GIT-READ-TREE(1) Git Manual GIT-READ-TREE(1)
NAME
git-read-tree - Reads tree information into the index
SYNOPSIS
git read-tree [[-m [--trivial] [--aggressive] | --reset | --prefix=<prefix>]
[-u [--exclude-per-directory=<gitignore>] | -i]]
[--index-output=<file>] [--no-sparse-checkout]
(--empty | <tree-ish1> [<tree-ish2> [<tree-ish3>]])
DESCRIPTION
Reads the tree information given by <tree-ish> into the index, but does
not actually update any of the files it "caches". (see: git-checkout-
index(1))
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
-m, 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 returns.
OPTIONS
-m
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.
--reset
Same as -m, except that unmerged entries are discarded instead of
failing. When used with -u, updates leading to loss of working tree
changes will not abort the operation.
-u
After a successful merge, update the files in the work tree with
the result of the merge.
-i
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 index file.
-n, --dry-run
Check if the command would error out, without updating the index or
the files in the working tree for real.
-v
Show the progress of checking files out.
--trivial
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.
--aggressive
Usually a three-way merge by git read-tree 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 internally:
o when one side removes a path and the other side leaves the path
unmodified. The resolution is to remove that path.
o when both sides remove a path. The resolution is to remove that
path.
o when both sides add a path identically. The resolution is to
add that path.
--prefix=<prefix>
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.
--exclude-per-directory=<gitignore>
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.
--index-output=<file>
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.
--[no-]recurse-submodules
Using --recurse-submodules will update the content of all active
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.
--no-sparse-checkout
Disable sparse checkout support even if core.sparseCheckout is
true.
--empty
Instead of reading tree object(s) into the index, just empty it.
-q, --quiet
Quiet, suppress feedback messages.
<tree-ish#>
The id of the tree object(s) to be read/merged.
MERGING
If -m 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.
Single Tree Merge
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> followed by a
git checkout-index -f -u -a, the git checkout-index only checks out the
stuff that really changed.
This is used to avoid unnecessary false hits when git diff-files is run
after git read-tree.
Two Tree Merge
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 the
following:
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 specified commit:
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
exists, fail
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
clean (H==M)
------
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 keeps the
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
output after 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.
3-Way Merge
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 normal use.
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":
o 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)
o 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 it)
o stage 1 and stage 3 are the same and stage 2 is different take
stage 2 (we did something while they did nothing)
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 stage 0.
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 works:
o if a file exists in identical format in all three trees, it will
automatically collapse to "merged" state by git read-tree.
o 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.
o 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 simple:
o you walk the index in order, and ignore all entries of stage 0,
since they've already been done.
o 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.
o 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 merge-one-file
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 following sequence:
$ 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 the merge.
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 (git read-tree 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
"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.
git read-tree and other merge-based commands (git merge, git
checkout...) 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.
While $GIT_DIR/info/sparse-checkout 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:
/*
!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 file
content as follows:
/*
Then you can disable sparse checkout. Sparse checkout support in git
read-tree and similar commands is disabled by default. You need to turn
core.sparseCheckout on in order to have sparse checkout support.
SEE ALSO
git-write-tree(1); git-ls-files(1); gitignore(5); git-sparse-
checkout(1);
GIT
Part of the git(1) suite
Git 2.27.0 06/01/2020 GIT-READ-TREE(1)