151 lines
7.0 KiB
Plaintext
151 lines
7.0 KiB
Plaintext
//po4a: entry man manual
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////
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SPDX-License-Identifier: MIT
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Copyright (C) 2008 - 2012 Julian Andres Klode. See hardlink.c for license.
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Copyright (C) 2021 Karel Zak <kzak@redhat.com>
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////
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= hardlink(1)
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:doctype: manpage
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:man manual: User Commands
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:man source: util-linux {release-version}
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:page-layout: base
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:command: hardlink
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== NAME
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hardlink - link multiple copies of a file
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== SYNOPSIS
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*hardlink* [options] [_directory_|_file_]...
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== DESCRIPTION
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*hardlink* is a tool that replaces copies of a file with either hardlinks
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or copy-on-write clones, thus saving space.
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*hardlink* first creates a binary tree of file sizes and then compares
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the content of files that have the same size. There are two basic content
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comparison methods. The *memcmp* method directly reads data blocks from
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files and compares them. The other method is based on checksums (like SHA256);
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in this case for each data block a checksum is calculated by the Linux kernel
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crypto API, and this checksum is stored in userspace and used for file
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comparisons.
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For each file also an "intro" buffer (32 bytes) is cached. This buffer is used
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independently from the comparison method and requested cache-size and io-size.
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The "intro" buffer dramatically reduces operations with data content as files
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are very often different from the beginning.
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== OPTIONS
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include::man-common/help-version.adoc[]
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*-v*, *--verbose*::
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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.
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*-q*, *--quiet*::
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Quiet mode, don't print anything.
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*-n*, *--dry-run*::
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Do not act, just print what would happen.
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*-y*, *--method* _name_::
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Set the file content comparison method. The currently supported methods are
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sha256, sha1, crc32c and memcmp. The default is sha256, or memcmp if Linux
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Crypto API is not available. The methods based on checksums are implemented in
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zero-copy way, in this case file contents are not copied to the userspace and all
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calculation is done in kernel.
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*--reflink*[=_when_]::
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Create copy-on-write clones (aka reflinks) rather than hardlinks. The reflinked files
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share only on-disk data, but the file mode and owner can be different. It's recommended
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to use it with *--ignore-owner* and *--ignore-mode* options. This option implies
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*--skip-reflinks* to ignore already cloned files.
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+
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The optional argument _when_ can be *never*, *always*, or *auto*. If the _when_ argument
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is omitted, it defaults to *auto*, in this case, *hardlink* checks filesystem type and
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uses reflinks on BTRFS and XFS only, and fallback to hardlinks when creating reflink is impossible.
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The argument *always* disables filesystem type detection and fallback to hardlinks, in this case,
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only reflinks are allowed.
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*--skip-reflinks*::
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Ignore already cloned files. This option may be used without *--reflink* when creating classic hardlinks.
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*-f*, *--respect-name*::
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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.
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*-p*, *--ignore-mode*::
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Link and compare files even if their mode is different. Results may be slightly unpredictable.
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*-o*, *--ignore-owner*::
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Link and compare files even if their owner information (user and group) differs. Results may be unpredictable.
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*-t*, *--ignore-time*::
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Link and compare files even if their time of modification is different. This is usually a good choice.
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*-c* *--content*::
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Consider only file content, not attributes, when determining whether two files are equal. Same as *-pot*.
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*-X*, *--respect-xattrs*::
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Only try to link files with the same extended attributes.
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*-m*, *--maximize*::
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Among equal files, keep the file with the highest link count.
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*-M*, *--minimize*::
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Among equal files, keep the file with the lowest link count.
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*-O*, *--keep-oldest*::
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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.
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*-x*, *--exclude* _regex_::
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A regular expression which excludes files from being compared and linked.
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*-i*, *--include* _regex_::
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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.
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*-s*, *--minimum-size* _size_::
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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").
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*-S*, *--maximum-size* _size_::
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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").
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*-b*, *--io-size* _size_::
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The size of the *read*(2) or *sendfile*(2) buffer used when comparing file contents.
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The _size_ argument may be followed by the multiplicative suffixes KiB, MiB,
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etc. The "iB" is optional, e.g., "K" has the same meaning as "KiB". The
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default is 8KiB for memcmp method and 1MiB for the other methods. The only
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memcmp method uses process memory for the buffer, other methods use zero-copy
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way and I/O operation is done in the kernel. The size may be altered on the fly
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to fit a number of cached content checksums.
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*-r*, *--cache-size* _size_::
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The size of the cache for content checksums. All non-memcmp methods calculate checksum for each
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file content block (see *--io-size*), these checksums are cached for the next comparison. The
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size is important for large files or a large sets of files of the same size. The default is
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10MiB.
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== ARGUMENTS
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*hardlink* takes one or more directories which will be searched for files to be linked.
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== BUGS
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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*.
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*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.
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== AUTHOR
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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.
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include::man-common/bugreports.adoc[]
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include::man-common/footer.adoc[]
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ifdef::translation[]
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include::man-common/translation.adoc[]
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endif::[]
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