hardlink(1) — Linux manual page


HARDLINK(1)                   User Commands                  HARDLINK(1)

NAME         top

       hardlink - link multiple copies of a file

SYNOPSIS         top

       hardlink [options] [directory|file]...

DESCRIPTION         top

       hardlink is a tool that replaces copies of a file with either
       hardlinks or copy-on-write clones, thus saving space.

       hardlink first creates a binary tree of file sizes and then
       compares the content of files that have the same size. There are
       two basic content comparison methods. The memcmp method directly
       reads data blocks from files and compares them. The other method
       is based on checksums (like SHA256); in this case for each data
       block a checksum is calculated by the Linux kernel crypto API,
       and this checksum is stored in userspace and used for file

       For each file also an "intro" buffer (32 bytes) is cached. This
       buffer is used independently from the comparison method and
       requested cache-size and io-size. The "intro" buffer dramatically
       reduces operations with data content as files are very often
       different from the beginning.

OPTIONS         top

       -h, --help
           Display help text and exit.

       -V, --version
           Print version and exit.

       -c, --content
           Consider only file content, not attributes, when determining
           whether two files are equal. Same as -pot.

       -b, --io-size size
           The size of the read(2) or sendfile(2) buffer used when
           comparing file contents. The size argument may be followed by
           the multiplicative suffixes KiB, MiB, etc. The "iB" is
           optional, e.g., "K" has the same meaning as "KiB". The
           default is 8KiB for memcmp method and 1MiB for the other
           methods. The only memcmp method uses process memory for the
           buffer, other methods use zero-copy way and I/O operation is
           done in the kernel. The size may be altered on the fly to fit
           a number of cached content checksums.

       -d, --respect-dir
           Only try to link files with the same directory name. The
           top-level directory (as specified on the hardlink command
           line) is ignored. For example, hardlink --respect-dir /foo
           /bar will link /foo/some/file with /bar/some/file, but not
           /bar/other/file. If combined with --respect-name, then entire
           paths (except the top-level directory) are compared.

       -f, --respect-name
           Only try to link files with the same (base)name. It’s
           strongly recommended to use long options rather than -f which
           is interpreted in a different way by other hardlink

       -i, --include regex
           A regular expression to include files. If the option
           --exclude has been given, this option re-includes files which
           would otherwise be excluded. If the option is used without
           --exclude, only files matched by the pattern are included.

       -m, --maximize
           Among equal files, keep the file with the highest link count.

       -M, --minimize
           Among equal files, keep the file with the lowest link count.

       -n, --dry-run
           Do not act, just print what would happen.

       -o, --ignore-owner
           Link and compare files even if their owner information (user
           and group) differs. Results may be unpredictable.

       -O, --keep-oldest
           Among equal files, keep the oldest file (least recent
           modification time). By default, the newest file is kept. If
           --maximize or --minimize is specified, the link count has a
           higher precedence than the time of modification.

       -p, --ignore-mode
           Link and compare files even if their mode is different.
           Results may be slightly unpredictable.

       -q, --quiet
           Quiet mode, don’t print anything.

       -r, --cache-size size
           The size of the cache for content checksums. All non-memcmp
           methods calculate checksum for each file content block (see
           --io-size), these checksums are cached for the next
           comparison. The size is important for large files or a large
           sets of files of the same size. The default is 10MiB.

       -s, --minimum-size size
           The minimum size to consider. By default this is 1, so empty
           files will not be linked. The size argument may be followed
           by the multiplicative suffixes KiB (=1024), MiB (=1024*1024),
           and so on for GiB, TiB, PiB, EiB, ZiB and YiB (the "iB" is
           optional, e.g., "K" has the same meaning as "KiB").

       -S, --maximum-size size
           The maximum size to consider. By default this is 0 and 0 has
           the special meaning of unlimited. The size argument may be
           followed by the multiplicative suffixes KiB (=1024), MiB
           (=1024*1024), and so on for GiB, TiB, PiB, EiB, ZiB and YiB
           (the "iB" is optional, e.g., "K" has the same meaning as

       -t, --ignore-time
           Link and compare files even if their time of modification is
           different. This is usually a good choice.

       -v, --verbose
           Verbose output, explain to the user what is being done. If
           specified once, every hardlinked file is displayed. If
           specified twice, it also shows every comparison.

       -x, --exclude regex
           A regular expression which excludes files from being compared
           and linked.

       -X, --respect-xattrs
           Only try to link files with the same extended attributes.

       -y, --method name
           Set the file content comparison method. The currently
           supported methods are sha256, sha1, crc32c and memcmp. The
           default is sha256, or memcmp if Linux Crypto API is not
           available. The methods based on checksums are implemented in
           zero-copy way, in this case file contents are not copied to
           the userspace and all calculation is done in kernel.

           Create copy-on-write clones (aka reflinks) rather than
           hardlinks. The reflinked files share only on-disk data, but
           the file mode and owner can be different. It’s recommended to
           use it with --ignore-owner and --ignore-mode options. This
           option implies --skip-reflinks to ignore already cloned

           The optional argument when can be never, always, or auto. If
           the when argument is omitted, it defaults to auto, in this
           case, hardlink checks filesystem type and uses reflinks on
           BTRFS and XFS only, and fallback to hardlinks when creating
           reflink is impossible. The argument always disables
           filesystem type detection and fallback to hardlinks, in this
           case, only reflinks are allowed.

           Ignore already cloned files. This option may be used without
           --reflink when creating classic hardlinks.

ARGUMENTS         top

       hardlink takes one or more directories which will be searched for
       files to be linked.

BUGS         top

       The original hardlink implementation uses the option -f to force
       hardlinks creation between filesystem. This very rarely usable
       feature is no more supported by the current hardlink.

       hardlink assumes that the trees it operates on do not change
       during operation. If a tree does change, the result is undefined
       and potentially dangerous. For example, if a regular file is
       replaced by a device, hardlink may start reading from the device.
       If a component of a path is replaced by a symbolic link or file
       permissions change, security may be compromised. Do not run
       hardlink on a changing tree or on a tree controlled by another

AUTHOR         top

       There are multiple hardlink implementations. The very first
       implementation is from Jakub Jelinek for Fedora distribution,
       this implementation has been used in util-linux between versions
       v2.34 to v2.36. The current implementations is based on Debian
       version from Julian Andres Klode.

REPORTING BUGS         top

       For bug reports, use the issue tracker at

AVAILABILITY         top

       The hardlink command is part of the util-linux package which can
       be downloaded from Linux Kernel Archive
       <https://www.kernel.org/pub/linux/utils/util-linux/>. This page
       is part of the util-linux (a random collection of Linux
       utilities) project. Information about the project can be found at
       ⟨https://www.kernel.org/pub/linux/utils/util-linux/⟩. If you have
       a bug report for this manual page, send it to
       util-linux@vger.kernel.org. This page was obtained from the
       project's upstream Git repository
       ⟨git://git.kernel.org/pub/scm/utils/util-linux/util-linux.git⟩ on
       2023-12-22. (At that time, the date of the most recent commit
       that was found in the repository was 2023-12-14.) If you discover
       any rendering problems in this HTML version of the page, or you
       believe there is a better or more up-to-date source for the page,
       or you have corrections or improvements to the information in
       this COLOPHON (which is not part of the original manual page),
       send a mail to man-pages@man7.org

util-linux 2.39.594-1e0ad      2023-08-25                    HARDLINK(1)