Provided by: util-linux_2.39.3-9ubuntu6.3_amd64 bug

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.

       -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.

       -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 "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 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.

       --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.

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 2.39.3                                  2023-11-21                                        HARDLINK(1)