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

6       git-read-tree - Reads tree information into the index
7

SYNOPSIS

9       git read-tree [[-m [--trivial] [--aggressive] | --reset | --prefix=<prefix>]
10                       [-u [--exclude-per-directory=<gitignore>] | -i]]
11                       [--index-output=<file>] [--no-sparse-checkout]
12                       (--empty | <tree-ish1> [<tree-ish2> [<tree-ish3>]])
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14

DESCRIPTION

16       Reads the tree information given by <tree-ish> into the index, but does
17       not actually update any of the files it "caches". (see: git-checkout-
18       index(1))
19
20       Optionally, it can merge a tree into the index, perform a fast-forward
21       (i.e. 2-way) merge, or a 3-way merge, with the -m flag. When used with
22       -m, the -u flag causes it to also update the files in the work tree
23       with the result of the merge.
24
25       Trivial merges are done by git read-tree itself. Only conflicting paths
26       will be in unmerged state when git read-tree returns.
27

OPTIONS

29       -m
30           Perform a merge, not just a read. The command will refuse to run if
31           your index file has unmerged entries, indicating that you have not
32           finished previous merge you started.
33
34       --reset
35           Same as -m, except that unmerged entries are discarded instead of
36           failing.
37
38       -u
39           After a successful merge, update the files in the work tree with
40           the result of the merge.
41
42       -i
43           Usually a merge requires the index file as well as the files in the
44           working tree to be up to date with the current head commit, in
45           order not to lose local changes. This flag disables the check with
46           the working tree and is meant to be used when creating a merge of
47           trees that are not directly related to the current working tree
48           status into a temporary index file.
49
50       -n, --dry-run
51           Check if the command would error out, without updating the index or
52           the files in the working tree for real.
53
54       -v
55           Show the progress of checking files out.
56
57       --trivial
58           Restrict three-way merge by git read-tree to happen only if there
59           is no file-level merging required, instead of resolving merge for
60           trivial cases and leaving conflicting files unresolved in the
61           index.
62
63       --aggressive
64           Usually a three-way merge by git read-tree resolves the merge for
65           really trivial cases and leaves other cases unresolved in the
66           index, so that porcelains can implement different merge policies.
67           This flag makes the command resolve a few more cases internally:
68
69           ·   when one side removes a path and the other side leaves the path
70               unmodified. The resolution is to remove that path.
71
72           ·   when both sides remove a path. The resolution is to remove that
73               path.
74
75           ·   when both sides add a path identically. The resolution is to
76               add that path.
77
78       --prefix=<prefix>
79           Keep the current index contents, and read the contents of the named
80           tree-ish under the directory at <prefix>. The command will refuse
81           to overwrite entries that already existed in the original index
82           file.
83
84       --exclude-per-directory=<gitignore>
85           When running the command with -u and -m options, the merge result
86           may need to overwrite paths that are not tracked in the current
87           branch. The command usually refuses to proceed with the merge to
88           avoid losing such a path. However this safety valve sometimes gets
89           in the way. For example, it often happens that the other branch
90           added a file that used to be a generated file in your branch, and
91           the safety valve triggers when you try to switch to that branch
92           after you ran make but before running make clean to remove the
93           generated file. This option tells the command to read per-directory
94           exclude file (usually .gitignore) and allows such an untracked but
95           explicitly ignored file to be overwritten.
96
97       --index-output=<file>
98           Instead of writing the results out to $GIT_INDEX_FILE, write the
99           resulting index in the named file. While the command is operating,
100           the original index file is locked with the same mechanism as usual.
101           The file must allow to be rename(2)ed into from a temporary file
102           that is created next to the usual index file; typically this means
103           it needs to be on the same filesystem as the index file itself, and
104           you need write permission to the directories the index file and
105           index output file are located in.
106
107       --[no-]recurse-submodules
108           Using --recurse-submodules will update the content of all
109           initialized submodules according to the commit recorded in the
110           superproject by calling read-tree recursively, also setting the
111           submodules HEAD to be detached at that commit.
112
113       --no-sparse-checkout
114           Disable sparse checkout support even if core.sparseCheckout is
115           true.
116
117       --empty
118           Instead of reading tree object(s) into the index, just empty it.
119
120       <tree-ish#>
121           The id of the tree object(s) to be read/merged.
122

MERGING

124       If -m is specified, git read-tree can perform 3 kinds of merge, a
125       single tree merge if only 1 tree is given, a fast-forward merge with 2
126       trees, or a 3-way merge if 3 or more trees are provided.
127
128   Single Tree Merge
129       If only 1 tree is specified, git read-tree operates as if the user did
130       not specify -m, except that if the original index has an entry for a
131       given pathname, and the contents of the path match with the tree being
132       read, the stat info from the index is used. (In other words, the
133       index’s stat()s take precedence over the merged tree’s).
134
135       That means that if you do a git read-tree -m <newtree> followed by a
136       git checkout-index -f -u -a, the git checkout-index only checks out the
137       stuff that really changed.
138
139       This is used to avoid unnecessary false hits when git diff-files is run
140       after git read-tree.
141
142   Two Tree Merge
143       Typically, this is invoked as git read-tree -m $H $M, where $H is the
144       head commit of the current repository, and $M is the head of a foreign
145       tree, which is simply ahead of $H (i.e. we are in a fast-forward
146       situation).
147
148       When two trees are specified, the user is telling git read-tree the
149       following:
150
151        1. The current index and work tree is derived from $H, but the user
152           may have local changes in them since $H.
153
154        2. The user wants to fast-forward to $M.
155
156       In this case, the git read-tree -m $H $M command makes sure that no
157       local change is lost as the result of this "merge". Here are the "carry
158       forward" rules, where "I" denotes the index, "clean" means that index
159       and work tree coincide, and "exists"/"nothing" refer to the presence of
160       a path in the specified commit:
161
162                   I                   H        M        Result
163                  -------------------------------------------------------
164                0  nothing             nothing  nothing  (does not happen)
165                1  nothing             nothing  exists   use M
166                2  nothing             exists   nothing  remove path from index
167                3  nothing             exists   exists,  use M if "initial checkout",
168                                                H == M   keep index otherwise
169                                                exists,  fail
170                                                H != M
171
172                   clean I==H  I==M
173                  ------------------
174                4  yes   N/A   N/A     nothing  nothing  keep index
175                5  no    N/A   N/A     nothing  nothing  keep index
176
177                6  yes   N/A   yes     nothing  exists   keep index
178                7  no    N/A   yes     nothing  exists   keep index
179                8  yes   N/A   no      nothing  exists   fail
180                9  no    N/A   no      nothing  exists   fail
181
182                10 yes   yes   N/A     exists   nothing  remove path from index
183                11 no    yes   N/A     exists   nothing  fail
184                12 yes   no    N/A     exists   nothing  fail
185                13 no    no    N/A     exists   nothing  fail
186
187                   clean (H==M)
188                  ------
189                14 yes                 exists   exists   keep index
190                15 no                  exists   exists   keep index
191
192                   clean I==H  I==M (H!=M)
193                  ------------------
194                16 yes   no    no      exists   exists   fail
195                17 no    no    no      exists   exists   fail
196                18 yes   no    yes     exists   exists   keep index
197                19 no    no    yes     exists   exists   keep index
198                20 yes   yes   no      exists   exists   use M
199                21 no    yes   no      exists   exists   fail
200
201       In all "keep index" cases, the index entry stays as in the original
202       index file. If the entry is not up to date, git read-tree keeps the
203       copy in the work tree intact when operating under the -u flag.
204
205       When this form of git read-tree returns successfully, you can see which
206       of the "local changes" that you made were carried forward by running
207       git diff-index --cached $M. Note that this does not necessarily match
208       what git diff-index --cached $H would have produced before such a two
209       tree merge. This is because of cases 18 and 19 --- if you already had
210       the changes in $M (e.g. maybe you picked it up via e-mail in a patch
211       form), git diff-index --cached $H would have told you about the change
212       before this merge, but it would not show in git diff-index --cached $M
213       output after the two-tree merge.
214
215       Case 3 is slightly tricky and needs explanation. The result from this
216       rule logically should be to remove the path if the user staged the
217       removal of the path and then switching to a new branch. That however
218       will prevent the initial checkout from happening, so the rule is
219       modified to use M (new tree) only when the content of the index is
220       empty. Otherwise the removal of the path is kept as long as $H and $M
221       are the same.
222
223   3-Way Merge
224       Each "index" entry has two bits worth of "stage" state. stage 0 is the
225       normal one, and is the only one you’d see in any kind of normal use.
226
227       However, when you do git read-tree with three trees, the "stage" starts
228       out at 1.
229
230       This means that you can do
231
232           $ git read-tree -m <tree1> <tree2> <tree3>
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234
235       and you will end up with an index with all of the <tree1> entries in
236       "stage1", all of the <tree2> entries in "stage2" and all of the <tree3>
237       entries in "stage3". When performing a merge of another branch into the
238       current branch, we use the common ancestor tree as <tree1>, the current
239       branch head as <tree2>, and the other branch head as <tree3>.
240
241       Furthermore, git read-tree has special-case logic that says: if you see
242       a file that matches in all respects in the following states, it
243       "collapses" back to "stage0":
244
245       ·   stage 2 and 3 are the same; take one or the other (it makes no
246           difference - the same work has been done on our branch in stage 2
247           and their branch in stage 3)
248
249       ·   stage 1 and stage 2 are the same and stage 3 is different; take
250           stage 3 (our branch in stage 2 did not do anything since the
251           ancestor in stage 1 while their branch in stage 3 worked on it)
252
253       ·   stage 1 and stage 3 are the same and stage 2 is different take
254           stage 2 (we did something while they did nothing)
255
256       The git write-tree command refuses to write a nonsensical tree, and it
257       will complain about unmerged entries if it sees a single entry that is
258       not stage 0.
259
260       OK, this all sounds like a collection of totally nonsensical rules, but
261       it’s actually exactly what you want in order to do a fast merge. The
262       different stages represent the "result tree" (stage 0, aka "merged"),
263       the original tree (stage 1, aka "orig"), and the two trees you are
264       trying to merge (stage 2 and 3 respectively).
265
266       The order of stages 1, 2 and 3 (hence the order of three <tree-ish>
267       command-line arguments) are significant when you start a 3-way merge
268       with an index file that is already populated. Here is an outline of how
269       the algorithm works:
270
271       ·   if a file exists in identical format in all three trees, it will
272           automatically collapse to "merged" state by git read-tree.
273
274       ·   a file that has any difference what-so-ever in the three trees will
275           stay as separate entries in the index. It’s up to "porcelain
276           policy" to determine how to remove the non-0 stages, and insert a
277           merged version.
278
279       ·   the index file saves and restores with all this information, so you
280           can merge things incrementally, but as long as it has entries in
281           stages 1/2/3 (i.e., "unmerged entries") you can’t write the result.
282           So now the merge algorithm ends up being really simple:
283
284           ·   you walk the index in order, and ignore all entries of stage 0,
285               since they’ve already been done.
286
287           ·   if you find a "stage1", but no matching "stage2" or "stage3",
288               you know it’s been removed from both trees (it only existed in
289               the original tree), and you remove that entry.
290
291           ·   if you find a matching "stage2" and "stage3" tree, you remove
292               one of them, and turn the other into a "stage0" entry. Remove
293               any matching "stage1" entry if it exists too. .. all the normal
294               trivial rules ..
295
296       You would normally use git merge-index with supplied git merge-one-file
297       to do this last step. The script updates the files in the working tree
298       as it merges each path and at the end of a successful merge.
299
300       When you start a 3-way merge with an index file that is already
301       populated, it is assumed that it represents the state of the files in
302       your work tree, and you can even have files with changes unrecorded in
303       the index file. It is further assumed that this state is "derived" from
304       the stage 2 tree. The 3-way merge refuses to run if it finds an entry
305       in the original index file that does not match stage 2.
306
307       This is done to prevent you from losing your work-in-progress changes,
308       and mixing your random changes in an unrelated merge commit. To
309       illustrate, suppose you start from what has been committed last to your
310       repository:
311
312           $ JC=`git rev-parse --verify "HEAD^0"`
313           $ git checkout-index -f -u -a $JC
314
315
316       You do random edits, without running git update-index. And then you
317       notice that the tip of your "upstream" tree has advanced since you
318       pulled from him:
319
320           $ git fetch git://.... linus
321           $ LT=`git rev-parse FETCH_HEAD`
322
323
324       Your work tree is still based on your HEAD ($JC), but you have some
325       edits since. Three-way merge makes sure that you have not added or
326       modified index entries since $JC, and if you haven’t, then does the
327       right thing. So with the following sequence:
328
329           $ git read-tree -m -u `git merge-base $JC $LT` $JC $LT
330           $ git merge-index git-merge-one-file -a
331           $ echo "Merge with Linus" | \
332             git commit-tree `git write-tree` -p $JC -p $LT
333
334
335       what you would commit is a pure merge between $JC and $LT without your
336       work-in-progress changes, and your work tree would be updated to the
337       result of the merge.
338
339       However, if you have local changes in the working tree that would be
340       overwritten by this merge, git read-tree will refuse to run to prevent
341       your changes from being lost.
342
343       In other words, there is no need to worry about what exists only in the
344       working tree. When you have local changes in a part of the project that
345       is not involved in the merge, your changes do not interfere with the
346       merge, and are kept intact. When they do interfere, the merge does not
347       even start (git read-tree complains loudly and fails without modifying
348       anything). In such a case, you can simply continue doing what you were
349       in the middle of doing, and when your working tree is ready (i.e. you
350       have finished your work-in-progress), attempt the merge again.
351

SPARSE CHECKOUT

353       "Sparse checkout" allows populating the working directory sparsely. It
354       uses the skip-worktree bit (see git-update-index(1)) to tell Git
355       whether a file in the working directory is worth looking at.
356
357       git read-tree and other merge-based commands (git merge, git
358       checkout...) can help maintaining the skip-worktree bitmap and working
359       directory update. $GIT_DIR/info/sparse-checkout is used to define the
360       skip-worktree reference bitmap. When git read-tree needs to update the
361       working directory, it resets the skip-worktree bit in the index based
362       on this file, which uses the same syntax as .gitignore files. If an
363       entry matches a pattern in this file, skip-worktree will not be set on
364       that entry. Otherwise, skip-worktree will be set.
365
366       Then it compares the new skip-worktree value with the previous one. If
367       skip-worktree turns from set to unset, it will add the corresponding
368       file back. If it turns from unset to set, that file will be removed.
369
370       While $GIT_DIR/info/sparse-checkout is usually used to specify what
371       files are in, you can also specify what files are not in, using negate
372       patterns. For example, to remove the file unwanted:
373
374           /*
375           !unwanted
376
377
378       Another tricky thing is fully repopulating the working directory when
379       you no longer want sparse checkout. You cannot just disable "sparse
380       checkout" because skip-worktree bits are still in the index and your
381       working directory is still sparsely populated. You should re-populate
382       the working directory with the $GIT_DIR/info/sparse-checkout file
383       content as follows:
384
385           /*
386
387
388       Then you can disable sparse checkout. Sparse checkout support in git
389       read-tree and similar commands is disabled by default. You need to turn
390       core.sparseCheckout on in order to have sparse checkout support.
391

SEE ALSO

393       git-write-tree(1); git-ls-files(1); gitignore(5)
394

GIT

396       Part of the git(1) suite
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400Git 2.20.1                        12/15/2018                  GIT-READ-TREE(1)
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