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

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

       git read-tree [[-m [--trivial] [--aggressive] | --reset | --prefix=<prefix>]
		       [-u | -i]] [--index-output=<file>] [--no-sparse-checkout]
		       (--empty | <tree-ish1> [<tree-ish2> [<tree-ish3>]])

       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-

       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.

	   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.

	   Same as -m, except that unmerged entries are discarded instead of
	   failing. When used with -u, updates leading to loss of working tree
	   changes or untracked files or directories will not abort the

	   After a successful merge, update the files in the work tree with
	   the result of the merge.

	   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 or
	   the files in the working tree for real.

	   Show the progress of checking files out.

	   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

	   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

	   o   when both sides add a path identically. The resolution is to
	       add that path.

	   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

	   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.

	   Using --recurse-submodules will update the content of all active
	   submodules according to the commit recorded in the superproject by
	   calling read-tree recursively, also setting the submodules' HEAD to
	   be detached at that commit.

	   Disable sparse checkout support even if core.sparseCheckout is

	   Instead of reading tree object(s) into the index, just empty it.

       -q, --quiet
	   Quiet, suppress feedback messages.

	   The id of the tree object(s) to be read/merged.

       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 or more 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

       When two trees are specified, the user is telling git read-tree the

	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

	   $ 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.

       Note: The skip-worktree capabilities in git-update-index(1) and
       read-tree predated the introduction of git-sparse-checkout(1). Users
       are encouraged to use the sparse-checkout command in preference to
       these plumbing commands for sparse-checkout/skip-worktree related
       needs. However, the information below might be useful to users trying
       to understand the pattern style used in non-cone mode of the
       sparse-checkout command.

       "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, or the entry corresponds to a
       file present in the working tree, then 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:


       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.

       git-write-tree(1), git-ls-files(1), gitignore(5), git-sparse-

       Part of the git(1) suite

Git 2.38.4			  05/16/2024		      GIT-READ-TREE(1)