# Bash 的 hardlink 命令

hardlink [OPTIONS] [FILE...]

$ echo Primers 编程伙伴 | tee 1.txt 2.txt       # 同时向两个文件写入相同内容
Primers 编程伙伴
$ ls -i                                         # 查看文件的 inode 号
9851498 1.txt  9851499 2.txt
$ hardlink .                                    # 合并相同文件
Mode:                     real
Method:                   sha256
Files:                    4
Linked:                   1 files
Compared:                 0 xattrs
Compared:                 1 files
Saved:                    21 B
Duration:                 0.000573 seconds
$ ls -i                                         # 查看文件的 inode 号
9851498 1.txt  9851498 2.txt

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.

       -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)