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 | -i]] [--index-output=<file>] [--no-sparse-checkout]
11                       (--empty | <tree-ish1> [<tree-ish2> [<tree-ish3>]])
12

DESCRIPTION

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

OPTIONS

27       -m
28           Perform a merge, not just a read. The command will refuse to run if
29           your index file has unmerged entries, indicating that you have not
30           finished previous merge you started.
31
32       --reset
33           Same as -m, except that unmerged entries are discarded instead of
34           failing. When used with -u, updates leading to loss of working tree
35           changes or untracked files or directories will not abort the
36           operation.
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       --index-output=<file>
85           Instead of writing the results out to $GIT_INDEX_FILE, write the
86           resulting index in the named file. While the command is operating,
87           the original index file is locked with the same mechanism as usual.
88           The file must allow to be rename(2)ed into from a temporary file
89           that is created next to the usual index file; typically this means
90           it needs to be on the same filesystem as the index file itself, and
91           you need write permission to the directories the index file and
92           index output file are located in.
93
94       --[no-]recurse-submodules
95           Using --recurse-submodules will update the content of all active
96           submodules according to the commit recorded in the superproject by
97           calling read-tree recursively, also setting the submodules' HEAD to
98           be detached at that commit.
99
100       --no-sparse-checkout
101           Disable sparse checkout support even if core.sparseCheckout is
102           true.
103
104       --empty
105           Instead of reading tree object(s) into the index, just empty it.
106
107       -q, --quiet
108           Quiet, suppress feedback messages.
109
110       <tree-ish#>
111           The id of the tree object(s) to be read/merged.
112

MERGING

114       If -m is specified, git read-tree can perform 3 kinds of merge, a
115       single tree merge if only 1 tree is given, a fast-forward merge with 2
116       trees, or a 3-way merge if 3 or more trees are provided.
117
118   Single Tree Merge
119       If only 1 tree is specified, git read-tree operates as if the user did
120       not specify -m, except that if the original index has an entry for a
121       given pathname, and the contents of the path match with the tree being
122       read, the stat info from the index is used. (In other words, the
123       index’s stat()s take precedence over the merged tree’s).
124
125       That means that if you do a git read-tree -m <newtree> followed by a
126       git checkout-index -f -u -a, the git checkout-index only checks out the
127       stuff that really changed.
128
129       This is used to avoid unnecessary false hits when git diff-files is run
130       after git read-tree.
131
132   Two Tree Merge
133       Typically, this is invoked as git read-tree -m $H $M, where $H is the
134       head commit of the current repository, and $M is the head of a foreign
135       tree, which is simply ahead of $H (i.e. we are in a fast-forward
136       situation).
137
138       When two trees are specified, the user is telling git read-tree the
139       following:
140
141        1. The current index and work tree is derived from $H, but the user
142           may have local changes in them since $H.
143
144        2. The user wants to fast-forward to $M.
145
146       In this case, the git read-tree -m $H $M command makes sure that no
147       local change is lost as the result of this "merge". Here are the "carry
148       forward" rules, where "I" denotes the index, "clean" means that index
149       and work tree coincide, and "exists"/"nothing" refer to the presence of
150       a path in the specified commit:
151
152                   I                   H        M        Result
153                  -------------------------------------------------------
154                0  nothing             nothing  nothing  (does not happen)
155                1  nothing             nothing  exists   use M
156                2  nothing             exists   nothing  remove path from index
157                3  nothing             exists   exists,  use M if "initial checkout",
158                                                H == M   keep index otherwise
159                                                exists,  fail
160                                                H != M
161
162                   clean I==H  I==M
163                  ------------------
164                4  yes   N/A   N/A     nothing  nothing  keep index
165                5  no    N/A   N/A     nothing  nothing  keep index
166
167                6  yes   N/A   yes     nothing  exists   keep index
168                7  no    N/A   yes     nothing  exists   keep index
169                8  yes   N/A   no      nothing  exists   fail
170                9  no    N/A   no      nothing  exists   fail
171
172                10 yes   yes   N/A     exists   nothing  remove path from index
173                11 no    yes   N/A     exists   nothing  fail
174                12 yes   no    N/A     exists   nothing  fail
175                13 no    no    N/A     exists   nothing  fail
176
177                   clean (H==M)
178                  ------
179                14 yes                 exists   exists   keep index
180                15 no                  exists   exists   keep index
181
182                   clean I==H  I==M (H!=M)
183                  ------------------
184                16 yes   no    no      exists   exists   fail
185                17 no    no    no      exists   exists   fail
186                18 yes   no    yes     exists   exists   keep index
187                19 no    no    yes     exists   exists   keep index
188                20 yes   yes   no      exists   exists   use M
189                21 no    yes   no      exists   exists   fail
190
191       In all "keep index" cases, the index entry stays as in the original
192       index file. If the entry is not up to date, git read-tree keeps the
193       copy in the work tree intact when operating under the -u flag.
194
195       When this form of git read-tree returns successfully, you can see which
196       of the "local changes" that you made were carried forward by running
197       git diff-index --cached $M. Note that this does not necessarily match
198       what git diff-index --cached $H would have produced before such a two
199       tree merge. This is because of cases 18 and 19 --- if you already had
200       the changes in $M (e.g. maybe you picked it up via e-mail in a patch
201       form), git diff-index --cached $H would have told you about the change
202       before this merge, but it would not show in git diff-index --cached $M
203       output after the two-tree merge.
204
205       Case 3 is slightly tricky and needs explanation. The result from this
206       rule logically should be to remove the path if the user staged the
207       removal of the path and then switching to a new branch. That however
208       will prevent the initial checkout from happening, so the rule is
209       modified to use M (new tree) only when the content of the index is
210       empty. Otherwise the removal of the path is kept as long as $H and $M
211       are the same.
212
213   3-Way Merge
214       Each "index" entry has two bits worth of "stage" state. stage 0 is the
215       normal one, and is the only one you’d see in any kind of normal use.
216
217       However, when you do git read-tree with three trees, the "stage" starts
218       out at 1.
219
220       This means that you can do
221
222           $ git read-tree -m <tree1> <tree2> <tree3>
223
224       and you will end up with an index with all of the <tree1> entries in
225       "stage1", all of the <tree2> entries in "stage2" and all of the <tree3>
226       entries in "stage3". When performing a merge of another branch into the
227       current branch, we use the common ancestor tree as <tree1>, the current
228       branch head as <tree2>, and the other branch head as <tree3>.
229
230       Furthermore, git read-tree has special-case logic that says: if you see
231       a file that matches in all respects in the following states, it
232       "collapses" back to "stage0":
233
234       •   stage 2 and 3 are the same; take one or the other (it makes no
235           difference - the same work has been done on our branch in stage 2
236           and their branch in stage 3)
237
238       •   stage 1 and stage 2 are the same and stage 3 is different; take
239           stage 3 (our branch in stage 2 did not do anything since the
240           ancestor in stage 1 while their branch in stage 3 worked on it)
241
242       •   stage 1 and stage 3 are the same and stage 2 is different take
243           stage 2 (we did something while they did nothing)
244
245       The git write-tree command refuses to write a nonsensical tree, and it
246       will complain about unmerged entries if it sees a single entry that is
247       not stage 0.
248
249       OK, this all sounds like a collection of totally nonsensical rules, but
250       it’s actually exactly what you want in order to do a fast merge. The
251       different stages represent the "result tree" (stage 0, aka "merged"),
252       the original tree (stage 1, aka "orig"), and the two trees you are
253       trying to merge (stage 2 and 3 respectively).
254
255       The order of stages 1, 2 and 3 (hence the order of three <tree-ish>
256       command-line arguments) are significant when you start a 3-way merge
257       with an index file that is already populated. Here is an outline of how
258       the algorithm works:
259
260       •   if a file exists in identical format in all three trees, it will
261           automatically collapse to "merged" state by git read-tree.
262
263       •   a file that has any difference what-so-ever in the three trees will
264           stay as separate entries in the index. It’s up to "porcelain
265           policy" to determine how to remove the non-0 stages, and insert a
266           merged version.
267
268       •   the index file saves and restores with all this information, so you
269           can merge things incrementally, but as long as it has entries in
270           stages 1/2/3 (i.e., "unmerged entries") you can’t write the result.
271           So now the merge algorithm ends up being really simple:
272
273           •   you walk the index in order, and ignore all entries of stage 0,
274               since they’ve already been done.
275
276           •   if you find a "stage1", but no matching "stage2" or "stage3",
277               you know it’s been removed from both trees (it only existed in
278               the original tree), and you remove that entry.
279
280           •   if you find a matching "stage2" and "stage3" tree, you remove
281               one of them, and turn the other into a "stage0" entry. Remove
282               any matching "stage1" entry if it exists too. .. all the normal
283               trivial rules ..
284
285       You would normally use git merge-index with supplied git merge-one-file
286       to do this last step. The script updates the files in the working tree
287       as it merges each path and at the end of a successful merge.
288
289       When you start a 3-way merge with an index file that is already
290       populated, it is assumed that it represents the state of the files in
291       your work tree, and you can even have files with changes unrecorded in
292       the index file. It is further assumed that this state is "derived" from
293       the stage 2 tree. The 3-way merge refuses to run if it finds an entry
294       in the original index file that does not match stage 2.
295
296       This is done to prevent you from losing your work-in-progress changes,
297       and mixing your random changes in an unrelated merge commit. To
298       illustrate, suppose you start from what has been committed last to your
299       repository:
300
301           $ JC=`git rev-parse --verify "HEAD^0"`
302           $ git checkout-index -f -u -a $JC
303
304       You do random edits, without running git update-index. And then you
305       notice that the tip of your "upstream" tree has advanced since you
306       pulled from him:
307
308           $ git fetch git://.... linus
309           $ LT=`git rev-parse FETCH_HEAD`
310
311       Your work tree is still based on your HEAD ($JC), but you have some
312       edits since. Three-way merge makes sure that you have not added or
313       modified index entries since $JC, and if you haven’t, then does the
314       right thing. So with the following sequence:
315
316           $ git read-tree -m -u `git merge-base $JC $LT` $JC $LT
317           $ git merge-index git-merge-one-file -a
318           $ echo "Merge with Linus" | \
319             git commit-tree `git write-tree` -p $JC -p $LT
320
321       what you would commit is a pure merge between $JC and $LT without your
322       work-in-progress changes, and your work tree would be updated to the
323       result of the merge.
324
325       However, if you have local changes in the working tree that would be
326       overwritten by this merge, git read-tree will refuse to run to prevent
327       your changes from being lost.
328
329       In other words, there is no need to worry about what exists only in the
330       working tree. When you have local changes in a part of the project that
331       is not involved in the merge, your changes do not interfere with the
332       merge, and are kept intact. When they do interfere, the merge does not
333       even start (git read-tree complains loudly and fails without modifying
334       anything). In such a case, you can simply continue doing what you were
335       in the middle of doing, and when your working tree is ready (i.e. you
336       have finished your work-in-progress), attempt the merge again.
337

SPARSE CHECKOUT

339       Note: The update-index and read-tree primitives for supporting the
340       skip-worktree bit predated the introduction of git-sparse-checkout(1).
341       Users are encouraged to use sparse-checkout in preference to these
342       low-level primitives.
343
344       "Sparse checkout" allows populating the working directory sparsely. It
345       uses the skip-worktree bit (see git-update-index(1)) to tell Git
346       whether a file in the working directory is worth looking at.
347
348       git read-tree and other merge-based commands (git merge, git
349       checkout...) can help maintaining the skip-worktree bitmap and working
350       directory update. $GIT_DIR/info/sparse-checkout is used to define the
351       skip-worktree reference bitmap. When git read-tree needs to update the
352       working directory, it resets the skip-worktree bit in the index based
353       on this file, which uses the same syntax as .gitignore files. If an
354       entry matches a pattern in this file, or the entry corresponds to a
355       file present in the working tree, then skip-worktree will not be set on
356       that entry. Otherwise, skip-worktree will be set.
357
358       Then it compares the new skip-worktree value with the previous one. If
359       skip-worktree turns from set to unset, it will add the corresponding
360       file back. If it turns from unset to set, that file will be removed.
361
362       While $GIT_DIR/info/sparse-checkout is usually used to specify what
363       files are in, you can also specify what files are not in, using negate
364       patterns. For example, to remove the file unwanted:
365
366           /*
367           !unwanted
368
369       Another tricky thing is fully repopulating the working directory when
370       you no longer want sparse checkout. You cannot just disable "sparse
371       checkout" because skip-worktree bits are still in the index and your
372       working directory is still sparsely populated. You should re-populate
373       the working directory with the $GIT_DIR/info/sparse-checkout file
374       content as follows:
375
376           /*
377
378       Then you can disable sparse checkout. Sparse checkout support in git
379       read-tree and similar commands is disabled by default. You need to turn
380       core.sparseCheckout on in order to have sparse checkout support.
381

SEE ALSO

383       git-write-tree(1), git-ls-files(1), gitignore(5), git-sparse-
384       checkout(1)
385

GIT

387       Part of the git(1) suite
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389
390
391Git 2.36.1                        2022-05-05                  GIT-READ-TREE(1)
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