ?

Let's assume we have partition mounted on /mnt/X with directories sharing files with hardlinks. How to move such directories to another partition , let it be /mnt/Y with preserving those hardlinks.

For better illustration what do I mean by "directories sharing files in common with hardlinks", here is an example:

# let's create three of directories and files
mkdir -p a/{b,c,d}/{x,y,z}
touch a/{b,c,d}/{x,y,z}/f{1,2,3,4,5}
# and copy it with hardlinks
cp -r -l a hardlinks_of_a

To be more specific, let's assume that total size of files is 10G and each file has 10 hardlinks. The question is how to move it to destination with using 10G (someone might say about copying it with 100G and then running deduplication - it is not what I am asking about)

Improve this question

6 Answers 6

Reset to default
46

rsync has a -H or --hard-links option for this, and has the usual rsync benefits of being able to be stopped and restarted, and to be re-run to efficiently deal with any files that were changed during/after the previous run.

-H, --hard-links
    This tells rsync to look for hard-linked files in
    the source and link together the corresponding
    files on the destination.  Without  this option,
    hard-linked files in the source are treated as
    though they were separate files. [...]

Read the rsync man page and search for -H. There is a lot more detail there about particular caveats.

Improve this answer
10
  • 2
    I've checked - it works.
    – Grzegorz Wierzowiecki
    Jul 31, 2012 at 18:16
  • yep, i know. I've been using it for years in my backup scripts. also to move files between filesystems as in your question.
    – cas
    Jul 31, 2012 at 22:03
  • rsync uses gobs of memory when building its file list. For me after many hours of "Building file list..." it filled up my 16GB of memory and bailed having copied nothing. YMMV.
    – msc
    Feb 2, 2018 at 1:57
  • 3
    From man rsync: Beginning with rsync 3.0.0, the recursive algorithm used is now an incremental scan that uses much less memory than before and begins the transfer after the scanning of the first few directories have been completed. This incremental scan only affects our recursion algorithm, and does not change a non-recursive transfer. It is also only possible when both ends of the transfer are at least version 3.0.0. Note that both --delete-before and --delete-after disable this improved algorithm.
    – cas
    Feb 2, 2018 at 2:18
  • Also, while rsync is an incredibly useful too, it isn't always the best tool for every job. These days, I prefer to use ZFS datasets so I can snapshot and zfs send them - I mostly use rsync on non-ZFS filesystems. btrfs has a similar snapshot + send capability.
    – cas
    Feb 2, 2018 at 2:22
41

First answer: The GNU Way

GNU cp -a copies recursively preserving as much structure and metadata as possible. Hard links between files in the source directory are included in that. To select hard link preservation specifically without all the other features of -a, use --preserve=links.

mkdir src
cd src
mkdir -p a/{b,c,d}/{x,y,z}
touch a/{b,c,d}/{x,y,z}/f{1,2,3,4,5}
cp -r -l a hardlinks_of_a
cd ..
cp -a src dst
Improve this answer
10
  • 3
    +1 on tar, -1 for using gnu-specific arguments for cp.
    – WhyNotHugo
    Jul 30, 2012 at 15:11
  • You gave three answers in one. Could you split them into three so they can be commented and evaluated separately ? (Tip: You can edit this, to leave only one - for example "cp -a". Later add two more, for "tar" and "pax")
    – Grzegorz Wierzowiecki
    Jul 31, 2012 at 13:03
  • 1
    @GrzegorzWierzowiecki split accomplished
    – Alan Curry
    Jul 31, 2012 at 18:42
  • 6
    @Hugo: there's nothing wrong with using GNU-specific args to standard tools. GNU versions are the de-facto standard these days, and even when they weren't pre-installed, it was common practice to install GNU tools (I know I always did - they were simply better than, e.g, solaris and *bsd versions, and they provided consistency between different *nixes). It's probably good practice to point out GNUisms when you use them but not required. Also Grzegorz didn't say "not on linux" so it's reasonable to assume that that's the environment he's talking about.
    – cas
    Jul 31, 2012 at 21:57
  • 1
    @WhyNotHugo: How is POSIX "may more standard?". POSIX is the stuff which brought us where we are. Did you know that all Windows versions since Windows NT are fully POSIX compliant? They have a path length limitation of 255 characters when using the POSIX file I/O functions, which renders them useless. Did you know that Solaris, Irix, HP-UX are all POSIX compliant, and yet all the arguments to their tools differ (e.g. tar). cp -a is a minimum requirement for any cp version which wants to replace GNU copy.
    – Johannes Overmann
    Feb 27, 2019 at 22:24
19

Third answer: The POSIX Way

POSIX hasn't standardized the tar utility, although they have standardized the tar archive format. The POSIX utility for manipulating tar archives is called pax and it has the bonus feature of being able to do the pack and unpack operation in a single process.

mkdir dst
pax -rw src dst
Improve this answer
0
16

Second answer: The Ancient UNIX Way

Create a tar archive in the source directory, send it over a pipe, and unpack it in the destination directory.

# create src as before
(cd src;tar cf - .) | (mkdir dst;cd dst;tar xf -)
Improve this answer
4
  • 1
    checked -> works. Hardlinks preserved.
    – Grzegorz Wierzowiecki
    Jul 31, 2012 at 20:33
  • 1
    Any insight into why this actually does preserve hardlinks?
    – peterph
    Aug 5, 2015 at 21:02
  • 1
    Because tar preserves hard-links. In GNU tar, at least, you can disable this behaviour with --hard-dereference
    – cas
    Sep 2, 2015 at 8:11
  • 1
    In my case, attempting to copy a large directory hierarchy (a TimeMachine backup), tar preserved some hard links but replicated the file in some cases. I think this is because the tar x does not have the full file list as files are still being piped in from the tar c. Probably if you saved the entire archive before extracting it, it would be okay. I'd be very happy if someone could confirm that theory.
    – msc
    Feb 2, 2018 at 2:04
10

Source: http://www.cyberciti.biz/faq/linux-unix-apple-osx-bsd-rsync-copy-hard-links/

What you need to make an exact copy is

rsync -az -H --delete --numeric-ids /path/to/source/ /path/to/dest/
Improve this answer
3
  • See my comment about rsync above.
    – msc
    Feb 2, 2018 at 1:57
  • 2
    I suspect this won't copy ACLs, extended attributes, and so forth. The Linux version also has the -A and -X options to preserve these, but I think you're out of luck on MacOS.
    – Edward Falk
    Dec 7, 2018 at 0:02
  • Can you explain your rationale of using the -z option (compress) when using rsync to copy between two mounted folders (since this is what was asked) ?
    – jmr
    May 16, 2020 at 0:01
0

the commands you can use to copy a file and preserve hardlinks in macOS CLI:

cp -r -l
Improve this answer

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .