git-read-tree
(1)
名前
git-read-tree - Reads tree information into the index
形式
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>]])
説明
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.
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-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. Note that
the <prefix>/ value must end with a slash.
--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
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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.
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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.
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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
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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
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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
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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.
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ATTRIBUTES
See attributes(5) for descriptions of the following
attributes:
+---------------+--------------------------+
|ATTRIBUTE TYPE | ATTRIBUTE VALUE |
+---------------+--------------------------+
|Availability | developer/versioning/git |
+---------------+--------------------------+
|Stability | Uncommitted |
+---------------+--------------------------+
SEE ALSO
git-write-tree(1); git-ls-files(1); gitignore(4)
GIT
Part of the git(1) suite
NOTES
This software was built from source available at
https://java.net/projects/solaris-userland. The original
community source was downloaded from http://git-
core.googlecode.com/files/git-1.7.9.2.tar.gz
Further information about this software can be found on the
open source community website at http://git-scm.com/.
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