HARDLINK(1) User Commands HARDLINK(1)
NAME
hardlink - link multiple copies of a file
SYNOPSIS
hardlink [options] [directory|file]...
DESCRIPTION
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.
OPTIONS
-h, --help
Display help text and exit.
-V, --version
Print version and exit.
-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.
-q, --quiet
Quiet mode, don’t print anything.
-n, --dry-run
Do not act, just print what would happen.
-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.
--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 it with
--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, 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.
--skip-reflinks
Ignore already cloned files. This option may be used without
--reflink when creating classic hardlinks.
-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.
-p, --ignore-mode
Link and compare files even if their mode is different. Results may
be slightly unpredictable.
-o, --ignore-owner
Link and compare files even if their owner information (user and
group) differs. Results may be unpredictable.
-t, --ignore-time
Link and compare files even if their time of modification is
different. This is usually a good choice.
-c --content
Consider only file content, not attributes, when determining
whether two files are equal. Same as -pot.
-X, --respect-xattrs
Only try to link files with the same extended attributes.
-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.
-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.
-x, --exclude regex
A regular expression which excludes files from being compared and
linked.
-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.
-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").
-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.
-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.
ARGUMENTS
hardlink takes one or more directories which will be searched for files
to be linked.
BUGS
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.
AUTHOR
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
For bug reports, use the issue tracker at
https://github.com/util-linux/util-linux/issues.
AVAILABILITY
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/>.
util-linux 2.38.1 2022-08-04 HARDLINK(1)
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