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.
Display help text and exit.
Print version and exit.
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.
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.
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.
Among equal files, keep the file with the highest link count.
Among equal files, keep the file with the lowest link count.
Do not act, just print what would happen.
Link and compare files even if their owner information (user
and group) differs. Results may be unpredictable.
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.
Link and compare files even if their mode is different.
Results may be slightly unpredictable.
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
Link and compare files even if their time of modification is
different. This is usually a good choice.
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
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.
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
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.
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
firstname.lastname@example.org. This page was obtained from the
project's upstream Git repository
2023-06-23. (At that time, the date of the most recent commit
that was found in the repository was 2023-06-22.) 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 email@example.com
util-linux 2.39.268-ae62d 2023-06-22 HARDLINK(1)