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.
- -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.
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.
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.
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 at 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.