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HTML rendering created 2024-06-26 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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GITDIFFCORE(7) Git Manual GITDIFFCORE(7)
gitdiffcore - Tweaking diff output
git diff *
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 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.
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%).
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 are 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
%%%SH%%%
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 a 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 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 the old version prefixed with
-, followed by the entire contents of the new version prefixed
with +.
DIFFCORE-PICKAXE: FOR DETECTING ADDITION/DELETION OF SPECIFIED STRING
%%%SH%%%
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.
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
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.
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]
Part of the git(1) suite
1. The Git User’s Manual
file:///home/mtk/share/doc/git-doc/user-manual.html
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 2024-06-14. (At that time,
the date of the most recent commit that was found in the
repository was 2024-06-12.) 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 2.45.2.492.gd63586 2024-06-12 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)
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HTML rendering created 2024-06-26 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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