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NAME | SYNOPSIS | DESCRIPTION | OPTIONS | ARGUMENTS | BUGS | AUTHOR | REPORTING BUGS | AVAILABILITY |
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HARDLINK(1) User Commands HARDLINK(1)
hardlink - link multiple copies of a file
hardlink [options] directory|file...
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
comparisons.
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.
-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
implementations.
-F, --prioritize-trees
Keep files found in the earliest specified top-level directory
if there are multiple identical files in different trees. For
example, hardlink foo/ bar/ will link files in bar/ to equal
files in the foo/ directory.
-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.
-l, --list-duplicates
Don’t link anything, but list the absolute path of every
duplicate file, one per line, preceded by a unique 16-byte
discriminator and a tab.
-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.
--mount
Do not traverse directories on different filesystems (remain
within the same filesystem).
-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.
--reflink[=when]
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 this option together with the --ignore-owner and
--ignore-mode options. This option implies --skip-reflinks to
ignore already cloned files.
The optional argument when can be never, always, or auto. If
the when argument is omitted, it defaults to auto, which means
that hardlink checks the filesystem type and uses reflinks on
BTRFS and XFS only, and falls back to hardlinks when creating
a reflink is impossible. The argument always disables
filesystem-type detection and the fallback to hardlinks, which
means that only reflinks are allowed.
--skip-reflinks
Ignore already cloned files. This option may be used without
--reflink when creating classic hardlinks.
-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
"KiB").
-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 that excludes files from being compared
and linked. This option can be used multiple times.
--exclude-subtree regex
A regular expression that excludes entire directories from
being compared and linked. This option can also be used
multiple times.
-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 the Linux Crypto API is not
available. The methods based on checksums are implemented in a
zero-copy way, which means that file contents are not copied
to userspace and all calculation is done in the kernel.
-z, --zero
Separate lines with a NUL byte instead of a newline (for -l).
-h, --help
Display help text and exit.
-V, --version
Display version and exit.
hardlink takes one or more directories which will be searched for
files to be linked.
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
user.
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.
For bug reports, use the issue tracker
<https://github.com/util-linux/util-linux/issues>.
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
[email protected]. This page was obtained from the
project's upstream Git repository
⟨git://git.kernel.org/pub/scm/utils/util-linux/util-linux.git⟩ on
2025-08-11. (At that time, the date of the most recent commit that
was found in the repository was 2025-08-05.) 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 protected]
util-linux 2.42-start-521-ec46 2025-08-09 HARDLINK(1)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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