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GIT-READ-TREE(1)		  Git Manual		      GIT-READ-TREE(1)

NAME
       git-read-tree - Reads tree information into the index

SYNOPSIS
       git-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 ALSO
       git-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)
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