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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 [[-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:

	   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 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
	     -------------------------------------------------------
	   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
       Part of the git(1) suite

Git 1.8.3.1				    06/10/2014				 GIT-READ-TREE(1)
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