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    1 git-read-tree(1)
    2 ================
    4 NAME
    5 ----
    6 git-read-tree - Reads tree information into the index
   10 --------
   11 [verse]
   12 'git read-tree' [[-m [--trivial] [--aggressive] | --reset | --prefix=<prefix>]
   13 		[-u [--exclude-per-directory=<gitignore>] | -i]]
   14 		[--index-output=<file>] [--no-sparse-checkout]
   15 		(--empty | <tree-ish1> [<tree-ish2> [<tree-ish3>]])
   19 -----------
   20 Reads the tree information given by <tree-ish> into the index,
   21 but does not actually *update* any of the files it "caches". (see:
   22 linkgit:git-checkout-index[1])
   24 Optionally, it can merge a tree into the index, perform a
   25 fast-forward (i.e. 2-way) merge, or a 3-way merge, with the `-m`
   26 flag.  When used with `-m`, the `-u` flag causes it to also update
   27 the files in the work tree with the result of the merge.
   29 Trivial merges are done by 'git read-tree' itself.  Only conflicting paths
   30 will be in unmerged state when 'git read-tree' returns.
   33 -------
   34 -m::
   35 	Perform a merge, not just a read.  The command will
   36 	refuse to run if your index file has unmerged entries,
   37 	indicating that you have not finished previous merge you
   38 	started.
   40 --reset::
   41 	Same as -m, except that unmerged entries are discarded instead
   42 	of failing. When used with `-u`, updates leading to loss of
   43 	working tree changes will not abort the operation.
   45 -u::
   46 	After a successful merge, update the files in the work
   47 	tree with the result of the merge.
   49 -i::
   50 	Usually a merge requires the index file as well as the
   51 	files in the working tree to be up to date with the
   52 	current head commit, in order not to lose local
   53 	changes.  This flag disables the check with the working
   54 	tree and is meant to be used when creating a merge of
   55 	trees that are not directly related to the current
   56 	working tree status into a temporary index file.
   58 -n::
   59 --dry-run::
   60 	Check if the command would error out, without updating the index
   61 	or the files in the working tree for real.
   63 -v::
   64 	Show the progress of checking files out.
   66 --trivial::
   67 	Restrict three-way merge by 'git read-tree' to happen
   68 	only if there is no file-level merging required, instead
   69 	of resolving merge for trivial cases and leaving
   70 	conflicting files unresolved in the index.
   72 --aggressive::
   73 	Usually a three-way merge by 'git read-tree' resolves
   74 	the merge for really trivial cases and leaves other
   75 	cases unresolved in the index, so that porcelains can
   76 	implement different merge policies.  This flag makes the
   77 	command resolve a few more cases internally:
   78 +
   79 * when one side removes a path and the other side leaves the path
   80   unmodified.  The resolution is to remove that path.
   81 * when both sides remove a path.  The resolution is to remove that path.
   82 * when both sides add a path identically.  The resolution
   83   is to add that path.
   85 --prefix=<prefix>::
   86 	Keep the current index contents, and read the contents
   87 	of the named tree-ish under the directory at `<prefix>`.
   88 	The command will refuse to overwrite entries that already
   89 	existed in the original index file.
   91 --exclude-per-directory=<gitignore>::
   92 	When running the command with `-u` and `-m` options, the
   93 	merge result may need to overwrite paths that are not
   94 	tracked in the current branch.  The command usually
   95 	refuses to proceed with the merge to avoid losing such a
   96 	path.  However this safety valve sometimes gets in the
   97 	way.  For example, it often happens that the other
   98 	branch added a file that used to be a generated file in
   99 	your branch, and the safety valve triggers when you try
  100 	to switch to that branch after you ran `make` but before
  101 	running `make clean` to remove the generated file.  This
  102 	option tells the command to read per-directory exclude
  103 	file (usually '.gitignore') and allows such an untracked
  104 	but explicitly ignored file to be overwritten.
  106 --index-output=<file>::
  107 	Instead of writing the results out to `$GIT_INDEX_FILE`,
  108 	write the resulting index in the named file.  While the
  109 	command is operating, the original index file is locked
  110 	with the same mechanism as usual.  The file must allow
  111 	to be rename(2)ed into from a temporary file that is
  112 	created next to the usual index file; typically this
  113 	means it needs to be on the same filesystem as the index
  114 	file itself, and you need write permission to the
  115 	directories the index file and index output file are
  116 	located in.
  118 --[no-]recurse-submodules::
  119 	Using --recurse-submodules will update the content of all initialized
  120 	submodules according to the commit recorded in the superproject by
  121 	calling read-tree recursively, also setting the submodules HEAD to be
  122 	detached at that commit.
  124 --no-sparse-checkout::
  125 	Disable sparse checkout support even if `core.sparseCheckout`
  126 	is true.
  128 --empty::
  129 	Instead of reading tree object(s) into the index, just empty
  130 	it.
  132 -q::
  133 --quiet::
  134 	Quiet, suppress feedback messages.
  136 <tree-ish#>::
  137 	The id of the tree object(s) to be read/merged.
  141 -------
  142 If `-m` is specified, 'git read-tree' can perform 3 kinds of
  143 merge, a single tree merge if only 1 tree is given, a
  144 fast-forward merge with 2 trees, or a 3-way merge if 3 or more trees are
  145 provided.
  148 Single Tree Merge
  149 ~~~~~~~~~~~~~~~~~
  150 If only 1 tree is specified, 'git read-tree' operates as if the user did not
  151 specify `-m`, except that if the original index has an entry for a
  152 given pathname, and the contents of the path match with the tree
  153 being read, the stat info from the index is used. (In other words, the
  154 index's stat()s take precedence over the merged tree's).
  156 That means that if you do a `git read-tree -m <newtree>` followed by a
  157 `git checkout-index -f -u -a`, the 'git checkout-index' only checks out
  158 the stuff that really changed.
  160 This is used to avoid unnecessary false hits when 'git diff-files' is
  161 run after 'git read-tree'.
  164 Two Tree Merge
  165 ~~~~~~~~~~~~~~
  167 Typically, this is invoked as `git read-tree -m $H $M`, where $H
  168 is the head commit of the current repository, and $M is the head
  169 of a foreign tree, which is simply ahead of $H (i.e. we are in a
  170 fast-forward situation).
  172 When two trees are specified, the user is telling 'git read-tree'
  173 the following:
  175      1. The current index and work tree is derived from $H, but
  176 	the user may have local changes in them since $H.
  178      2. The user wants to fast-forward to $M.
  180 In this case, the `git read-tree -m $H $M` command makes sure
  181 that no local change is lost as the result of this "merge".
  182 Here are the "carry forward" rules, where "I" denotes the index,
  183 "clean" means that index and work tree coincide, and "exists"/"nothing"
  184 refer to the presence of a path in the specified commit:
  186 ....
  187 	I                   H        M        Result
  188        -------------------------------------------------------
  189      0  nothing             nothing  nothing  (does not happen)
  190      1  nothing             nothing  exists   use M
  191      2  nothing             exists   nothing  remove path from index
  192      3  nothing             exists   exists,  use M if "initial checkout",
  193 				     H == M   keep index otherwise
  194 				     exists,  fail
  195 				     H != M
  197         clean I==H  I==M
  198        ------------------
  199      4  yes   N/A   N/A     nothing  nothing  keep index
  200      5  no    N/A   N/A     nothing  nothing  keep index
  202      6  yes   N/A   yes     nothing  exists   keep index
  203      7  no    N/A   yes     nothing  exists   keep index
  204      8  yes   N/A   no      nothing  exists   fail
  205      9  no    N/A   no      nothing  exists   fail
  207      10 yes   yes   N/A     exists   nothing  remove path from index
  208      11 no    yes   N/A     exists   nothing  fail
  209      12 yes   no    N/A     exists   nothing  fail
  210      13 no    no    N/A     exists   nothing  fail
  212 	clean (H==M)
  213        ------
  214      14 yes                 exists   exists   keep index
  215      15 no                  exists   exists   keep index
  217         clean I==H  I==M (H!=M)
  218        ------------------
  219      16 yes   no    no      exists   exists   fail
  220      17 no    no    no      exists   exists   fail
  221      18 yes   no    yes     exists   exists   keep index
  222      19 no    no    yes     exists   exists   keep index
  223      20 yes   yes   no      exists   exists   use M
  224      21 no    yes   no      exists   exists   fail
  225 ....
  227 In all "keep index" cases, the index entry stays as in the
  228 original index file.  If the entry is not up to date,
  229 'git read-tree' keeps the copy in the work tree intact when
  230 operating under the -u flag.
  232 When this form of 'git read-tree' returns successfully, you can
  233 see which of the "local changes" that you made were carried forward by running
  234 `git diff-index --cached $M`.  Note that this does not
  235 necessarily match what `git diff-index --cached $H` would have
  236 produced before such a two tree merge.  This is because of cases
  237 18 and 19 --- if you already had the changes in $M (e.g. maybe
  238 you picked it up via e-mail in a patch form), `git diff-index
  239 --cached $H` would have told you about the change before this
  240 merge, but it would not show in `git diff-index --cached $M`
  241 output after the two-tree merge.
  243 Case 3 is slightly tricky and needs explanation.  The result from this
  244 rule logically should be to remove the path if the user staged the removal
  245 of the path and then switching to a new branch.  That however will prevent
  246 the initial checkout from happening, so the rule is modified to use M (new
  247 tree) only when the content of the index is empty.  Otherwise the removal
  248 of the path is kept as long as $H and $M are the same.
  250 3-Way Merge
  251 ~~~~~~~~~~~
  252 Each "index" entry has two bits worth of "stage" state. stage 0 is the
  253 normal one, and is the only one you'd see in any kind of normal use.
  255 However, when you do 'git read-tree' with three trees, the "stage"
  256 starts out at 1.
  258 This means that you can do
  260 ----------------
  261 $ git read-tree -m <tree1> <tree2> <tree3>
  262 ----------------
  264 and you will end up with an index with all of the <tree1> entries in
  265 "stage1", all of the <tree2> entries in "stage2" and all of the
  266 <tree3> entries in "stage3".  When performing a merge of another
  267 branch into the current branch, we use the common ancestor tree
  268 as <tree1>, the current branch head as <tree2>, and the other
  269 branch head as <tree3>.
  271 Furthermore, 'git read-tree' has special-case logic that says: if you see
  272 a file that matches in all respects in the following states, it
  273 "collapses" back to "stage0":
  275    - stage 2 and 3 are the same; take one or the other (it makes no
  276      difference - the same work has been done on our branch in
  277      stage 2 and their branch in stage 3)
  279    - stage 1 and stage 2 are the same and stage 3 is different; take
  280      stage 3 (our branch in stage 2 did not do anything since the
  281      ancestor in stage 1 while their branch in stage 3 worked on
  282      it)
  284    - stage 1 and stage 3 are the same and stage 2 is different take
  285      stage 2 (we did something while they did nothing)
  287 The 'git write-tree' command refuses to write a nonsensical tree, and it
  288 will complain about unmerged entries if it sees a single entry that is not
  289 stage 0.
  291 OK, this all sounds like a collection of totally nonsensical rules,
  292 but it's actually exactly what you want in order to do a fast
  293 merge. The different stages represent the "result tree" (stage 0, aka
  294 "merged"), the original tree (stage 1, aka "orig"), and the two trees
  295 you are trying to merge (stage 2 and 3 respectively).
  297 The order of stages 1, 2 and 3 (hence the order of three
  298 <tree-ish> command-line arguments) are significant when you
  299 start a 3-way merge with an index file that is already
  300 populated.  Here is an outline of how the algorithm works:
  302 - if a file exists in identical format in all three trees, it will
  303   automatically collapse to "merged" state by 'git read-tree'.
  305 - a file that has _any_ difference what-so-ever in the three trees
  306   will stay as separate entries in the index. It's up to "porcelain
  307   policy" to determine how to remove the non-0 stages, and insert a
  308   merged version.
  310 - the index file saves and restores with all this information, so you
  311   can merge things incrementally, but as long as it has entries in
  312   stages 1/2/3 (i.e., "unmerged entries") you can't write the result. So
  313   now the merge algorithm ends up being really simple:
  315   * you walk the index in order, and ignore all entries of stage 0,
  316     since they've already been done.
  318   * if you find a "stage1", but no matching "stage2" or "stage3", you
  319     know it's been removed from both trees (it only existed in the
  320     original tree), and you remove that entry.
  322   * if you find a matching "stage2" and "stage3" tree, you remove one
  323     of them, and turn the other into a "stage0" entry. Remove any
  324     matching "stage1" entry if it exists too.  .. all the normal
  325     trivial rules ..
  327 You would normally use 'git merge-index' with supplied
  328 'git merge-one-file' to do this last step.  The script updates
  329 the files in the working tree as it merges each path and at the
  330 end of a successful merge.
  332 When you start a 3-way merge with an index file that is already
  333 populated, it is assumed that it represents the state of the
  334 files in your work tree, and you can even have files with
  335 changes unrecorded in the index file.  It is further assumed
  336 that this state is "derived" from the stage 2 tree.  The 3-way
  337 merge refuses to run if it finds an entry in the original index
  338 file that does not match stage 2.
  340 This is done to prevent you from losing your work-in-progress
  341 changes, and mixing your random changes in an unrelated merge
  342 commit.  To illustrate, suppose you start from what has been
  343 committed last to your repository:
  345 ----------------
  346 $ JC=`git rev-parse --verify "HEAD^0"`
  347 $ git checkout-index -f -u -a $JC
  348 ----------------
  350 You do random edits, without running 'git update-index'.  And then
  351 you notice that the tip of your "upstream" tree has advanced
  352 since you pulled from him:
  354 ----------------
  355 $ git fetch git://.... linus
  356 $ LT=`git rev-parse FETCH_HEAD`
  357 ----------------
  359 Your work tree is still based on your HEAD ($JC), but you have
  360 some edits since.  Three-way merge makes sure that you have not
  361 added or modified index entries since $JC, and if you haven't,
  362 then does the right thing.  So with the following sequence:
  364 ----------------
  365 $ git read-tree -m -u `git merge-base $JC $LT` $JC $LT
  366 $ git merge-index git-merge-one-file -a
  367 $ echo "Merge with Linus" | \
  368   git commit-tree `git write-tree` -p $JC -p $LT
  369 ----------------
  371 what you would commit is a pure merge between $JC and $LT without
  372 your work-in-progress changes, and your work tree would be
  373 updated to the result of the merge.
  375 However, if you have local changes in the working tree that
  376 would be overwritten by this merge, 'git read-tree' will refuse
  377 to run to prevent your changes from being lost.
  379 In other words, there is no need to worry about what exists only
  380 in the working tree.  When you have local changes in a part of
  381 the project that is not involved in the merge, your changes do
  382 not interfere with the merge, and are kept intact.  When they
  383 *do* interfere, the merge does not even start ('git read-tree'
  384 complains loudly and fails without modifying anything).  In such
  385 a case, you can simply continue doing what you were in the
  386 middle of doing, and when your working tree is ready (i.e. you
  387 have finished your work-in-progress), attempt the merge again.
  391 ---------------
  393 "Sparse checkout" allows populating the working directory sparsely.
  394 It uses the skip-worktree bit (see linkgit:git-update-index[1]) to tell
  395 Git whether a file in the working directory is worth looking at.
  397 'git read-tree' and other merge-based commands ('git merge', 'git
  398 checkout'...) can help maintaining the skip-worktree bitmap and working
  399 directory update. `$GIT_DIR/info/sparse-checkout` is used to
  400 define the skip-worktree reference bitmap. When 'git read-tree' needs
  401 to update the working directory, it resets the skip-worktree bit in the index
  402 based on this file, which uses the same syntax as .gitignore files.
  403 If an entry matches a pattern in this file, skip-worktree will not be
  404 set on that entry. Otherwise, skip-worktree will be set.
  406 Then it compares the new skip-worktree value with the previous one. If
  407 skip-worktree turns from set to unset, it will add the corresponding
  408 file back. If it turns from unset to set, that file will be removed.
  410 While `$GIT_DIR/info/sparse-checkout` is usually used to specify what
  411 files are in, you can also specify what files are _not_ in, using
  412 negate patterns. For example, to remove the file `unwanted`:
  414 ----------------
  415 /*
  416 !unwanted
  417 ----------------
  419 Another tricky thing is fully repopulating the working directory when you
  420 no longer want sparse checkout. You cannot just disable "sparse
  421 checkout" because skip-worktree bits are still in the index and your working
  422 directory is still sparsely populated. You should re-populate the working
  423 directory with the `$GIT_DIR/info/sparse-checkout` file content as
  424 follows:
  426 ----------------
  427 /*
  428 ----------------
  430 Then you can disable sparse checkout. Sparse checkout support in 'git
  431 read-tree' and similar commands is disabled by default. You need to
  432 turn `core.sparseCheckout` on in order to have sparse checkout
  433 support.
  436 SEE ALSO
  437 --------
  438 linkgit:git-write-tree[1]; linkgit:git-ls-files[1];
  439 linkgit:gitignore[5]
  441 GIT
  442 ---
  443 Part of the linkgit:git[1] suite