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. 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.

       -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/>.