gitdiffcore(7) — Linux manual page

NAME | SYNOPSIS | DESCRIPTION | THE CHAIN OF OPERATION | DIFFCORE-BREAK: FOR SPLITTING UP COMPLETE REWRITES | DIFFCORE-RENAME: FOR DETECTING RENAMES AND COPIES | | | DIFFCORE-ORDER: FOR SORTING THE OUTPUT BASED ON FILENAMES | DIFFCORE-ROTATE: FOR CHANGING AT WHICH PATH OUTPUT STARTS | SEE ALSO | GIT | NOTES | COLOPHON

GITDIFFCORE(7)                 Git Manual                 GITDIFFCORE(7)

NAME         top

       gitdiffcore - Tweaking diff output

SYNOPSIS         top

       git diff *

DESCRIPTION         top

       The diff commands git diff-index, git diff-files, and git
       diff-tree can be told to manipulate differences they find in
       unconventional ways before showing diff output. The manipulation
       is collectively called "diffcore transformation". This short note
       describes what they are and how to use them to produce diff
       output that is easier to understand than the conventional kind.

THE CHAIN OF OPERATION         top

       The git diff-* family works by first comparing two sets of files:

       •   git diff-index compares contents of a "tree" object and the
           working directory (when --cached flag is not used) or a
           "tree" object and the index file (when --cached flag is
           used);

       •   git diff-files compares contents of the index file and the
           working directory;

       •   git diff-tree compares contents of two "tree" objects;

       In all of these cases, the commands themselves first optionally
       limit the two sets of files by any pathspecs given on their
       command-lines, and compare corresponding paths in the two
       resulting sets of files.

       The pathspecs are used to limit the world diff operates in. They
       remove the filepairs outside the specified sets of pathnames.
       E.g. If the input set of filepairs included:

           :100644 100644 bcd1234... 0123456... M junkfile

       but the command invocation was git diff-files myfile, then the
       junkfile entry would be removed from the list because only
       "myfile" is under consideration.

       The result of comparison is passed from these commands to what is
       internally called "diffcore", in a format similar to what is
       output when the -p option is not used. E.g.

           in-place edit  :100644 100644 bcd1234... 0123456... M file0
           create         :000000 100644 0000000... 1234567... A file4
           delete         :100644 000000 1234567... 0000000... D file5
           unmerged       :000000 000000 0000000... 0000000... U file6

       The diffcore mechanism is fed a list of such comparison results
       (each of which is called "filepair", although at this point each
       of them talks about a single file), and transforms such a list
       into another list. There are currently 5 such transformations:

       •   diffcore-break

       •   diffcore-rename

       •   diffcore-merge-broken

       •   diffcore-pickaxe

       •   diffcore-order

       •   diffcore-rotate

       These are applied in sequence. The set of filepairs git diff-*
       commands find are used as the input to diffcore-break, and the
       output from diffcore-break is used as the input to the next
       transformation. The final result is then passed to the output
       routine and generates either diff-raw format (see Output format
       sections of the manual for git diff-* commands) or diff-patch
       format.

DIFFCORE-BREAK: FOR SPLITTING UP COMPLETE REWRITES         top

       The second transformation in the chain is diffcore-break, and is
       controlled by the -B option to the git diff-* commands. This is
       used to detect a filepair that represents "complete rewrite" and
       break such filepair into two filepairs that represent delete and
       create. E.g. If the input contained this filepair:

           :100644 100644 bcd1234... 0123456... M file0

       and if it detects that the file "file0" is completely rewritten,
       it changes it to:

           :100644 000000 bcd1234... 0000000... D file0
           :000000 100644 0000000... 0123456... A file0

       For the purpose of breaking a filepair, diffcore-break examines
       the extent of changes between the contents of the files before
       and after modification (i.e. the contents that have "bcd1234..."
       and "0123456..." as their SHA-1 content ID, in the above
       example). The amount of deletion of original contents and
       insertion of new material are added together, and if it exceeds
       the "break score", the filepair is broken into two. The break
       score defaults to 50% of the size of the smaller of the original
       and the result (i.e. if the edit shrinks the file, the size of
       the result is used; if the edit lengthens the file, the size of
       the original is used), and can be customized by giving a number
       after "-B" option (e.g. "-B75" to tell it to use 75%).

DIFFCORE-RENAME: FOR DETECTING RENAMES AND COPIES         top

       This transformation is used to detect renames and copies, and is
       controlled by the -M option (to detect renames) and the -C option
       (to detect copies as well) to the git diff-* commands. If the
       input contained these filepairs:

           :100644 000000 0123456... 0000000... D fileX
           :000000 100644 0000000... 0123456... A file0

       and the contents of the deleted file fileX is similar enough to
       the contents of the created file file0, then rename detection
       merges these filepairs and creates:

           :100644 100644 0123456... 0123456... R100 fileX file0

       When the "-C" option is used, the original contents of modified
       files, and deleted files (and also unmodified files, if the
       "--find-copies-harder" option is used) are considered as
       candidates of the source files in rename/copy operation. If the
       input were like these filepairs, that talk about a modified file
       fileY and a newly created file file0:

           :100644 100644 0123456... 1234567... M fileY
           :000000 100644 0000000... bcd3456... A file0

       the original contents of fileY and the resulting contents of
       file0 are compared, and if they are similar enough, they are
       changed to:

           :100644 100644 0123456... 1234567... M fileY
           :100644 100644 0123456... bcd3456... C100 fileY file0

       In both rename and copy detection, the same "extent of changes"
       algorithm used in diffcore-break is used to determine if two
       files are "similar enough", and can be customized to use a
       similarity score different from the default of 50% by giving a
       number after the "-M" or "-C" option (e.g. "-M8" to tell it to
       use 8/10 = 80%).

       Note that when rename detection is on but both copy and break
       detection are off, rename detection adds a preliminary step that
       first checks if files are moved across directories while keeping
       their filename the same. If there is a file added to a directory
       whose contents is sufficiently similar to a file with the same
       name that got deleted from a different directory, it will mark
       them as renames and exclude them from the later quadratic step
       (the one that pairwise compares all unmatched files to find the
       "best" matches, determined by the highest content similarity).
       So, for example, if a deleted docs/ext.txt and an added
       docs/config/ext.txt are similar enough, they will be marked as a
       rename and prevent an added docs/ext.md that may be even more
       similar to the deleted docs/ext.txt from being considered as the
       rename destination in the later step. For this reason, the
       preliminary "match same filename" step uses a bit higher
       threshold to mark a file pair as a rename and stop considering
       other candidates for better matches. At most, one comparison is
       done per file in this preliminary pass; so if there are several
       remaining ext.txt files throughout the directory hierarchy after
       exact rename detection, this preliminary step may be skipped for
       those files.

       Note. When the "-C" option is used with --find-copies-harder
       option, git diff-* commands feed unmodified filepairs to diffcore
       mechanism as well as modified ones. This lets the copy detector
       consider unmodified files as copy source candidates at the
       expense of making it slower. Without --find-copies-harder, git
       diff-* commands can detect copies only if the file that was
       copied happened to have been modified in the same changeset.

DIFFCORE-MERGE-BROKEN: FOR PUTTING COMPLETE REWRITES BACK TOGETHER

        top

       This transformation is used to merge filepairs broken by
       diffcore-break, and not transformed into rename/copy by
       diffcore-rename, back into a single modification. This always
       runs when diffcore-break is used.

       For the purpose of merging broken filepairs back, it uses a
       different "extent of changes" computation from the ones used by
       diffcore-break and diffcore-rename. It counts only the deletion
       from the original, and does not count insertion. If you removed
       only 10 lines from a 100-line document, even if you added 910 new
       lines to make a new 1000-line document, you did not do a complete
       rewrite. diffcore-break breaks such a case in order to help
       diffcore-rename to consider such filepairs as candidate of
       rename/copy detection, but if filepairs broken that way were not
       matched with other filepairs to create rename/copy, then this
       transformation merges them back into the original "modification".

       The "extent of changes" parameter can be tweaked from the default
       80% (that is, unless more than 80% of the original material is
       deleted, the broken pairs are merged back into a single
       modification) by giving a second number to -B option, like these:

       •   -B50/60 (give 50% "break score" to diffcore-break, use 60%
           for diffcore-merge-broken).

       •   -B/60 (the same as above, since diffcore-break defaults to
           50%).

       Note that earlier implementation left a broken pair as a separate
       creation and deletion patches. This was an unnecessary hack and
       the latest implementation always merges all the broken pairs back
       into modifications, but the resulting patch output is formatted
       differently for easier review in case of such a complete rewrite
       by showing the entire contents of old version prefixed with -,
       followed by the entire contents of new version prefixed with +.

DIFFCORE-PICKAXE: FOR DETECTING ADDITION/DELETION OF SPECIFIED STRING

        top

       This transformation limits the set of filepairs to those that
       change specified strings between the preimage and the postimage
       in a certain way. -S<block of text> and -G<regular expression>
       options are used to specify different ways these strings are
       sought.

       "-S<block of text>" detects filepairs whose preimage and
       postimage have different number of occurrences of the specified
       block of text. By definition, it will not detect in-file moves.
       Also, when a changeset moves a file wholesale without affecting
       the interesting string, diffcore-rename kicks in as usual, and -S
       omits the filepair (since the number of occurrences of that
       string didn’t change in that rename-detected filepair). When used
       with --pickaxe-regex, treat the <block of text> as an extended
       POSIX regular expression to match, instead of a literal string.

       "-G<regular expression>" (mnemonic: grep) detects filepairs whose
       textual diff has an added or a deleted line that matches the
       given regular expression. This means that it will detect in-file
       (or what rename-detection considers the same file) moves, which
       is noise. The implementation runs diff twice and greps, and this
       can be quite expensive. To speed things up binary files without
       textconv filters will be ignored.

       When -S or -G are used without --pickaxe-all, only filepairs that
       match their respective criterion are kept in the output. When
       --pickaxe-all is used, if even one filepair matches their
       respective criterion in a changeset, the entire changeset is
       kept. This behavior is designed to make reviewing changes in the
       context of the whole changeset easier.

DIFFCORE-ORDER: FOR SORTING THE OUTPUT BASED ON FILENAMES         top

       This is used to reorder the filepairs according to the user’s (or
       project’s) taste, and is controlled by the -O option to the git
       diff-* commands.

       This takes a text file each of whose lines is a shell glob
       pattern. Filepairs that match a glob pattern on an earlier line
       in the file are output before ones that match a later line, and
       filepairs that do not match any glob pattern are output last.

       As an example, a typical orderfile for the core Git probably
       would look like this:

           README
           Makefile
           Documentation
           *.h
           *.c
           t

DIFFCORE-ROTATE: FOR CHANGING AT WHICH PATH OUTPUT STARTS         top

       This transformation takes one pathname, and rotates the set of
       filepairs so that the filepair for the given pathname comes
       first, optionally discarding the paths that come before it. This
       is used to implement the --skip-to and the --rotate-to options.
       It is an error when the specified pathname is not in the set of
       filepairs, but it is not useful to error out when used with "git
       log" family of commands, because it is unreasonable to expect
       that a given path would be modified by each and every commit
       shown by the "git log" command. For this reason, when used with
       "git log", the filepair that sorts the same as, or the first one
       that sorts after, the given pathname is where the output starts.

       Use of this transformation combined with diffcore-order will
       produce unexpected results, as the input to this transformation
       is likely not sorted when diffcore-order is in effect.

SEE ALSO         top

       git-diff(1), git-diff-files(1), git-diff-index(1),
       git-diff-tree(1), git-format-patch(1), git-log(1),
       gitglossary(7), The Git User’s Manual[1]

GIT         top

       Part of the git(1) suite

NOTES         top

        1. The Git User’s Manual
           file:///usr/local/share/doc/git/user-manual.html

COLOPHON         top

       This page is part of the git (Git distributed version control
       system) project.  Information about the project can be found at
       ⟨http://git-scm.com/⟩.  If you have a bug report for this manual
       page, see ⟨http://git-scm.com/community⟩.  This page was obtained
       from the project's upstream Git repository
       ⟨https://github.com/git/git.git⟩ on 2021-04-01.  (At that time,
       the date of the most recent commit that was found in the
       repository was 2021-03-30.)  If you discover any rendering
       problems in this HTML version of the page, or you believe there
       is a better or more up-to-date source for the page, or you have
       corrections or improvements to the information in this COLOPHON
       (which is not part of the original manual page), send a mail to
       man-pages@man7.org

Git 2.31.1.163.ga65ce7         04/01/2021                 GITDIFFCORE(7)

Pages that refer to this page: git(1)git-diff(1)git-diff-files(1)git-diff-index(1)git-diff-tree(1)git-format-patch(1)git-log(1)git-show(1)gitweb.conf(5)