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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 [--exclude-per-directory=<gitignore>] | -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 will	not abort the operation.

	   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

	   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.

	   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.

       "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:


       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 2.28.0			  07/26/2020		      GIT-READ-TREE(1)


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