1GIT-READ-TREE(1) Git Manual GIT-READ-TREE(1)
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6 git-read-tree - Reads tree information into the index
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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
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 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 Only trivial merges are done by git read-tree itself. Only conflicting
24 paths will be in an unmerged state when git read-tree returns.
25
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 a previous merge you started.
31 --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 -u
39 After a successful merge, update the files in the work tree with
40 the result of the merge.
41 -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 -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 -v
55 Show the progress of checking files out.
56 --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 --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 • when one side removes a path and the other side leaves the path
70 unmodified. The resolution is to remove that path.
71 • when both sides remove a path. The resolution is to remove that
73 path.
74 • when both sides add a path identically. The resolution is to
76 add that path.
77 --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 --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 --[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 --no-sparse-checkout
101 Disable sparse checkout support even if core.sparseCheckout is
102 true.
103 --empty
105 Instead of reading tree object(s) into the index, just empty it.
106 -q, --quiet
108 Quiet, suppress feedback messages.
109 <tree-ish#>
111 The id of the tree object(s) to be read/merged.
112
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 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 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 This is used to avoid unnecessary false hits when git diff-files is run
130 after git read-tree.
131 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 When two trees are specified, the user is telling git read-tree the
139 following:
140 1. The current index and work tree is derived from $H, but the user
142 may have local changes in them since $H.
143 2. The user wants to fast-forward to $M.
145 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 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 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 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 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 clean (H==M)
178 ------
179 14 yes exists exists keep index
180 15 no exists exists keep index
181 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 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 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 the
200 changes in $M (e.g. maybe you picked it up via e-mail in a patch form),
201 git diff-index --cached $H would have told you about the change before
202 this merge, but it would not show in git diff-index --cached $M output
203 after the two-tree merge.
204 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 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 However, when you do git read-tree with three trees, the "stage" starts
218 out at 1.
219 This means that you can do
221 $ git read-tree -m <tree1> <tree2> <tree3>
223 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 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 • 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 • 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 • 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 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 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 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 • if a file exists in identical format in all three trees, it will
261 automatically collapse to "merged" state by git read-tree.
262 • 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 • 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 • you walk the index in order, and ignore all entries of stage 0,
274 since they’ve already been done.
275 • 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 • 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 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 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 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 $ JC=`git rev-parse --verify "HEAD^0"`
302 $ git checkout-index -f -u -a $JC
303 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 $ git fetch git://.... linus
309 $ LT=`git rev-parse FETCH_HEAD`
310 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 $ 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 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 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 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
339 Note: The skip-worktree capabilities in git-update-index(1) and
340 read-tree predated the introduction of git-sparse-checkout(1). Users
341 are encouraged to use the sparse-checkout command in preference to
342 these plumbing commands for sparse-checkout/skip-worktree related
343 needs. However, the information below might be useful to users trying
344 to understand the pattern style used in non-cone mode of the
345 sparse-checkout command.
346 "Sparse checkout" allows populating the working directory sparsely. It
348 uses the skip-worktree bit (see git-update-index(1)) to tell Git
349 whether a file in the working directory is worth looking at.
350 git read-tree and other merge-based commands (git merge, git
352 checkout...) can help maintaining the skip-worktree bitmap and working
353 directory update. $GIT_DIR/info/sparse-checkout is used to define the
354 skip-worktree reference bitmap. When git read-tree needs to update the
355 working directory, it resets the skip-worktree bit in the index based
356 on this file, which uses the same syntax as .gitignore files. If an
357 entry matches a pattern in this file, or the entry corresponds to a
358 file present in the working tree, then skip-worktree will not be set on
359 that entry. Otherwise, skip-worktree will be set.
360 Then it compares the new skip-worktree value with the previous one. If
362 skip-worktree turns from set to unset, it will add the corresponding
363 file back. If it turns from unset to set, that file will be removed.
364 While $GIT_DIR/info/sparse-checkout is usually used to specify what
366 files are in, you can also specify what files are not in, using negate
367 patterns. For example, to remove the file unwanted:
368 /*
370 !unwanted
371 Another tricky thing is fully repopulating the working directory when
373 you no longer want sparse checkout. You cannot just disable "sparse
374 checkout" because skip-worktree bits are still in the index and your
375 working directory is still sparsely populated. You should re-populate
376 the working directory with the $GIT_DIR/info/sparse-checkout file
377 content as follows:
378 /*
380 Then you can disable sparse checkout. Sparse checkout support in git
382 read-tree and similar commands is disabled by default. You need to turn
383 core.sparseCheckout on in order to have sparse checkout support.
384
386 git-write-tree(1), git-ls-files(1), gitignore(5), git-sparse-
387 checkout(1)
388
390 Part of the git(1) suite
391Git 2.43.0 11/20/2023 GIT-READ-TREE(1)