1GIT-READ-TREE(1) Git Manual GIT-READ-TREE(1)
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6 git-read-tree - Reads tree information into the index
7
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
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 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 Trivial merges are done by git read-tree itself. Only conflicting paths
25 will be in unmerged state when git read-tree returns.
26
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 --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 -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 --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 --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 --[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 --no-sparse-checkout
114 Disable sparse checkout support even if core.sparseCheckout is
115 true.
116 --empty
118 Instead of reading tree object(s) into the index, just empty it.
119 -q, --quiet
121 Quiet, suppress feedback messages.
122 <tree-ish#>
124 The id of the tree object(s) to be read/merged.
125
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 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 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 This is used to avoid unnecessary false hits when git diff-files is run
143 after git read-tree.
144 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 When two trees are specified, the user is telling git read-tree the
152 following:
153 1. The current index and work tree is derived from $H, but the user
155 may have local changes in them since $H.
156 2. The user wants to fast-forward to $M.
158 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 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 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 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 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 clean (H==M)
191 ------
192 14 yes exists exists keep index
193 15 no exists exists keep index
194 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 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 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 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 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 However, when you do git read-tree with three trees, the "stage" starts
231 out at 1.
232 This means that you can do
234 $ git read-tree -m <tree1> <tree2> <tree3>
236 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 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 • 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 • 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 • 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 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 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 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 • if a file exists in identical format in all three trees, it will
274 automatically collapse to "merged" state by git read-tree.
275 • 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 • 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 • you walk the index in order, and ignore all entries of stage 0,
287 since they’ve already been done.
288 • 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 • 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 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 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 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 $ JC=`git rev-parse --verify "HEAD^0"`
315 $ git checkout-index -f -u -a $JC
316 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 $ git fetch git://.... linus
322 $ LT=`git rev-parse FETCH_HEAD`
323 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 $ 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 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 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 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
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 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 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 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 /*
374 !unwanted
375 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 /*
384 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
390 git-write-tree(1); git-ls-files(1); gitignore(5); git-sparse-
391 checkout(1);
392
394 Part of the git(1) suite
395Git 2.33.1 2021-10-12 GIT-READ-TREE(1)