NAME | SYNOPSIS | DESCRIPTION | THE CHAIN OF OPERATION | DIFFCORE-BREAK: FOR SPLITTING UP COMPLETE REWRITES | DIFFCORE-RENAME: FOR DETECTING RENAMES AND COPIES | DIFFCORE-MERGE-BROKEN: FOR PUTTING COMPLETE REWRITES BACK TOGETHER | | DIFFCORE-ORDER: FOR SORTING THE OUTPUT BASED ON FILENAMES | 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

       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. 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.

       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

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
       2017-09-15.  If you discover any rendering problems in this HTML ver‐
       sion 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 man‐
       ual page), send a mail to man-pages@man7.org

Git 2.12.0.244.g625568           03/12/2017                   GITDIFFCORE(7)

Pages that refer to this page: 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)