1HARDLINK(1) User Commands HARDLINK(1)
2
6 hardlink - link multiple copies of a file
7
9 hardlink [options] [directory|file]...
10
12 hardlink is a tool that replaces copies of a file with either hardlinks
13 or copy-on-write clones, thus saving space.
14 hardlink first creates a binary tree of file sizes and then compares
16 the content of files that have the same size. There are two basic
17 content comparison methods. The memcmp method directly reads data
18 blocks from files and compares them. The other method is based on
19 checksums (like SHA256); in this case for each data block a checksum is
20 calculated by the Linux kernel crypto API, and this checksum is stored
21 in userspace and used for file comparisons.
22 For each file also an "intro" buffer (32 bytes) is cached. This buffer
24 is used independently from the comparison method and requested
25 cache-size and io-size. The "intro" buffer dramatically reduces
26 operations with data content as files are very often different from the
27 beginning.
28
30 -h, --help
31 Display help text and exit.
32 -V, --version
34 Print version and exit.
35 -c, --content
37 Consider only file content, not attributes, when determining
38 whether two files are equal. Same as -pot.
39 -b, --io-size size
41 The size of the read(2) or sendfile(2) buffer used when comparing
42 file contents. The size argument may be followed by the
43 multiplicative suffixes KiB, MiB, etc. The "iB" is optional, e.g.,
44 "K" has the same meaning as "KiB". The default is 8KiB for memcmp
45 method and 1MiB for the other methods. The only memcmp method uses
46 process memory for the buffer, other methods use zero-copy way and
47 I/O operation is done in the kernel. The size may be altered on the
48 fly to fit a number of cached content checksums.
49 -d, --respect-dir
51 Only try to link files with the same directory name. The top-level
52 directory (as specified on the hardlink command line) is ignored.
53 For example, hardlink --respect-dir /foo /bar will link
54 /foo/some/file with /bar/some/file, but not /bar/other/file. If
55 combined with --respect-name, then entire paths (except the
56 top-level directory) are compared.
57 -f, --respect-name
59 Only try to link files with the same (base)name. It’s strongly
60 recommended to use long options rather than -f which is interpreted
61 in a different way by other hardlink implementations.
62 -i, --include regex
64 A regular expression to include files. If the option --exclude has
65 been given, this option re-includes files which would otherwise be
66 excluded. If the option is used without --exclude, only files
67 matched by the pattern are included.
68 -m, --maximize
70 Among equal files, keep the file with the highest link count.
71 -M, --minimize
73 Among equal files, keep the file with the lowest link count.
74 -n, --dry-run
76 Do not act, just print what would happen.
77 -o, --ignore-owner
79 Link and compare files even if their owner information (user and
80 group) differs. Results may be unpredictable.
81 -O, --keep-oldest
83 Among equal files, keep the oldest file (least recent modification
84 time). By default, the newest file is kept. If --maximize or
85 --minimize is specified, the link count has a higher precedence
86 than the time of modification.
87 -p, --ignore-mode
89 Link and compare files even if their mode is different. Results may
90 be slightly unpredictable.
91 -q, --quiet
93 Quiet mode, don’t print anything.
94 -r, --cache-size size
96 The size of the cache for content checksums. All non-memcmp methods
97 calculate checksum for each file content block (see --io-size),
98 these checksums are cached for the next comparison. The size is
99 important for large files or a large sets of files of the same
100 size. The default is 10MiB.
101 -s, --minimum-size size
103 The minimum size to consider. By default this is 1, so empty files
104 will not be linked. The size argument may be followed by the
105 multiplicative suffixes KiB (=1024), MiB (=1024*1024), and so on
106 for GiB, TiB, PiB, EiB, ZiB and YiB (the "iB" is optional, e.g.,
107 "K" has the same meaning as "KiB").
108 -S, --maximum-size size
110 The maximum size to consider. By default this is 0 and 0 has the
111 special meaning of unlimited. The size argument may be followed by
112 the multiplicative suffixes KiB (=1024), MiB (=1024*1024), and so
113 on for GiB, TiB, PiB, EiB, ZiB and YiB (the "iB" is optional, e.g.,
114 "K" has the same meaning as "KiB").
115 -t, --ignore-time
117 Link and compare files even if their time of modification is
118 different. This is usually a good choice.
119 -v, --verbose
121 Verbose output, explain to the user what is being done. If
122 specified once, every hardlinked file is displayed. If specified
123 twice, it also shows every comparison.
124 -x, --exclude regex
126 A regular expression which excludes files from being compared and
127 linked.
128 -X, --respect-xattrs
130 Only try to link files with the same extended attributes.
131 -y, --method name
133 Set the file content comparison method. The currently supported
134 methods are sha256, sha1, crc32c and memcmp. The default is sha256,
135 or memcmp if Linux Crypto API is not available. The methods based
136 on checksums are implemented in zero-copy way, in this case file
137 contents are not copied to the userspace and all calculation is
138 done in kernel.
139 --reflink[=when]
141 Create copy-on-write clones (aka reflinks) rather than hardlinks.
142 The reflinked files share only on-disk data, but the file mode and
143 owner can be different. It’s recommended to use it with
144 --ignore-owner and --ignore-mode options. This option implies
145 --skip-reflinks to ignore already cloned files.
146 The optional argument when can be never, always, or auto. If the
148 when argument is omitted, it defaults to auto, in this case,
149 hardlink checks filesystem type and uses reflinks on BTRFS and XFS
150 only, and fallback to hardlinks when creating reflink is
151 impossible. The argument always disables filesystem type detection
152 and fallback to hardlinks, in this case, only reflinks are allowed.
153 --skip-reflinks
155 Ignore already cloned files. This option may be used without
156 --reflink when creating classic hardlinks.
157
159 hardlink takes one or more directories which will be searched for files
160 to be linked.
161
163 The original hardlink implementation uses the option -f to force
164 hardlinks creation between filesystem. This very rarely usable feature
165 is no more supported by the current hardlink.
166 hardlink assumes that the trees it operates on do not change during
168 operation. If a tree does change, the result is undefined and
169 potentially dangerous. For example, if a regular file is replaced by a
170 device, hardlink may start reading from the device. If a component of a
171 path is replaced by a symbolic link or file permissions change,
172 security may be compromised. Do not run hardlink on a changing tree or
173 on a tree controlled by another user.
174
176 There are multiple hardlink implementations. The very first
177 implementation is from Jakub Jelinek for Fedora distribution, this
178 implementation has been used in util-linux between versions v2.34 to
179 v2.36. The current implementations is based on Debian version from
180 Julian Andres Klode.
181
183 For bug reports, use the issue tracker at
184 https://github.com/util-linux/util-linux/issues.
185
187 The hardlink command is part of the util-linux package which can be
188 downloaded from Linux Kernel Archive
189 <https://www.kernel.org/pub/linux/utils/util-linux/>.
190util-linux 2.39.2 2023-07-12 HARDLINK(1)