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
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 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 Trivial merges are done by git read-tree itself. Only conflicting paths
26 will be in unmerged state when git read-tree returns.
27
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 --reset
35 Same as -m, except that unmerged entries are discarded instead of
36 failing. When used with -u, updates leading to loss of working tree
37 changes will not abort the operation.
38 -u
40 After a successful merge, update the files in the work tree with
41 the result of the merge.
42 -i
44 Usually a merge requires the index file as well as the files in the
45 working tree to be up to date with the current head commit, in
46 order not to lose local changes. This flag disables the check with
47 the working tree and is meant to be used when creating a merge of
48 trees that are not directly related to the current working tree
49 status into a temporary index file.
50 -n, --dry-run
52 Check if the command would error out, without updating the index or
53 the files in the working tree for real.
54 -v
56 Show the progress of checking files out.
57 --trivial
59 Restrict three-way merge by git read-tree to happen only if there
60 is no file-level merging required, instead of resolving merge for
61 trivial cases and leaving conflicting files unresolved in the
62 index.
63 --aggressive
65 Usually a three-way merge by git read-tree resolves the merge for
66 really trivial cases and leaves other cases unresolved in the
67 index, so that porcelains can implement different merge policies.
68 This flag makes the command resolve a few more cases internally:
69 · when one side removes a path and the other side leaves the path
71 unmodified. The resolution is to remove that path.
72 · when both sides remove a path. The resolution is to remove that
74 path.
75 · when both sides add a path identically. The resolution is to
77 add that path.
78 --prefix=<prefix>
80 Keep the current index contents, and read the contents of the named
81 tree-ish under the directory at <prefix>. The command will refuse
82 to overwrite entries that already existed in the original index
83 file.
84 --exclude-per-directory=<gitignore>
86 When running the command with -u and -m options, the merge result
87 may need to overwrite paths that are not tracked in the current
88 branch. The command usually refuses to proceed with the merge to
89 avoid losing such a path. However this safety valve sometimes gets
90 in the way. For example, it often happens that the other branch
91 added a file that used to be a generated file in your branch, and
92 the safety valve triggers when you try to switch to that branch
93 after you ran make but before running make clean to remove the
94 generated file. This option tells the command to read per-directory
95 exclude file (usually .gitignore) and allows such an untracked but
96 explicitly ignored file to be overwritten.
97 --index-output=<file>
99 Instead of writing the results out to $GIT_INDEX_FILE, write the
100 resulting index in the named file. While the command is operating,
101 the original index file is locked with the same mechanism as usual.
102 The file must allow to be rename(2)ed into from a temporary file
103 that is created next to the usual index file; typically this means
104 it needs to be on the same filesystem as the index file itself, and
105 you need write permission to the directories the index file and
106 index output file are located in.
107 --[no-]recurse-submodules
109 Using --recurse-submodules will update the content of all
110 initialized submodules according to the commit recorded in the
111 superproject by calling read-tree recursively, also setting the
112 submodules HEAD to be detached at that commit.
113 --no-sparse-checkout
115 Disable sparse checkout support even if core.sparseCheckout is
116 true.
117 --empty
119 Instead of reading tree object(s) into the index, just empty it.
120 -q, --quiet
122 Quiet, suppress feedback messages.
123 <tree-ish#>
125 The id of the tree object(s) to be read/merged.
126
128 If -m is specified, git read-tree can perform 3 kinds of merge, a
129 single tree merge if only 1 tree is given, a fast-forward merge with 2
130 trees, or a 3-way merge if 3 or more trees are provided.
131 Single Tree Merge
133 If only 1 tree is specified, git read-tree operates as if the user did
134 not specify -m, except that if the original index has an entry for a
135 given pathname, and the contents of the path match with the tree being
136 read, the stat info from the index is used. (In other words, the
137 index’s stat()s take precedence over the merged tree’s).
138 That means that if you do a git read-tree -m <newtree> followed by a
140 git checkout-index -f -u -a, the git checkout-index only checks out the
141 stuff that really changed.
142 This is used to avoid unnecessary false hits when git diff-files is run
144 after git read-tree.
145 Two Tree Merge
147 Typically, this is invoked as git read-tree -m $H $M, where $H is the
148 head commit of the current repository, and $M is the head of a foreign
149 tree, which is simply ahead of $H (i.e. we are in a fast-forward
150 situation).
151 When two trees are specified, the user is telling git read-tree the
153 following:
154 1. The current index and work tree is derived from $H, but the user
156 may have local changes in them since $H.
157 2. The user wants to fast-forward to $M.
159 In this case, the git read-tree -m $H $M command makes sure that no
161 local change is lost as the result of this "merge". Here are the "carry
162 forward" rules, where "I" denotes the index, "clean" means that index
163 and work tree coincide, and "exists"/"nothing" refer to the presence of
164 a path in the specified commit:
165 I H M Result
167 -------------------------------------------------------
168 0 nothing nothing nothing (does not happen)
169 1 nothing nothing exists use M
170 2 nothing exists nothing remove path from index
171 3 nothing exists exists, use M if "initial checkout",
172 H == M keep index otherwise
173 exists, fail
174 H != M
175 clean I==H I==M
177 ------------------
178 4 yes N/A N/A nothing nothing keep index
179 5 no N/A N/A nothing nothing keep index
180 6 yes N/A yes nothing exists keep index
182 7 no N/A yes nothing exists keep index
183 8 yes N/A no nothing exists fail
184 9 no N/A no nothing exists fail
185 10 yes yes N/A exists nothing remove path from index
187 11 no yes N/A exists nothing fail
188 12 yes no N/A exists nothing fail
189 13 no no N/A exists nothing fail
190 clean (H==M)
192 ------
193 14 yes exists exists keep index
194 15 no exists exists keep index
195 clean I==H I==M (H!=M)
197 ------------------
198 16 yes no no exists exists fail
199 17 no no no exists exists fail
200 18 yes no yes exists exists keep index
201 19 no no yes exists exists keep index
202 20 yes yes no exists exists use M
203 21 no yes no exists exists fail
204 In all "keep index" cases, the index entry stays as in the original
206 index file. If the entry is not up to date, git read-tree keeps the
207 copy in the work tree intact when operating under the -u flag.
208 When this form of git read-tree returns successfully, you can see which
210 of the "local changes" that you made were carried forward by running
211 git diff-index --cached $M. Note that this does not necessarily match
212 what git diff-index --cached $H would have produced before such a two
213 tree merge. This is because of cases 18 and 19 --- if you already had
214 the changes in $M (e.g. maybe you picked it up via e-mail in a patch
215 form), git diff-index --cached $H would have told you about the change
216 before this merge, but it would not show in git diff-index --cached $M
217 output after the two-tree merge.
218 Case 3 is slightly tricky and needs explanation. The result from this
220 rule logically should be to remove the path if the user staged the
221 removal of the path and then switching to a new branch. That however
222 will prevent the initial checkout from happening, so the rule is
223 modified to use M (new tree) only when the content of the index is
224 empty. Otherwise the removal of the path is kept as long as $H and $M
225 are the same.
226 3-Way Merge
228 Each "index" entry has two bits worth of "stage" state. stage 0 is the
229 normal one, and is the only one you’d see in any kind of normal use.
230 However, when you do git read-tree with three trees, the "stage" starts
232 out at 1.
233 This means that you can do
235 $ git read-tree -m <tree1> <tree2> <tree3>
237 and you will end up with an index with all of the <tree1> entries in
240 "stage1", all of the <tree2> entries in "stage2" and all of the <tree3>
241 entries in "stage3". When performing a merge of another branch into the
242 current branch, we use the common ancestor tree as <tree1>, the current
243 branch head as <tree2>, and the other branch head as <tree3>.
244 Furthermore, git read-tree has special-case logic that says: if you see
246 a file that matches in all respects in the following states, it
247 "collapses" back to "stage0":
248 · stage 2 and 3 are the same; take one or the other (it makes no
250 difference - the same work has been done on our branch in stage 2
251 and their branch in stage 3)
252 · stage 1 and stage 2 are the same and stage 3 is different; take
254 stage 3 (our branch in stage 2 did not do anything since the
255 ancestor in stage 1 while their branch in stage 3 worked on it)
256 · stage 1 and stage 3 are the same and stage 2 is different take
258 stage 2 (we did something while they did nothing)
259 The git write-tree command refuses to write a nonsensical tree, and it
261 will complain about unmerged entries if it sees a single entry that is
262 not stage 0.
263 OK, this all sounds like a collection of totally nonsensical rules, but
265 it’s actually exactly what you want in order to do a fast merge. The
266 different stages represent the "result tree" (stage 0, aka "merged"),
267 the original tree (stage 1, aka "orig"), and the two trees you are
268 trying to merge (stage 2 and 3 respectively).
269 The order of stages 1, 2 and 3 (hence the order of three <tree-ish>
271 command-line arguments) are significant when you start a 3-way merge
272 with an index file that is already populated. Here is an outline of how
273 the algorithm works:
274 · if a file exists in identical format in all three trees, it will
276 automatically collapse to "merged" state by git read-tree.
277 · a file that has any difference what-so-ever in the three trees will
279 stay as separate entries in the index. It’s up to "porcelain
280 policy" to determine how to remove the non-0 stages, and insert a
281 merged version.
282 · the index file saves and restores with all this information, so you
284 can merge things incrementally, but as long as it has entries in
285 stages 1/2/3 (i.e., "unmerged entries") you can’t write the result.
286 So now the merge algorithm ends up being really simple:
287 · you walk the index in order, and ignore all entries of stage 0,
289 since they’ve already been done.
290 · if you find a "stage1", but no matching "stage2" or "stage3",
292 you know it’s been removed from both trees (it only existed in
293 the original tree), and you remove that entry.
294 · if you find a matching "stage2" and "stage3" tree, you remove
296 one of them, and turn the other into a "stage0" entry. Remove
297 any matching "stage1" entry if it exists too. .. all the normal
298 trivial rules ..
299 You would normally use git merge-index with supplied git merge-one-file
301 to do this last step. The script updates the files in the working tree
302 as it merges each path and at the end of a successful merge.
303 When you start a 3-way merge with an index file that is already
305 populated, it is assumed that it represents the state of the files in
306 your work tree, and you can even have files with changes unrecorded in
307 the index file. It is further assumed that this state is "derived" from
308 the stage 2 tree. The 3-way merge refuses to run if it finds an entry
309 in the original index file that does not match stage 2.
310 This is done to prevent you from losing your work-in-progress changes,
312 and mixing your random changes in an unrelated merge commit. To
313 illustrate, suppose you start from what has been committed last to your
314 repository:
315 $ JC=`git rev-parse --verify "HEAD^0"`
317 $ git checkout-index -f -u -a $JC
318 You do random edits, without running git update-index. And then you
321 notice that the tip of your "upstream" tree has advanced since you
322 pulled from him:
323 $ git fetch git://.... linus
325 $ LT=`git rev-parse FETCH_HEAD`
326 Your work tree is still based on your HEAD ($JC), but you have some
329 edits since. Three-way merge makes sure that you have not added or
330 modified index entries since $JC, and if you haven’t, then does the
331 right thing. So with the following sequence:
332 $ git read-tree -m -u `git merge-base $JC $LT` $JC $LT
334 $ git merge-index git-merge-one-file -a
335 $ echo "Merge with Linus" | \
336 git commit-tree `git write-tree` -p $JC -p $LT
337 what you would commit is a pure merge between $JC and $LT without your
340 work-in-progress changes, and your work tree would be updated to the
341 result of the merge.
342 However, if you have local changes in the working tree that would be
344 overwritten by this merge, git read-tree will refuse to run to prevent
345 your changes from being lost.
346 In other words, there is no need to worry about what exists only in the
348 working tree. When you have local changes in a part of the project that
349 is not involved in the merge, your changes do not interfere with the
350 merge, and are kept intact. When they do interfere, the merge does not
351 even start (git read-tree complains loudly and fails without modifying
352 anything). In such a case, you can simply continue doing what you were
353 in the middle of doing, and when your working tree is ready (i.e. you
354 have finished your work-in-progress), attempt the merge again.
355
357 "Sparse checkout" allows populating the working directory sparsely. It
358 uses the skip-worktree bit (see git-update-index(1)) to tell Git
359 whether a file in the working directory is worth looking at.
360 git read-tree and other merge-based commands (git merge, git
362 checkout...) can help maintaining the skip-worktree bitmap and working
363 directory update. $GIT_DIR/info/sparse-checkout is used to define the
364 skip-worktree reference bitmap. When git read-tree needs to update the
365 working directory, it resets the skip-worktree bit in the index based
366 on this file, which uses the same syntax as .gitignore files. If an
367 entry matches a pattern in this file, skip-worktree will not be set on
368 that entry. Otherwise, skip-worktree will be set.
369 Then it compares the new skip-worktree value with the previous one. If
371 skip-worktree turns from set to unset, it will add the corresponding
372 file back. If it turns from unset to set, that file will be removed.
373 While $GIT_DIR/info/sparse-checkout is usually used to specify what
375 files are in, you can also specify what files are not in, using negate
376 patterns. For example, to remove the file unwanted:
377 /*
379 !unwanted
380 Another tricky thing is fully repopulating the working directory when
383 you no longer want sparse checkout. You cannot just disable "sparse
384 checkout" because skip-worktree bits are still in the index and your
385 working directory is still sparsely populated. You should re-populate
386 the working directory with the $GIT_DIR/info/sparse-checkout file
387 content as follows:
388 /*
390 Then you can disable sparse checkout. Sparse checkout support in git
393 read-tree and similar commands is disabled by default. You need to turn
394 core.sparseCheckout on in order to have sparse checkout support.
395
397 git-write-tree(1); git-ls-files(1); gitignore(5)
398
400 Part of the git(1) suite
401Git 2.24.1 12/10/2019 GIT-READ-TREE(1)