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 [--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.
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
52 nor 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. Note that the <prefix>/ value must end with a slash.
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-sparse-checkout
108 Disable sparse checkout support even if core.sparseCheckout is
109 true.
110 --empty
112 Instead of reading tree object(s) into the index, just empty it.
113 <tree-ish#>
115 The id of the tree object(s) to be read/merged.
116
118 If -m is specified, git read-tree can perform 3 kinds of merge, a
119 single tree merge if only 1 tree is given, a fast-forward merge with 2
120 trees, or a 3-way merge if 3 trees are provided.
121 Single Tree Merge
123 If only 1 tree is specified, git read-tree operates as if the user did
124 not specify -m, except that if the original index has an entry for a
125 given pathname, and the contents of the path match with the tree being
126 read, the stat info from the index is used. (In other words, the
127 index’s stat()s take precedence over the merged tree’s).
128 That means that if you do a git read-tree -m <newtree> followed by a
130 git checkout-index -f -u -a, the git checkout-index only checks out the
131 stuff that really changed.
132 This is used to avoid unnecessary false hits when git diff-files is run
134 after git read-tree.
135 Two Tree Merge
137 Typically, this is invoked as git read-tree -m $H $M, where $H is the
138 head commit of the current repository, and $M is the head of a foreign
139 tree, which is simply ahead of $H (i.e. we are in a fast-forward
140 situation).
141 When two trees are specified, the user is telling git read-tree the
143 following:
144 1. The current index and work tree is derived from $H, but the user
146 may have local changes in them since $H.
147 2. The user wants to fast-forward to $M.
149 In this case, the git read-tree -m $H $M command makes sure that no
151 local change is lost as the result of this "merge". Here are the "carry
152 forward" rules, where "I" denotes the index, "clean" means that index
153 and work tree coincide, and "exists"/"nothing" refer to the presence of
154 a path in the specified commit:
155 I H M Result
157 -------------------------------------------------------
158 0 nothing nothing nothing (does not happen)
159 1 nothing nothing exists use M
160 2 nothing exists nothing remove path from index
161 3 nothing exists exists, use M if "initial checkout",
162 H == M keep index otherwise
163 exists, fail
164 H != M
165 clean I==H I==M
167 ------------------
168 4 yes N/A N/A nothing nothing keep index
169 5 no N/A N/A nothing nothing keep index
170 6 yes N/A yes nothing exists keep index
172 7 no N/A yes nothing exists keep index
173 8 yes N/A no nothing exists fail
174 9 no N/A no nothing exists fail
175 10 yes yes N/A exists nothing remove path from index
177 11 no yes N/A exists nothing fail
178 12 yes no N/A exists nothing fail
179 13 no no N/A exists nothing fail
180 clean (H==M)
182 ------
183 14 yes exists exists keep index
184 15 no exists exists keep index
185 clean I==H I==M (H!=M)
187 ------------------
188 16 yes no no exists exists fail
189 17 no no no exists exists fail
190 18 yes no yes exists exists keep index
191 19 no no yes exists exists keep index
192 20 yes yes no exists exists use M
193 21 no yes no exists exists fail
194 In all "keep index" cases, the index entry stays as in the original
196 index file. If the entry is not up to date, git read-tree keeps the
197 copy in the work tree intact when operating under the -u flag.
198 When this form of git read-tree returns successfully, you can see which
200 of the "local changes" that you made were carried forward by running
201 git diff-index --cached $M. Note that this does not necessarily match
202 what git diff-index --cached $H would have produced before such a two
203 tree merge. This is because of cases 18 and 19 --- if you already had
204 the changes in $M (e.g. maybe you picked it up via e-mail in a patch
205 form), git diff-index --cached $H would have told you about the change
206 before this merge, but it would not show in git diff-index --cached $M
207 output after the two-tree merge.
208 Case 3 is slightly tricky and needs explanation. The result from this
210 rule logically should be to remove the path if the user staged the
211 removal of the path and then switching to a new branch. That however
212 will prevent the initial checkout from happening, so the rule is
213 modified to use M (new tree) only when the content of the index is
214 empty. Otherwise the removal of the path is kept as long as $H and $M
215 are the same.
216 3-Way Merge
218 Each "index" entry has two bits worth of "stage" state. stage 0 is the
219 normal one, and is the only one you’d see in any kind of normal use.
220 However, when you do git read-tree with three trees, the "stage" starts
222 out at 1.
223 This means that you can do
225 $ git read-tree -m <tree1> <tree2> <tree3>
227 and you will end up with an index with all of the <tree1> entries in
230 "stage1", all of the <tree2> entries in "stage2" and all of the <tree3>
231 entries in "stage3". When performing a merge of another branch into the
232 current branch, we use the common ancestor tree as <tree1>, the current
233 branch head as <tree2>, and the other branch head as <tree3>.
234 Furthermore, git read-tree has special-case logic that says: if you see
236 a file that matches in all respects in the following states, it
237 "collapses" back to "stage0":
238 · stage 2 and 3 are the same; take one or the other (it makes no
240 difference - the same work has been done on our branch in stage 2
241 and their branch in stage 3)
242 · stage 1 and stage 2 are the same and stage 3 is different; take
244 stage 3 (our branch in stage 2 did not do anything since the
245 ancestor in stage 1 while their branch in stage 3 worked on it)
246 · stage 1 and stage 3 are the same and stage 2 is different take
248 stage 2 (we did something while they did nothing)
249 The git write-tree command refuses to write a nonsensical tree, and it
251 will complain about unmerged entries if it sees a single entry that is
252 not stage 0.
253 OK, this all sounds like a collection of totally nonsensical rules, but
255 it’s actually exactly what you want in order to do a fast merge. The
256 different stages represent the "result tree" (stage 0, aka "merged"),
257 the original tree (stage 1, aka "orig"), and the two trees you are
258 trying to merge (stage 2 and 3 respectively).
259 The order of stages 1, 2 and 3 (hence the order of three <tree-ish>
261 command line arguments) are significant when you start a 3-way merge
262 with an index file that is already populated. Here is an outline of how
263 the algorithm works:
264 · if a file exists in identical format in all three trees, it will
266 automatically collapse to "merged" state by git read-tree.
267 · a file that has any difference what-so-ever in the three trees will
269 stay as separate entries in the index. It’s up to "porcelain
270 policy" to determine how to remove the non-0 stages, and insert a
271 merged version.
272 · the index file saves and restores with all this information, so you
274 can merge things incrementally, but as long as it has entries in
275 stages 1/2/3 (i.e., "unmerged entries") you can’t write the result.
276 So now the merge algorithm ends up being really simple:
277 · you walk the index in order, and ignore all entries of stage 0,
279 since they’ve already been done.
280 · if you find a "stage1", but no matching "stage2" or "stage3",
282 you know it’s been removed from both trees (it only existed in
283 the original tree), and you remove that entry.
284 · if you find a matching "stage2" and "stage3" tree, you remove
286 one of them, and turn the other into a "stage0" entry. Remove
287 any matching "stage1" entry if it exists too. .. all the normal
288 trivial rules ..
289 You would normally use git merge-index with supplied git merge-one-file
291 to do this last step. The script updates the files in the working tree
292 as it merges each path and at the end of a successful merge.
293 When you start a 3-way merge with an index file that is already
295 populated, it is assumed that it represents the state of the files in
296 your work tree, and you can even have files with changes unrecorded in
297 the index file. It is further assumed that this state is "derived" from
298 the stage 2 tree. The 3-way merge refuses to run if it finds an entry
299 in the original index file that does not match stage 2.
300 This is done to prevent you from losing your work-in-progress changes,
302 and mixing your random changes in an unrelated merge commit. To
303 illustrate, suppose you start from what has been committed last to your
304 repository:
305 $ JC=`git rev-parse --verify "HEAD^0"`
307 $ git checkout-index -f -u -a $JC
308 You do random edits, without running git update-index. And then you
311 notice that the tip of your "upstream" tree has advanced since you
312 pulled from him:
313 $ git fetch git://.... linus
315 $ LT=`git rev-parse FETCH_HEAD`
316 Your work tree is still based on your HEAD ($JC), but you have some
319 edits since. Three-way merge makes sure that you have not added or
320 modified index entries since $JC, and if you haven’t, then does the
321 right thing. So with the following sequence:
322 $ git read-tree -m -u `git merge-base $JC $LT` $JC $LT
324 $ git merge-index git-merge-one-file -a
325 $ echo "Merge with Linus" | \
326 git commit-tree `git write-tree` -p $JC -p $LT
327 what you would commit is a pure merge between $JC and $LT without your
330 work-in-progress changes, and your work tree would be updated to the
331 result of the merge.
332 However, if you have local changes in the working tree that would be
334 overwritten by this merge, git read-tree will refuse to run to prevent
335 your changes from being lost.
336 In other words, there is no need to worry about what exists only in the
338 working tree. When you have local changes in a part of the project that
339 is not involved in the merge, your changes do not interfere with the
340 merge, and are kept intact. When they do interfere, the merge does not
341 even start (git read-tree complains loudly and fails without modifying
342 anything). In such a case, you can simply continue doing what you were
343 in the middle of doing, and when your working tree is ready (i.e. you
344 have finished your work-in-progress), attempt the merge again.
345
347 "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, skip-worktree will not be set on
358 that entry. Otherwise, skip-worktree will be set.
359 Then it compares the new skip-worktree value with the previous one. If
361 skip-worktree turns from set to unset, it will add the corresponding
362 file back. If it turns from unset to set, that file will be removed.
363 While $GIT_DIR/info/sparse-checkout is usually used to specify what
365 files are in, you can also specify what files are not in, using negate
366 patterns. For example, to remove the file unwanted:
367 /*
369 !unwanted
370 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
383 read-tree and similar commands is disabled by default. You need to turn
384 core.sparseCheckout on in order to have sparse checkout support.
385
387 git-write-tree(1); git-ls-files(1); gitignore(5)
388
390 Part of the git(1) suite
391Git 1.8.3.1 11/19/2018 GIT-READ-TREE(1)