1GIT-READ-TREE(1) Git Manual GIT-READ-TREE(1)
2
6 git-read-tree - Reads tree information into the index
7
9 git read-tree [[-m [--trivial] [--aggressive] | --reset |
10 --prefix=<prefix>] [-u [--exclude-per-directory=<gitignore>] | -i]]
11 [--index-output=<file>] [--no-sparse-checkout] <tree-ish1> [<tree-ish2>
12 [<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.
36 -u
38 After a successful merge, update the files in the work tree with
39 the result of the merge.
40 -i
42 Usually a merge requires the index file as well as the files in the
43 working tree are up to date with the current head commit, in order
44 not to lose local changes. This flag disables the check with the
45 working tree and is meant to be used when creating a merge of trees
46 that are not directly related to the current working tree status
47 into a temporary index file.
48 -v
50 Show the progress of checking files out.
51 --trivial
53 Restrict three-way merge by git read-tree to happen only if there
54 is no file-level merging required, instead of resolving merge for
55 trivial cases and leaving conflicting files unresolved in the
56 index.
57 --aggressive
59 Usually a three-way merge by git read-tree resolves the merge for
60 really trivial cases and leaves other cases unresolved in the
61 index, so that Porcelains can implement different merge policies.
62 This flag makes the command to resolve a few more cases internally:
63 · when one side removes a path and the other side leaves the path
65 unmodified. The resolution is to remove that path.
66 · when both sides remove a path. The resolution is to remove that
68 path.
69 · when both sides adds a path identically. The resolution is to
71 add that path.
72 --prefix=<prefix>/
74 Keep the current index contents, and read the contents of named
75 tree-ish under directory at <prefix>. The original index file
76 cannot have anything at the path <prefix> itself, and have nothing
77 in <prefix>/ directory. Note that the <prefix>/ value must end with
78 a slash.
79 --exclude-per-directory=<gitignore>
81 When running the command with -u and -m options, the merge result
82 may need to overwrite paths that are not tracked in the current
83 branch. The command usually refuses to proceed with the merge to
84 avoid losing such a path. However this safety valve sometimes gets
85 in the way. For example, it often happens that the other branch
86 added a file that used to be a generated file in your branch, and
87 the safety valve triggers when you try to switch to that branch
88 after you ran make but before running make clean to remove the
89 generated file. This option tells the command to read per-directory
90 exclude file (usually .gitignore) and allows such an untracked but
91 explicitly ignored file to be overwritten.
92 --index-output=<file>
94 Instead of writing the results out to $GIT_INDEX_FILE, write the
95 resulting index in the named file. While the command is operating,
96 the original index file is locked with the same mechanism as usual.
97 The file must allow to be rename(2)ed into from a temporary file
98 that is created next to the usual index file; typically this means
99 it needs to be on the same filesystem as the index file itself, and
100 you need write permission to the directories the index file and
101 index output file are located in.
102 --no-sparse-checkout
104 Disable sparse checkout support even if core.sparseCheckout is
105 true.
106 <tree-ish#>
108 The id of the tree object(s) to be read/merged.
109
111 If -m is specified, git read-tree can perform 3 kinds of merge, a
112 single tree merge if only 1 tree is given, a fast-forward merge with 2
113 trees, or a 3-way merge if 3 trees are provided.
114 Single Tree Merge
116 If only 1 tree is specified, git read-tree operates as if the user did
117 not specify -m, except that if the original index has an entry for a
118 given pathname, and the contents of the path match with the tree being
119 read, the stat info from the index is used. (In other words, the
120 index’s stat()s take precedence over the merged tree’s).
121 That means that if you do a git read-tree -m <newtree> followed by a
123 git checkout-index -f -u -a, the git checkout-index only checks out the
124 stuff that really changed.
125 This is used to avoid unnecessary false hits when git diff-files is run
127 after git read-tree.
128 Two Tree Merge
130 Typically, this is invoked as git read-tree -m $H $M, where $H is the
131 head commit of the current repository, and $M is the head of a foreign
132 tree, which is simply ahead of $H (i.e. we are in a fast-forward
133 situation).
134 When two trees are specified, the user is telling git read-tree the
136 following:
137 1. The current index and work tree is derived from $H, but the user
139 may have local changes in them since $H.
140 2. The user wants to fast-forward to $M.
142 In this case, the git read-tree -m $H $M command makes sure that no
144 local change is lost as the result of this "merge". Here are the "carry
145 forward" rules, where "I" denotes the index, "clean" means that index
146 and work tree coincide, and "exists"/"nothing" refer to the presence of
147 a path in the specified commit:
148 I H M Result
150 -------------------------------------------------------
151 0 nothing nothing nothing (does not happen)
152 1 nothing nothing exists use M
153 2 nothing exists nothing remove path from index
154 3 nothing exists exists, use M if "initial checkout",
155 H == M keep index otherwise
156 exists, fail
157 H != M
158 clean I==H I==M
160 ------------------
161 4 yes N/A N/A nothing nothing keep index
162 5 no N/A N/A nothing nothing keep index
163 6 yes N/A yes nothing exists keep index
165 7 no N/A yes nothing exists keep index
166 8 yes N/A no nothing exists fail
167 9 no N/A no nothing exists fail
168 10 yes yes N/A exists nothing remove path from index
170 11 no yes N/A exists nothing fail
171 12 yes no N/A exists nothing fail
172 13 no no N/A exists nothing fail
173 clean (H==M)
175 ------
176 14 yes exists exists keep index
177 15 no exists exists keep index
178 clean I==H I==M (H!=M)
180 ------------------
181 16 yes no no exists exists fail
182 17 no no no exists exists fail
183 18 yes no yes exists exists keep index
184 19 no no yes exists exists keep index
185 20 yes yes no exists exists use M
186 21 no yes no exists exists fail
187 In all "keep index" cases, the index entry stays as in the original
189 index file. If the entry is not up to date, git read-tree keeps the
190 copy in the work tree intact when operating under the -u flag.
191 When this form of git read-tree returns successfully, you can see which
193 of the "local changes" that you made were carried forward by running
194 git diff-index --cached $M. Note that this does not necessarily match
195 what git diff-index --cached $H would have produced before such a two
196 tree merge. This is because of cases 18 and 19 --- if you already had
197 the changes in $M (e.g. maybe you picked it up via e-mail in a patch
198 form), git diff-index --cached $H would have told you about the change
199 before this merge, but it would not show in git diff-index --cached $M
200 output after the two-tree merge.
201 Case 3 is slightly tricky and needs explanation. The result from this
203 rule logically should be to remove the path if the user staged the
204 removal of the path and then switching to a new branch. That however
205 will prevent the initial checkout from happening, so the rule is
206 modified to use M (new tree) only when the content of the index is
207 empty. Otherwise the removal of the path is kept as long as $H and $M
208 are the same.
209 3-Way Merge
211 Each "index" entry has two bits worth of "stage" state. stage 0 is the
212 normal one, and is the only one you’d see in any kind of normal use.
213 However, when you do git read-tree with three trees, the "stage" starts
215 out at 1.
216 This means that you can do
218 $ git read-tree -m <tree1> <tree2> <tree3>
220 and you will end up with an index with all of the <tree1> entries in
223 "stage1", all of the <tree2> entries in "stage2" and all of the <tree3>
224 entries in "stage3". When performing a merge of another branch into the
225 current branch, we use the common ancestor tree as <tree1>, the current
226 branch head as <tree2>, and the other branch head as <tree3>.
227 Furthermore, git read-tree has special-case logic that says: if you see
229 a file that matches in all respects in the following states, it
230 "collapses" back to "stage0":
231 · stage 2 and 3 are the same; take one or the other (it makes no
233 difference - the same work has been done on our branch in stage 2
234 and their branch in stage 3)
235 · stage 1 and stage 2 are the same and stage 3 is different; take
237 stage 3 (our branch in stage 2 did not do anything since the
238 ancestor in stage 1 while their branch in stage 3 worked on it)
239 · stage 1 and stage 3 are the same and stage 2 is different take
241 stage 2 (we did something while they did nothing)
242 The git write-tree command refuses to write a nonsensical tree, and it
244 will complain about unmerged entries if it sees a single entry that is
245 not stage 0.
246 OK, this all sounds like a collection of totally nonsensical rules, but
248 it’s actually exactly what you want in order to do a fast merge. The
249 different stages represent the "result tree" (stage 0, aka "merged"),
250 the original tree (stage 1, aka "orig"), and the two trees you are
251 trying to merge (stage 2 and 3 respectively).
252 The order of stages 1, 2 and 3 (hence the order of three <tree-ish>
254 command line arguments) are significant when you start a 3-way merge
255 with an index file that is already populated. Here is an outline of how
256 the algorithm works:
257 · if a file exists in identical format in all three trees, it will
259 automatically collapse to "merged" state by git read-tree.
260 · a file that has any difference what-so-ever in the three trees will
262 stay as separate entries in the index. It’s up to "porcelain
263 policy" to determine how to remove the non-0 stages, and insert a
264 merged version.
265 · the index file saves and restores with all this information, so you
267 can merge things incrementally, but as long as it has entries in
268 stages 1/2/3 (i.e., "unmerged entries") you can’t write the result.
269 So now the merge algorithm ends up being really simple:
270 · you walk the index in order, and ignore all entries of stage 0,
272 since they’ve already been done.
273 · if you find a "stage1", but no matching "stage2" or "stage3",
275 you know it’s been removed from both trees (it only existed in
276 the original tree), and you remove that entry.
277 · if you find a matching "stage2" and "stage3" tree, you remove
279 one of them, and turn the other into a "stage0" entry. Remove
280 any matching "stage1" entry if it exists too. .. all the normal
281 trivial rules ..
282 You would normally use git merge-index with supplied git merge-one-file
284 to do this last step. The script updates the files in the working tree
285 as it merges each path and at the end of a successful merge.
286 When you start a 3-way merge with an index file that is already
288 populated, it is assumed that it represents the state of the files in
289 your work tree, and you can even have files with changes unrecorded in
290 the index file. It is further assumed that this state is "derived" from
291 the stage 2 tree. The 3-way merge refuses to run if it finds an entry
292 in the original index file that does not match stage 2.
293 This is done to prevent you from losing your work-in-progress changes,
295 and mixing your random changes in an unrelated merge commit. To
296 illustrate, suppose you start from what has been committed last to your
297 repository:
298 $ JC=`git rev-parse --verify "HEAD^0"`
300 $ git checkout-index -f -u -a $JC
301 You do random edits, without running git update-index. And then you
304 notice that the tip of your "upstream" tree has advanced since you
305 pulled from him:
306 $ git fetch git://.... linus
308 $ LT=`cat .git/FETCH_HEAD`
309 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
323 work-in-progress changes, and your work tree would be updated to the
324 result of the merge.
325 However, if you have local changes in the working tree that would be
327 overwritten by this merge, git read-tree will refuse to run to prevent
328 your changes from being lost.
329 In other words, there is no need to worry about what exists only in the
331 working tree. When you have local changes in a part of the project that
332 is not involved in the merge, your changes do not interfere with the
333 merge, and are kept intact. When they do interfere, the merge does not
334 even start (git read-tree complains loudly and fails without modifying
335 anything). In such a case, you can simply continue doing what you were
336 in the middle of doing, and when your working tree is ready (i.e. you
337 have finished your work-in-progress), attempt the merge again.
338
340 "Sparse checkout" allows to sparsely populate working directory. It
341 uses skip-worktree bit (see git-update-index(1)) to tell Git whether a
342 file on working directory is worth looking at.
343 "git read-tree" and other merge-based commands ("git merge", "git
345 checkout"...) can help maintaining skip-worktree bitmap and working
346 directory update. $GIT_DIR/info/sparse-checkout is used to define the
347 skip-worktree reference bitmap. When "git read-tree" needs to update
348 working directory, it will reset skip-worktree bit in index based on
349 this file, which uses the same syntax as .gitignore files. If an entry
350 matches a pattern in this file, skip-worktree will be set on that
351 entry. Otherwise, skip-worktree will be unset.
352 Then it compares the new skip-worktree value with the previous one. If
354 skip-worktree turns from unset to set, it will add the corresponding
355 file back. If it turns from set to unset, that file will be removed.
356 While $GIT_DIR/info/sparse-checkout is usually used to specify what
358 files are in. You can also specify what files are not in, using negate
359 patterns. For example, to remove file "unwanted":
360 *
362 !unwanted
363 Another tricky thing is fully repopulating working directory when you
366 no longer want sparse checkout. You cannot just disable "sparse
367 checkout" because skip-worktree are still in the index and you working
368 directory is still sparsely populated. You should re-populate working
369 directory with the $GIT_DIR/info/sparse-checkout file content as
370 follows:
371 *
373 Then you can disable sparse checkout. Sparse checkout support in "git
376 read-tree" and similar commands is disabled by default. You need to
377 turn core.sparseCheckout on in order to have sparse checkout support.
378
380 git-write-tree(1); git-ls-files(1); gitignore(5)
381
383 Written by Linus Torvalds <torvalds@osdl.org[1]>
384
386 Documentation by David Greaves, Junio C Hamano and the git-list
387 <git@vger.kernel.org[2]>.
388
390 Part of the git(1) suite
391
393 1. torvalds@osdl.org
394 mailto:torvalds@osdl.org
395 2. git@vger.kernel.org
397 mailto:git@vger.kernel.org
398Git 1.7.1 08/16/2017 GIT-READ-TREE(1)