GIT-READ-TREE(1) Git Manual GIT-READ-TREE(1)NAMEgit-read-tree - Reads tree information into the index
SYNOPSISgit-read-tree (<tree-ish> | [[-m [--trivial] [--aggressive] | --reset |
--prefix=<prefix>] [-u | -i]] [--exclude-per-directory=<gitignore>]
[--index-output=<file>] <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 are 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.
--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 to 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.
· when both sides remove a path. The resolution is to remove
that path.
· when both sides adds a path identically. The resolution is to
add that path.
--prefix=<prefix>/
Keep the current index contents, and read the contents of named
tree-ish under directory at <prefix>. The original index file
cannot have anything at the path <prefix> itself, and have
nothing in <prefix>/ directory. 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 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.
<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 matches 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:
I (index) 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
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 were 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
what "local changes" you made are carried forward by running
git-diff-index --cached $M. Note that this does not necessarily
match 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 two-tree merge.
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":
· 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 it)
· 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:
· 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=`cat .git/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.
SEE ALSOgit-write-tree(1); git-ls-files(1); gitignore(5)AUTHOR
Written by Linus Torvalds <torvalds@osdl.org>
DOCUMENTATION
Documentation by David Greaves, Junio C Hamano and the git-list
<git@vger.kernel.org>.
GIT
Part of the git(7) suite
Git 1.5.5.2 10/21/2008 GIT-READ-TREE(1)