fetch.c: do not call process_tree() from process_tree().
[git/git.git] / Documentation / git-read-tree.txt
1 git-read-tree(1)
2 ================
3
4 NAME
5 ----
6 git-read-tree - Reads tree information into the index
7
8
9 SYNOPSIS
10 --------
11 'git-read-tree' (<tree-ish> | [[-m [--aggressive]| --reset] [-u | -i]] <tree-ish1> [<tree-ish2> [<tree-ish3>]])
12
13
14 DESCRIPTION
15 -----------
16 Reads the tree information given by <tree-ish> into the index,
17 but does not actually *update* any of the files it "caches". (see:
18 gitlink:git-checkout-index[1])
19
20 Optionally, it can merge a tree into the index, perform a
21 fast-forward (i.e. 2-way) merge, or a 3-way merge, with the `-m`
22 flag. When used with `-m`, the `-u` flag causes it to also update
23 the files in the work tree 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
28 OPTIONS
29 -------
30 -m::
31 Perform a merge, not just a read. The command will
32 refuse to run if your index file has unmerged entries,
33 indicating that you have not finished previous merge you
34 started.
35
36 --reset::
37 Same as -m, except that unmerged entries are discarded
38 instead of failing.
39
40 -u::
41 After a successful merge, update the files in the work
42 tree with the result of the merge.
43
44 -i::
45 Usually a merge requires the index file as well as the
46 files in the working tree are up to date with the
47 current head commit, in order not to lose local
48 changes. This flag disables the check with the working
49 tree and is meant to be used when creating a merge of
50 trees that are not directly related to the current
51 working tree status into a temporary index file.
52
53 --aggressive::
54 Usually a three-way merge by `git-read-tree` resolves
55 the merge for really trivial cases and leaves other
56 cases unresolved in the index, so that Porcelains can
57 implement different merge policies. This flag makes the
58 command to resolve a few more cases internally:
59 +
60 * when one side removes a path and the other side leaves the path
61 unmodified. The resolution is to remove that path.
62 * when both sides remove a path. The resolution is to remove that path.
63 * when both sides adds a path identically. The resolution
64 is to add that path.
65
66 <tree-ish#>::
67 The id of the tree object(s) to be read/merged.
68
69
70 Merging
71 -------
72 If `-m` is specified, `git-read-tree` can perform 3 kinds of
73 merge, a single tree merge if only 1 tree is given, a
74 fast-forward merge with 2 trees, or a 3-way merge if 3 trees are
75 provided.
76
77
78 Single Tree Merge
79 ~~~~~~~~~~~~~~~~~
80 If only 1 tree is specified, git-read-tree operates as if the user did not
81 specify `-m`, except that if the original index has an entry for a
82 given pathname, and the contents of the path matches with the tree
83 being read, the stat info from the index is used. (In other words, the
84 index's stat()s take precedence over the merged tree's).
85
86 That means that if you do a `git-read-tree -m <newtree>` followed by a
87 `git-checkout-index -f -u -a`, the `git-checkout-index` only checks out
88 the stuff that really changed.
89
90 This is used to avoid unnecessary false hits when `git-diff-files` is
91 run after `git-read-tree`.
92
93
94 Two Tree Merge
95 ~~~~~~~~~~~~~~
96
97 Typically, this is invoked as `git-read-tree -m $H $M`, where $H
98 is the head commit of the current repository, and $M is the head
99 of a foreign tree, which is simply ahead of $H (i.e. we are in a
100 fast forward situation).
101
102 When two trees are specified, the user is telling git-read-tree
103 the following:
104
105 1. The current index and work tree is derived from $H, but
106 the user may have local changes in them since $H;
107
108 2. The user wants to fast-forward to $M.
109
110 In this case, the `git-read-tree -m $H $M` command makes sure
111 that no local change is lost as the result of this "merge".
112 Here are the "carry forward" rules:
113
114 I (index) H M Result
115 -------------------------------------------------------
116 0 nothing nothing nothing (does not happen)
117 1 nothing nothing exists use M
118 2 nothing exists nothing remove path from index
119 3 nothing exists exists use M
120
121 clean I==H I==M
122 ------------------
123 4 yes N/A N/A nothing nothing keep index
124 5 no N/A N/A nothing nothing keep index
125
126 6 yes N/A yes nothing exists keep index
127 7 no N/A yes nothing exists keep index
128 8 yes N/A no nothing exists fail
129 9 no N/A no nothing exists fail
130
131 10 yes yes N/A exists nothing remove path from index
132 11 no yes N/A exists nothing fail
133 12 yes no N/A exists nothing fail
134 13 no no N/A exists nothing fail
135
136 clean (H=M)
137 ------
138 14 yes exists exists keep index
139 15 no exists exists keep index
140
141 clean I==H I==M (H!=M)
142 ------------------
143 16 yes no no exists exists fail
144 17 no no no exists exists fail
145 18 yes no yes exists exists keep index
146 19 no no yes exists exists keep index
147 20 yes yes no exists exists use M
148 21 no yes no exists exists fail
149
150 In all "keep index" cases, the index entry stays as in the
151 original index file. If the entry were not up to date,
152 git-read-tree keeps the copy in the work tree intact when
153 operating under the -u flag.
154
155 When this form of git-read-tree returns successfully, you can
156 see what "local changes" you made are carried forward by running
157 `git-diff-index --cached $M`. Note that this does not
158 necessarily match `git-diff-index --cached $H` would have
159 produced before such a two tree merge. This is because of cases
160 18 and 19 --- if you already had the changes in $M (e.g. maybe
161 you picked it up via e-mail in a patch form), `git-diff-index
162 --cached $H` would have told you about the change before this
163 merge, but it would not show in `git-diff-index --cached $M`
164 output after two-tree merge.
165
166
167 3-Way Merge
168 ~~~~~~~~~~~
169 Each "index" entry has two bits worth of "stage" state. stage 0 is the
170 normal one, and is the only one you'd see in any kind of normal use.
171
172 However, when you do `git-read-tree` with three trees, the "stage"
173 starts out at 1.
174
175 This means that you can do
176
177 ----------------
178 $ git-read-tree -m <tree1> <tree2> <tree3>
179 ----------------
180
181 and you will end up with an index with all of the <tree1> entries in
182 "stage1", all of the <tree2> entries in "stage2" and all of the
183 <tree3> entries in "stage3". When performing a merge of another
184 branch into the current branch, we use the common ancestor tree
185 as <tree1>, the current branch head as <tree2>, and the other
186 branch head as <tree3>.
187
188 Furthermore, `git-read-tree` has special-case logic that says: if you see
189 a file that matches in all respects in the following states, it
190 "collapses" back to "stage0":
191
192 - stage 2 and 3 are the same; take one or the other (it makes no
193 difference - the same work has been done on our branch in
194 stage 2 and their branch in stage 3)
195
196 - stage 1 and stage 2 are the same and stage 3 is different; take
197 stage 3 (our branch in stage 2 did not do anything since the
198 ancestor in stage 1 while their branch in stage 3 worked on
199 it)
200
201 - stage 1 and stage 3 are the same and stage 2 is different take
202 stage 2 (we did something while they did nothing)
203
204 The `git-write-tree` command refuses to write a nonsensical tree, and it
205 will complain about unmerged entries if it sees a single entry that is not
206 stage 0.
207
208 Ok, this all sounds like a collection of totally nonsensical rules,
209 but it's actually exactly what you want in order to do a fast
210 merge. The different stages represent the "result tree" (stage 0, aka
211 "merged"), the original tree (stage 1, aka "orig"), and the two trees
212 you are trying to merge (stage 2 and 3 respectively).
213
214 The order of stages 1, 2 and 3 (hence the order of three
215 <tree-ish> command line arguments) are significant when you
216 start a 3-way merge with an index file that is already
217 populated. Here is an outline of how the algorithm works:
218
219 - if a file exists in identical format in all three trees, it will
220 automatically collapse to "merged" state by git-read-tree.
221
222 - a file that has _any_ difference what-so-ever in the three trees
223 will stay as separate entries in the index. It's up to "porcelain
224 policy" to determine how to remove the non-0 stages, and insert a
225 merged version.
226
227 - the index file saves and restores with all this information, so you
228 can merge things incrementally, but as long as it has entries in
229 stages 1/2/3 (ie "unmerged entries") you can't write the result. So
230 now the merge algorithm ends up being really simple:
231
232 * you walk the index in order, and ignore all entries of stage 0,
233 since they've already been done.
234
235 * if you find a "stage1", but no matching "stage2" or "stage3", you
236 know it's been removed from both trees (it only existed in the
237 original tree), and you remove that entry.
238
239 * if you find a matching "stage2" and "stage3" tree, you remove one
240 of them, and turn the other into a "stage0" entry. Remove any
241 matching "stage1" entry if it exists too. .. all the normal
242 trivial rules ..
243
244 You would normally use `git-merge-index` with supplied
245 `git-merge-one-file` to do this last step. The script updates
246 the files in the working tree as it merges each path and at the
247 end of a successful merge.
248
249 When you start a 3-way merge with an index file that is already
250 populated, it is assumed that it represents the state of the
251 files in your work tree, and you can even have files with
252 changes unrecorded in the index file. It is further assumed
253 that this state is "derived" from the stage 2 tree. The 3-way
254 merge refuses to run if it finds an entry in the original index
255 file that does not match stage 2.
256
257 This is done to prevent you from losing your work-in-progress
258 changes, and mixing your random changes in an unrelated merge
259 commit. To illustrate, suppose you start from what has been
260 commited last to your repository:
261
262 ----------------
263 $ JC=`git-rev-parse --verify "HEAD^0"`
264 $ git-checkout-index -f -u -a $JC
265 ----------------
266
267 You do random edits, without running git-update-index. And then
268 you notice that the tip of your "upstream" tree has advanced
269 since you pulled from him:
270
271 ----------------
272 $ git-fetch git://.... linus
273 $ LT=`cat .git/FETCH_HEAD`
274 ----------------
275
276 Your work tree is still based on your HEAD ($JC), but you have
277 some edits since. Three-way merge makes sure that you have not
278 added or modified index entries since $JC, and if you haven't,
279 then does the right thing. So with the following sequence:
280
281 ----------------
282 $ git-read-tree -m -u `git-merge-base $JC $LT` $JC $LT
283 $ git-merge-index git-merge-one-file -a
284 $ echo "Merge with Linus" | \
285 git-commit-tree `git-write-tree` -p $JC -p $LT
286 ----------------
287
288 what you would commit is a pure merge between $JC and $LT without
289 your work-in-progress changes, and your work tree would be
290 updated to the result of the merge.
291
292 However, if you have local changes in the working tree that
293 would be overwritten by this merge,`git-read-tree` will refuse
294 to run to prevent your changes from being lost.
295
296 In other words, there is no need to worry about what exists only
297 in the working tree. When you have local changes in a part of
298 the project that is not involved in the merge, your changes do
299 not interfere with the merge, and are kept intact. When they
300 *do* interfere, the merge does not even start (`git-read-tree`
301 complains loudly and fails without modifying anything). In such
302 a case, you can simply continue doing what you were in the
303 middle of doing, and when your working tree is ready (i.e. you
304 have finished your work-in-progress), attempt the merge again.
305
306
307 See Also
308 --------
309 gitlink:git-write-tree[1]; gitlink:git-ls-files[1]
310
311
312 Author
313 ------
314 Written by Linus Torvalds <torvalds@osdl.org>
315
316 Documentation
317 --------------
318 Documentation by David Greaves, Junio C Hamano and the git-list <git@vger.kernel.org>.
319
320 GIT
321 ---
322 Part of the gitlink:git[7] suite
323