git-read-tree - Reads tree information into the index
git read-tree [[-m [--trivial] [--aggressive] | --reset | --prefix=<prefix>]
[-u [--exclude-per-directory=<gitignore>] | -i]]
(--empty | <tree-ish1> [<tree-ish2> [<tree-ish3>]])
Reads the tree information given by <tree-ish> into the index, but does
not actually update
any of the files it "caches". (see:
Optionally, it can merge a tree into the index, perform a fast-forward (i.e.
2-way) merge, or a 3-way merge, with the -m
flag. When used with
, the -u
flag causes it to also update the files in the work
tree with the result of the merge.
Trivial merges are done by git read-tree
itself. Only conflicting paths
will be in unmerged state when git read-tree
Perform a merge, not just a read. The command
will refuse to run if your index file has unmerged entries, indicating that
you have not finished previous merge you started.
Same as -m, except that unmerged entries are
discarded instead of failing.
After a successful merge, update the files in
the work tree with the result of the merge.
Usually a merge requires the index file as
well as the files in the working tree to be up to date with the current head
commit, in order not to lose local changes. This flag disables the check with
the working tree and is meant to be used when creating a merge of trees that
are not directly related to the current working tree status into a temporary
Check if the command would error out, without
updating the index or the files in the working tree for real.
Show the progress of checking files out.
Restrict three-way merge by git
read-tree to happen only if there is no file-level merging required,
instead of resolving merge for trivial cases and leaving conflicting files
unresolved in the index.
Usually a three-way merge by git
resolves the merge for really trivial cases and leaves other
cases unresolved in the index, so that porcelains can implement different
merge policies. This flag makes the command resolve a few more cases
•when one side removes a path and the
other side leaves the path unmodified. The resolution is to remove that
•when both sides remove a path. The
resolution is to remove that path.
•when both sides add a path
identically. The resolution is to add that path.
Keep the current index contents, and read the
contents of the named tree-ish under the directory at <prefix>.
The command will refuse to overwrite entries that already existed in the
original index file.
When running the command with -u and
-m options, the merge result may need to overwrite paths that are not
tracked in the current branch. The command usually refuses to proceed with the
merge to avoid losing such a path. However this safety valve sometimes gets in
the way. For example, it often happens that the other branch added a file that
used to be a generated file in your branch, and the safety valve triggers when
you try to switch to that branch after you ran make but before running
make clean to remove the generated file. This option tells the command
to read per-directory exclude file (usually .gitignore) and allows such
an untracked but explicitly ignored file to be overwritten.
Instead of writing the results out to
$GIT_INDEX_FILE, write the resulting index in the named file. While the
command is operating, the original index file is locked with the same
mechanism as usual. The file must allow to be rename(2)ed into from a
temporary file that is created next to the usual index file; typically this
means it needs to be on the same filesystem as the index file itself, and you
need write permission to the directories the index file and index output file
are located in.
Using --recurse-submodules will update the
content of all initialized submodules according to the commit recorded in the
superproject by calling read-tree recursively, also setting the submodules
HEAD to be detached at that commit.
Disable sparse checkout support even if
core.sparseCheckout is true.
Instead of reading tree object(s) into the
index, just empty it.
The id of the tree object(s) to be
is specified, git read-tree
can perform 3 kinds of merge, a
single tree merge if only 1 tree is given, a fast-forward merge with 2 trees,
or a 3-way merge if 3 or more trees are provided.
If only 1 tree is specified, git read-tree
operates as if the user did
not specify -m
, except that if the original index has an entry for a
given pathname, and the contents of the path match with the tree being read,
the stat info from the index is used. (In other words, the index’s
stat()s take precedence over the merged tree’s).
That means that if you do a git read-tree -m <newtree>
a git checkout-index -f -u -a
, the git checkout-index
checks out the stuff that really changed.
This is used to avoid unnecessary false hits when git diff-files
after git read-tree
Typically, this is invoked as git read-tree -m $H $M
, where $H is the
head commit of the current repository, and $M is the head of a foreign tree,
which is simply ahead of $H (i.e. we are in a fast-forward situation).
When two trees are specified, the user is telling git read-tree
1.The current index and work tree is derived
from $H, but the user may have local changes in them since $H.
2.The user wants to fast-forward to $M.
In this case, the git read-tree -m $H $M
command makes sure that no local
change is lost as the result of this "merge". Here are the
"carry forward" rules, where "I" denotes the index,
"clean" means that index and work tree coincide, and
"exists"/"nothing" refer to the presence of a path in the
I H M Result
0 nothing nothing nothing (does not happen)
1 nothing nothing exists use M
2 nothing exists nothing remove path from index
3 nothing exists exists, use M if "initial checkout",
H == M keep index otherwise
H != M
clean I==H I==M
4 yes N/A N/A nothing nothing keep index
5 no N/A N/A nothing nothing keep index
6 yes N/A yes nothing exists keep index
7 no N/A yes nothing exists keep index
8 yes N/A no nothing exists fail
9 no N/A no nothing exists fail
10 yes yes N/A exists nothing remove path from index
11 no yes N/A exists nothing fail
12 yes no N/A exists nothing fail
13 no no N/A exists nothing fail
14 yes exists exists keep index
15 no exists exists keep index
clean I==H I==M (H!=M)
16 yes no no exists exists fail
17 no no no exists exists fail
18 yes no yes exists exists keep index
19 no no yes exists exists keep index
20 yes yes no exists exists use M
21 no yes no exists exists fail
In all "keep index" cases, the index entry stays as in the original
index file. If the entry is not up to date, git read-tree
copy in the work tree intact when operating under the -u flag.
When this form of git read-tree
returns successfully, you can see which
of the "local changes" that you made were carried forward by running
git diff-index --cached $M
. Note that this does not necessarily match
what git diff-index --cached $H
would have produced before such a two
tree merge. This is because of cases 18 and 19 --- if you already had the
changes in $M (e.g. maybe you picked it up via e-mail in a patch form), git
diff-index --cached $H
would have told you about the change before this
merge, but it would not show in git diff-index --cached $M
the two-tree merge.
Case 3 is slightly tricky and needs explanation. The result from this rule
logically should be to remove the path if the user staged the removal of the
path and then switching to a new branch. That however will prevent the initial
checkout from happening, so the rule is modified to use M (new tree) only when
the content of the index is empty. Otherwise the removal of the path is kept
as long as $H and $M are the same.
Each "index" entry has two bits worth of "stage" state.
stage 0 is the normal one, and is the only one you’d see in any kind of
However, when you do git read-tree
with three trees, the
"stage" starts out at 1.
This means that you can do
$ git read-tree -m <tree1> <tree2> <tree3>
and you will end up with an index with all of the <tree1> entries in
"stage1", all of the <tree2> entries in "stage2" and
all of the <tree3> entries in "stage3". When performing a
merge of another branch into the current branch, we use the common ancestor
tree as <tree1>, the current branch head as <tree2>, and the other
branch head as <tree3>.
Furthermore, git read-tree
has special-case logic that says: if you see a
file that matches in all respects in the following states, it
"collapses" back to "stage0":
•stage 2 and 3 are the same; take one
or the other (it makes no difference - the same work has been done on our
branch in stage 2 and their branch in stage 3)
•stage 1 and stage 2 are the same and
stage 3 is different; take stage 3 (our branch in stage 2 did not do anything
since the ancestor in stage 1 while their branch in stage 3 worked on
•stage 1 and stage 3 are the same and
stage 2 is different take stage 2 (we did something while they did
The git write-tree
command refuses to write a nonsensical tree, and it
will complain about unmerged entries if it sees a single entry that is not
OK, this all sounds like a collection of totally nonsensical rules, but
it’s actually exactly what you want in order to do a fast merge. The
different stages represent the "result tree" (stage 0, aka
"merged"), the original tree (stage 1, aka "orig"), and
the two trees you are trying to merge (stage 2 and 3 respectively).
The order of stages 1, 2 and 3 (hence the order of three <tree-ish>
command-line arguments) are significant when you start a 3-way merge with an
index file that is already populated. Here is an outline of how the algorithm
•if a file exists in identical format
in all three trees, it will automatically collapse to "merged" state
by git read-tree.
•a file that has any difference
what-so-ever in the three trees will stay as separate entries in the index.
It’s up to "porcelain policy" to determine how to remove the
non-0 stages, and insert a merged version.
•the index file saves and restores with
all this information, so you can merge things incrementally, but as long as it
has entries in stages 1/2/3 (i.e., "unmerged entries") you
can’t write the result. So now the merge algorithm ends up being really
•you walk the index in order, and
ignore all entries of stage 0, since they’ve already been done.
•if you find a "stage1", but
no matching "stage2" or "stage3", you know it’s
been removed from both trees (it only existed in the original tree), and you
remove that entry.
•if you find a matching
"stage2" and "stage3" tree, you remove one of them, and
turn the other into a "stage0" entry. Remove any matching
"stage1" entry if it exists too. .. all the normal trivial rules
You would normally use git merge-index
with supplied git
to do this last step. The script updates the files in the
working tree as it merges each path and at the end of a successful merge.
When you start a 3-way merge with an index file that is already populated, it is
assumed that it represents the state of the files in your work tree, and you
can even have files with changes unrecorded in the index file. It is further
assumed that this state is "derived" from the stage 2 tree. The
3-way merge refuses to run if it finds an entry in the original index file
that does not match stage 2.
This is done to prevent you from losing your work-in-progress changes, and
mixing your random changes in an unrelated merge commit. To illustrate,
suppose you start from what has been committed last to your repository:
$ JC=`git rev-parse --verify "HEAD^0"`
$ git checkout-index -f -u -a $JC
You do random edits, without running git update-index
. And then you
notice that the tip of your "upstream" tree has advanced since you
pulled from him:
$ git fetch git://.... linus
$ LT=`git rev-parse FETCH_HEAD`
Your work tree is still based on your HEAD ($JC), but you have some edits since.
Three-way merge makes sure that you have not added or modified index entries
since $JC, and if you haven’t, then does the right thing. So with the
$ git read-tree -m -u `git merge-base $JC $LT` $JC $LT
$ git merge-index git-merge-one-file -a
$ echo "Merge with Linus" | \
git commit-tree `git write-tree` -p $JC -p $LT
what you would commit is a pure merge between $JC and $LT without your
work-in-progress changes, and your work tree would be updated to the result of
However, if you have local changes in the working tree that would be overwritten
by this merge, git read-tree
will refuse to run to prevent your changes
from being lost.
In other words, there is no need to worry about what exists only in the working
tree. When you have local changes in a part of the project that is not
involved in the merge, your changes do not interfere with the merge, and are
kept intact. When they do
interfere, the merge does not even start
complains loudly and fails without modifying anything).
In such a case, you can simply continue doing what you were in the middle of
doing, and when your working tree is ready (i.e. you have finished your
work-in-progress), attempt the merge again.
"Sparse checkout" allows populating the working directory sparsely. It
uses the skip-worktree bit (see git-update-index(1)
) to tell Git
whether a file in the working directory is worth looking at.
and other merge-based commands (git merge
...) can help maintaining the skip-worktree bitmap and working
directory update. $GIT_DIR/info/sparse-checkout
is used to define the
skip-worktree reference bitmap. When git read-tree
needs to update the
working directory, it resets the skip-worktree bit in the index based on this
file, which uses the same syntax as .gitignore files. If an entry matches a
pattern in this file, skip-worktree will not be set on that entry. Otherwise,
skip-worktree will be set.
Then it compares the new skip-worktree value with the previous one. If
skip-worktree turns from set to unset, it will add the corresponding file
back. If it turns from unset to set, that file will be removed.
is usually used to specify what files
are in, you can also specify what files are not
in, using negate
patterns. For example, to remove the file unwanted
Another tricky thing is fully repopulating the working directory when you no
longer want sparse checkout. You cannot just disable "sparse
checkout" because skip-worktree bits are still in the index and your
working directory is still sparsely populated. You should re-populate the
working directory with the $GIT_DIR/info/sparse-checkout
Then you can disable sparse checkout. Sparse checkout support in git
and similar commands is disabled by default. You need to turn
on in order to have sparse checkout support.
Part of the git(1)