1GIT-READ-TREE(1)                  Git Manual                  GIT-READ-TREE(1)
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NAME

6       git-read-tree - Reads tree information into the index
7

SYNOPSIS

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>]])
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14

DESCRIPTION

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
20       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
25       Trivial merges are done by git read-tree itself. Only conflicting paths
26       will be in unmerged state when git read-tree returns.
27

OPTIONS

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
34       --reset
35           Same as -m, except that unmerged entries are discarded instead of
36           failing.
37
38       -u
39           After a successful merge, update the files in the work tree with
40           the result of the merge.
41
42       -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
50       -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
54       -v
55           Show the progress of checking files out.
56
57       --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
63       --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
69           ·   when one side removes a path and the other side leaves the path
70               unmodified. The resolution is to remove that path.
71
72           ·   when both sides remove a path. The resolution is to remove that
73               path.
74
75           ·   when both sides add a path identically. The resolution is to
76               add that path.
77
78       --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
84       --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
97       --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
107       --no-sparse-checkout
108           Disable sparse checkout support even if core.sparseCheckout is
109           true.
110
111       --empty
112           Instead of reading tree object(s) into the index, just empty it.
113
114       <tree-ish#>
115           The id of the tree object(s) to be read/merged.
116

MERGING

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
122   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
129       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
133       This is used to avoid unnecessary false hits when git diff-files is run
134       after git read-tree.
135
136   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
142       When two trees are specified, the user is telling git read-tree the
143       following:
144
145        1. The current index and work tree is derived from $H, but the user
146           may have local changes in them since $H.
147
148        2. The user wants to fast-forward to $M.
149
150       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
156              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
166              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
171           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
176           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
181              clean (H==M)
182             ------
183           14 yes                 exists   exists   keep index
184           15 no                  exists   exists   keep index
185
186              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
195       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
199       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
209       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
217   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
221       However, when you do git read-tree with three trees, the "stage" starts
222       out at 1.
223
224       This means that you can do
225
226           $ git read-tree -m <tree1> <tree2> <tree3>
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228
229       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
235       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
239       ·   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
243       ·   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
247       ·   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
250       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
254       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
260       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
265       ·   if a file exists in identical format in all three trees, it will
266           automatically collapse to "merged" state by git read-tree.
267
268       ·   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
273       ·   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
278           ·   you walk the index in order, and ignore all entries of stage 0,
279               since they’ve already been done.
280
281           ·   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
285           ·   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
290       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
294       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
301       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
306           $ JC=`git rev-parse --verify "HEAD^0"`
307           $ git checkout-index -f -u -a $JC
308
309
310       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
314           $ git fetch git://.... linus
315           $ LT=`git rev-parse FETCH_HEAD`
316
317
318       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
323           $ 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
328
329       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
333       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
337       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

SPARSE CHECKOUT

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
351       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
360       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
364       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
368           /*
369           !unwanted
370
371
372       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
379           /*
380
381
382       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

SEE ALSO

387       git-write-tree(1); git-ls-files(1); gitignore(5)
388

GIT

390       Part of the git(1) suite
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392
393
394Git 1.8.3.1                       11/19/2018                  GIT-READ-TREE(1)
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