1GITDIFFCORE(7)                    Git Manual                    GITDIFFCORE(7)
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NAME

6       gitdiffcore - Tweaking diff output
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SYNOPSIS

9       git diff *
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DESCRIPTION

12       The diff commands git diff-index, git diff-files, and git diff-tree can
13       be told to manipulate differences they find in unconventional ways
14       before showing diff output. The manipulation is collectively called
15       "diffcore transformation". This short note describes what they are and
16       how to use them to produce diff output that is easier to understand
17       than the conventional kind.
18

THE CHAIN OF OPERATION

20       The git diff-* family works by first comparing two sets of files:
21
22       ·   git diff-index compares contents of a "tree" object and the working
23           directory (when --cached flag is not used) or a "tree" object and
24           the index file (when --cached flag is used);
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26       ·   git diff-files compares contents of the index file and the working
27           directory;
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29       ·   git diff-tree compares contents of two "tree" objects;
30
31       In all of these cases, the commands themselves first optionally limit
32       the two sets of files by any pathspecs given on their command-lines,
33       and compare corresponding paths in the two resulting sets of files.
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35       The pathspecs are used to limit the world diff operates in. They remove
36       the filepairs outside the specified sets of pathnames. E.g. If the
37       input set of filepairs included:
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39           :100644 100644 bcd1234... 0123456... M junkfile
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41       but the command invocation was git diff-files myfile, then the junkfile
42       entry would be removed from the list because only "myfile" is under
43       consideration.
44
45       The result of comparison is passed from these commands to what is
46       internally called "diffcore", in a format similar to what is output
47       when the -p option is not used. E.g.
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49           in-place edit  :100644 100644 bcd1234... 0123456... M file0
50           create         :000000 100644 0000000... 1234567... A file4
51           delete         :100644 000000 1234567... 0000000... D file5
52           unmerged       :000000 000000 0000000... 0000000... U file6
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54       The diffcore mechanism is fed a list of such comparison results (each
55       of which is called "filepair", although at this point each of them
56       talks about a single file), and transforms such a list into another
57       list. There are currently 5 such transformations:
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59       ·   diffcore-break
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61       ·   diffcore-rename
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63       ·   diffcore-merge-broken
64
65       ·   diffcore-pickaxe
66
67       ·   diffcore-order
68
69       These are applied in sequence. The set of filepairs git diff-* commands
70       find are used as the input to diffcore-break, and the output from
71       diffcore-break is used as the input to the next transformation. The
72       final result is then passed to the output routine and generates either
73       diff-raw format (see Output format sections of the manual for git
74       diff-* commands) or diff-patch format.
75

DIFFCORE-BREAK: FOR SPLITTING UP COMPLETE REWRITES

77       The second transformation in the chain is diffcore-break, and is
78       controlled by the -B option to the git diff-* commands. This is used to
79       detect a filepair that represents "complete rewrite" and break such
80       filepair into two filepairs that represent delete and create. E.g. If
81       the input contained this filepair:
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83           :100644 100644 bcd1234... 0123456... M file0
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85       and if it detects that the file "file0" is completely rewritten, it
86       changes it to:
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88           :100644 000000 bcd1234... 0000000... D file0
89           :000000 100644 0000000... 0123456... A file0
90
91       For the purpose of breaking a filepair, diffcore-break examines the
92       extent of changes between the contents of the files before and after
93       modification (i.e. the contents that have "bcd1234..." and "0123456..."
94       as their SHA-1 content ID, in the above example). The amount of
95       deletion of original contents and insertion of new material are added
96       together, and if it exceeds the "break score", the filepair is broken
97       into two. The break score defaults to 50% of the size of the smaller of
98       the original and the result (i.e. if the edit shrinks the file, the
99       size of the result is used; if the edit lengthens the file, the size of
100       the original is used), and can be customized by giving a number after
101       "-B" option (e.g. "-B75" to tell it to use 75%).
102

DIFFCORE-RENAME: FOR DETECTING RENAMES AND COPIES

104       This transformation is used to detect renames and copies, and is
105       controlled by the -M option (to detect renames) and the -C option (to
106       detect copies as well) to the git diff-* commands. If the input
107       contained these filepairs:
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109           :100644 000000 0123456... 0000000... D fileX
110           :000000 100644 0000000... 0123456... A file0
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112       and the contents of the deleted file fileX is similar enough to the
113       contents of the created file file0, then rename detection merges these
114       filepairs and creates:
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116           :100644 100644 0123456... 0123456... R100 fileX file0
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118       When the "-C" option is used, the original contents of modified files,
119       and deleted files (and also unmodified files, if the
120       "--find-copies-harder" option is used) are considered as candidates of
121       the source files in rename/copy operation. If the input were like these
122       filepairs, that talk about a modified file fileY and a newly created
123       file file0:
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125           :100644 100644 0123456... 1234567... M fileY
126           :000000 100644 0000000... bcd3456... A file0
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128       the original contents of fileY and the resulting contents of file0 are
129       compared, and if they are similar enough, they are changed to:
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131           :100644 100644 0123456... 1234567... M fileY
132           :100644 100644 0123456... bcd3456... C100 fileY file0
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134       In both rename and copy detection, the same "extent of changes"
135       algorithm used in diffcore-break is used to determine if two files are
136       "similar enough", and can be customized to use a similarity score
137       different from the default of 50% by giving a number after the "-M" or
138       "-C" option (e.g. "-M8" to tell it to use 8/10 = 80%).
139
140       Note. When the "-C" option is used with --find-copies-harder option,
141       git diff-* commands feed unmodified filepairs to diffcore mechanism as
142       well as modified ones. This lets the copy detector consider unmodified
143       files as copy source candidates at the expense of making it slower.
144       Without --find-copies-harder, git diff-* commands can detect copies
145       only if the file that was copied happened to have been modified in the
146       same changeset.
147

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

149       This transformation is used to merge filepairs broken by
150       diffcore-break, and not transformed into rename/copy by
151       diffcore-rename, back into a single modification. This always runs when
152       diffcore-break is used.
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154       For the purpose of merging broken filepairs back, it uses a different
155       "extent of changes" computation from the ones used by diffcore-break
156       and diffcore-rename. It counts only the deletion from the original, and
157       does not count insertion. If you removed only 10 lines from a 100-line
158       document, even if you added 910 new lines to make a new 1000-line
159       document, you did not do a complete rewrite. diffcore-break breaks such
160       a case in order to help diffcore-rename to consider such filepairs as
161       candidate of rename/copy detection, but if filepairs broken that way
162       were not matched with other filepairs to create rename/copy, then this
163       transformation merges them back into the original "modification".
164
165       The "extent of changes" parameter can be tweaked from the default 80%
166       (that is, unless more than 80% of the original material is deleted, the
167       broken pairs are merged back into a single modification) by giving a
168       second number to -B option, like these:
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170       ·   -B50/60 (give 50% "break score" to diffcore-break, use 60% for
171           diffcore-merge-broken).
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173       ·   -B/60 (the same as above, since diffcore-break defaults to 50%).
174
175       Note that earlier implementation left a broken pair as a separate
176       creation and deletion patches. This was an unnecessary hack and the
177       latest implementation always merges all the broken pairs back into
178       modifications, but the resulting patch output is formatted differently
179       for easier review in case of such a complete rewrite by showing the
180       entire contents of old version prefixed with -, followed by the entire
181       contents of new version prefixed with +.
182

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

184       This transformation limits the set of filepairs to those that change
185       specified strings between the preimage and the postimage in a certain
186       way. -S<block of text> and -G<regular expression> options are used to
187       specify different ways these strings are sought.
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189       "-S<block of text>" detects filepairs whose preimage and postimage have
190       different number of occurrences of the specified block of text. By
191       definition, it will not detect in-file moves. Also, when a changeset
192       moves a file wholesale without affecting the interesting string,
193       diffcore-rename kicks in as usual, and -S omits the filepair (since the
194       number of occurrences of that string didn’t change in that
195       rename-detected filepair). When used with --pickaxe-regex, treat the
196       <block of text> as an extended POSIX regular expression to match,
197       instead of a literal string.
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199       "-G<regular expression>" (mnemonic: grep) detects filepairs whose
200       textual diff has an added or a deleted line that matches the given
201       regular expression. This means that it will detect in-file (or what
202       rename-detection considers the same file) moves, which is noise. The
203       implementation runs diff twice and greps, and this can be quite
204       expensive. To speed things up binary files without textconv filters
205       will be ignored.
206
207       When -S or -G are used without --pickaxe-all, only filepairs that match
208       their respective criterion are kept in the output. When --pickaxe-all
209       is used, if even one filepair matches their respective criterion in a
210       changeset, the entire changeset is kept. This behavior is designed to
211       make reviewing changes in the context of the whole changeset easier.
212

DIFFCORE-ORDER: FOR SORTING THE OUTPUT BASED ON FILENAMES

214       This is used to reorder the filepairs according to the user’s (or
215       project’s) taste, and is controlled by the -O option to the git diff-*
216       commands.
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218       This takes a text file each of whose lines is a shell glob pattern.
219       Filepairs that match a glob pattern on an earlier line in the file are
220       output before ones that match a later line, and filepairs that do not
221       match any glob pattern are output last.
222
223       As an example, a typical orderfile for the core Git probably would look
224       like this:
225
226           README
227           Makefile
228           Documentation
229           *.h
230           *.c
231           t
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SEE ALSO

234       git-diff(1), git-diff-files(1), git-diff-index(1), git-diff-tree(1),
235       git-format-patch(1), git-log(1), gitglossary(7), The Git User’s
236       Manual[1]
237

GIT

239       Part of the git(1) suite
240

NOTES

242        1. The Git User’s Manual
243           file:///usr/share/doc/git/user-manual.html
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247Git 2.26.2                        2020-04-20                    GITDIFFCORE(7)
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