unchanged" bit to paths you have not changed to
cause Git not to do this check.  Note that setting this bit on a
path does not mean Git will check the contents of the file to
see if it has changed -- it makes Git to omit any checking and
assume it has *not* changed.  When you make changes to working
tree files, you have to explicitly tell Git about it by dropping
"assume unchanged" bit, either before or after you modify them.

In order to set "assume unchanged" bit, use `--assume-unchanged`
option.  To unset, use `--no-assume-unchanged`. To see which files
have the "assume unchanged" bit set, use `git ls-files -v`
(see linkgit:git-ls-files[1]).

The command looks at `core.ignorestat` configuration variable.  When
this is true, paths updated with `git update-index paths...` and
paths updated with other Git commands that update both index and
working tree (e.g. 'git apply --index', 'git checkout-index -u',
and 'git read-tree -u') are automatically marked as "assume
unchanged".  Note that "assume unchanged" bit is *not* set if
`git update-index --refresh` finds the working tree file matches
the index (use `git update-index --really-refresh` if you want
to mark them as "assume unchanged").

Sometimes users confuse the assume-unchanged bit with the
skip-worktree bit.  See the final paragraph in the "Skip-worktree bit"
section below for an explanation of the differences.


EXAMPLES
--------
To update and refresh only the files already checked out:

----------------
$ git checkout-index -n -f -a && git update-index --ignore-missing --refresh
----------------

On an inefficient filesystem with `core.ignorestat` set::
+
------------
$ git update-index --really-refresh              <1>
$ git update-index --no-assume-unchanged foo.c   <2>
$ git diff --name-only                           <3>
$ edit foo.c
$ git diff --name-only                           <4>
M foo.c
$ git update-index foo.c                         <5>
$ git diff --name-only                           <6>
$ edit foo.c
$ git diff --name-only                           <7>
$ git update-index --no-assume-unchanged foo.c   <8>
$ git diff --name-only                           <9>
M foo.c
------------
+
<1> forces lstat(2) to set "assume unchanged" bits for paths that match index.
<2> mark the path to be edited.
<3> this does lstat(2) and finds index matches the path.
<4> this does lstat(2) and finds index does *not* match the path.
<5> registering the new version to index sets "assume unchanged" bit.
<6> and it is assumed unchanged.
<7> even after you edit it.
<8> you can tell about the change after the fact.
<9> now it checks with lstat(2) and finds it has been changed.


SKIP-WORKTREE BIT
-----------------

Skip-worktree bit can be defined in one (long) sentence: Tell git to
avoid writing the file to the working directory when reasonably
possible, and treat the file as unchanged when it is not
present in the working directory.

Note that not all git commands will pay attention to this bit, and
some only partially support it.

The update-index flags and the read-tree capabilities relating to the
skip-worktree bit predated the introduction of the
linkgit:git-sparse-checkout[1] command, which provides a much easier
way to configure and handle the skip-worktree bits.  If you want to
reduce your working tree to only deal with a subset of the files in
the repository, we strongly encourage the use of
linkgit:git-sparse-checkout[1] in preference to the low-level
update-index and read-tree primitives.

The primary purpose of the skip-worktree bit is to enable sparse
checkouts, i.e. to have working directories with only a subset of
paths present.  When the skip-worktree bit is set, Git commands (such
as `switch`, `pull`, `merge`) will avoid writing these files.
However, these commands will sometimes write these files anyway in
important cases such as conflicts during a merge or rebase.  Git
commands will also avoid treating the lack of such files as an
intentional deletion; for example `git