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>]])
13

DESCRIPTION

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

OPTIONS

28       -m
29           Perform a merge, not just a read. The command will refuse to run if
30           your index file has unmerged entries, indicating that you have not
31           finished previous merge you started.
32
33       --reset
34           Same as -m, except that unmerged entries are discarded instead of
35           failing. When used with -u, updates leading to loss of working tree
36           changes will not abort the 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       --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 active
109           submodules according to the commit recorded in the superproject by
110           calling read-tree recursively, also setting the submodules' HEAD to
111           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       -q, --quiet
121           Quiet, suppress feedback messages.
122
123       <tree-ish#>
124           The id of the tree object(s) to be read/merged.
125

MERGING

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

SPARSE CHECKOUT

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

SEE ALSO

390       git-write-tree(1); git-ls-files(1); gitignore(5); git-sparse-
391       checkout(1);
392

GIT

394       Part of the git(1) suite
395
396
397
398Git 2.33.1                        2021-10-12                  GIT-READ-TREE(1)
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