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.
-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 nor 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:
· when one side removes a path and the other side leaves the path unmodified. The resolution is to
remove that path.
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-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.
<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 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
-------------------------------------------------------
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
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:
· 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 simple:
· 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 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`
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
Part of the git(1) suite
Git 1.8.3.1 03/23/2016 GIT-READ-TREE(1)
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