read-tree: migrate to parse-options
[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' [[-m [--trivial] [--aggressive] | --reset | --prefix=<prefix>]
12 [-u [--exclude-per-directory=<gitignore>] | -i]]
13 [--index-output=<file>]
14 <tree-ish1> [<tree-ish2> [<tree-ish3>]]
15
16
17 DESCRIPTION
18 -----------
19 Reads the tree information given by <tree-ish> into the index,
20 but does not actually *update* any of the files it "caches". (see:
21 linkgit:git-checkout-index[1])
22
23 Optionally, it can merge a tree into the index, perform a
24 fast-forward (i.e. 2-way) merge, or a 3-way merge, with the `-m`
25 flag. When used with `-m`, the `-u` flag causes it to also update
26 the files in the work tree with the result of the merge.
27
28 Trivial merges are done by 'git-read-tree' itself. Only conflicting paths
29 will be in unmerged state when 'git-read-tree' returns.
30
31 OPTIONS
32 -------
33 -m::
34 Perform a merge, not just a read. The command will
35 refuse to run if your index file has unmerged entries,
36 indicating that you have not finished previous merge you
37 started.
38
39 --reset::
40 Same as -m, except that unmerged entries are discarded
41 instead of failing.
42
43 -u::
44 After a successful merge, update the files in the work
45 tree with the result of the merge.
46
47 -i::
48 Usually a merge requires the index file as well as the
49 files in the working tree are up to date with the
50 current head commit, in order not to lose local
51 changes. This flag disables the check with the working
52 tree and is meant to be used when creating a merge of
53 trees that are not directly related to the current
54 working tree status into a temporary index file.
55
56 -v::
57 Show the progress of checking files out.
58
59 --trivial::
60 Restrict three-way merge by 'git-read-tree' to happen
61 only if there is no file-level merging required, instead
62 of resolving merge for trivial cases and leaving
63 conflicting files unresolved in the index.
64
65 --aggressive::
66 Usually a three-way merge by 'git-read-tree' resolves
67 the merge for really trivial cases and leaves other
68 cases unresolved in the index, so that Porcelains can
69 implement different merge policies. This flag makes the
70 command to resolve a few more cases internally:
71 +
72 * when one side removes a path and the other side leaves the path
73 unmodified. The resolution is to remove that path.
74 * when both sides remove a path. The resolution is to remove that path.
75 * when both sides adds a path identically. The resolution
76 is to add that path.
77
78 --prefix=<prefix>/::
79 Keep the current index contents, and read the contents
80 of named tree-ish under directory at `<prefix>`. The
81 original index file cannot have anything at the path
82 `<prefix>` itself, and have nothing in `<prefix>/`
83 directory. Note that the `<prefix>/` value must end
84 with a slash.
85
86 --exclude-per-directory=<gitignore>::
87 When running the command with `-u` and `-m` options, the
88 merge result may need to overwrite paths that are not
89 tracked in the current branch. The command usually
90 refuses to proceed with the merge to avoid losing such a
91 path. However this safety valve sometimes gets in the
92 way. For example, it often happens that the other
93 branch added a file that used to be a generated file in
94 your branch, and the safety valve triggers when you try
95 to switch to that branch after you ran `make` but before
96 running `make clean` to remove the generated file. This
97 option tells the command to read per-directory exclude
98 file (usually '.gitignore') and allows such an untracked
99 but explicitly ignored file to be overwritten.
100
101 --index-output=<file>::
102 Instead of writing the results out to `$GIT_INDEX_FILE`,
103 write the resulting index in the named file. While the
104 command is operating, the original index file is locked
105 with the same mechanism as usual. The file must allow
106 to be rename(2)ed into from a temporary file that is
107 created next to the usual index file; typically this
108 means it needs to be on the same filesystem as the index
109 file itself, and you need write permission to the
110 directories the index file and index output file are
111 located in.
112
113 <tree-ish#>::
114 The id of the tree object(s) to be read/merged.
115
116
117 Merging
118 -------
119 If `-m` is specified, 'git-read-tree' can perform 3 kinds of
120 merge, a single tree merge if only 1 tree is given, a
121 fast-forward merge with 2 trees, or a 3-way merge if 3 trees are
122 provided.
123
124
125 Single Tree Merge
126 ~~~~~~~~~~~~~~~~~
127 If only 1 tree is specified, 'git-read-tree' operates as if the user did not
128 specify `-m`, except that if the original index has an entry for a
129 given pathname, and the contents of the path matches with the tree
130 being read, the stat info from the index is used. (In other words, the
131 index's stat()s take precedence over the merged tree's).
132
133 That means that if you do a `git read-tree -m <newtree>` followed by a
134 `git checkout-index -f -u -a`, the 'git-checkout-index' only checks out
135 the stuff that really changed.
136
137 This is used to avoid unnecessary false hits when 'git-diff-files' is
138 run after 'git-read-tree'.
139
140
141 Two Tree Merge
142 ~~~~~~~~~~~~~~
143
144 Typically, this is invoked as `git read-tree -m $H $M`, where $H
145 is the head commit of the current repository, and $M is the head
146 of a foreign tree, which is simply ahead of $H (i.e. we are in a
147 fast forward situation).
148
149 When two trees are specified, the user is telling 'git-read-tree'
150 the following:
151
152 1. The current index and work tree is derived from $H, but
153 the user may have local changes in them since $H;
154
155 2. The user wants to fast-forward to $M.
156
157 In this case, the `git read-tree -m $H $M` command makes sure
158 that no local change is lost as the result of this "merge".
159 Here are the "carry forward" rules:
160
161 I (index) H M Result
162 -------------------------------------------------------
163 0 nothing nothing nothing (does not happen)
164 1 nothing nothing exists use M
165 2 nothing exists nothing remove path from index
166 3 nothing exists exists, use M if "initial checkout"
167 H == M keep index otherwise
168 exists fail
169 H != M
170
171 clean I==H I==M
172 ------------------
173 4 yes N/A N/A nothing nothing keep index
174 5 no N/A N/A nothing nothing keep index
175
176 6 yes N/A yes nothing exists keep index
177 7 no N/A yes nothing exists keep index
178 8 yes N/A no nothing exists fail
179 9 no N/A no nothing exists fail
180
181 10 yes yes N/A exists nothing remove path from index
182 11 no yes N/A exists nothing fail
183 12 yes no N/A exists nothing fail
184 13 no no N/A exists nothing fail
185
186 clean (H=M)
187 ------
188 14 yes exists exists keep index
189 15 no exists exists keep index
190
191 clean I==H I==M (H!=M)
192 ------------------
193 16 yes no no exists exists fail
194 17 no no no exists exists fail
195 18 yes no yes exists exists keep index
196 19 no no yes exists exists keep index
197 20 yes yes no exists exists use M
198 21 no yes no exists exists fail
199
200 In all "keep index" cases, the index entry stays as in the
201 original index file. If the entry were not up to date,
202 'git-read-tree' keeps the copy in the work tree intact when
203 operating under the -u flag.
204
205 When this form of 'git-read-tree' returns successfully, you can
206 see what "local changes" you made are carried forward by running
207 `git diff-index --cached $M`. Note that this does not
208 necessarily match `git diff-index --cached $H` would have
209 produced before such a two tree merge. This is because of cases
210 18 and 19 --- if you already had the changes in $M (e.g. maybe
211 you picked it up via e-mail in a patch form), `git diff-index
212 --cached $H` would have told you about the change before this
213 merge, but it would not show in `git diff-index --cached $M`
214 output after two-tree merge.
215
216 Case #3 is slightly tricky and needs explanation. The result from this
217 rule logically should be to remove the path if the user staged the removal
218 of the path and then switching to a new branch. That however will prevent
219 the initial checkout from happening, so the rule is modified to use M (new
220 tree) only when the contents of the index is empty. Otherwise the removal
221 of the path is kept as long as $H and $M are the same.
222
223 3-Way Merge
224 ~~~~~~~~~~~
225 Each "index" entry has two bits worth of "stage" state. stage 0 is the
226 normal one, and is the only one you'd see in any kind of normal use.
227
228 However, when you do 'git-read-tree' with three trees, the "stage"
229 starts out at 1.
230
231 This means that you can do
232
233 ----------------
234 $ git read-tree -m <tree1> <tree2> <tree3>
235 ----------------
236
237 and you will end up with an index with all of the <tree1> entries in
238 "stage1", all of the <tree2> entries in "stage2" and all of the
239 <tree3> entries in "stage3". When performing a merge of another
240 branch into the current branch, we use the common ancestor tree
241 as <tree1>, the current branch head as <tree2>, and the other
242 branch head as <tree3>.
243
244 Furthermore, 'git-read-tree' has special-case logic that says: if you see
245 a file that matches in all respects in the following states, it
246 "collapses" back to "stage0":
247
248 - stage 2 and 3 are the same; take one or the other (it makes no
249 difference - the same work has been done on our branch in
250 stage 2 and their branch in stage 3)
251
252 - stage 1 and stage 2 are the same and stage 3 is different; take
253 stage 3 (our branch in stage 2 did not do anything since the
254 ancestor in stage 1 while their branch in stage 3 worked on
255 it)
256
257 - 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
260 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 not
262 stage 0.
263
264 OK, this all sounds like a collection of totally nonsensical rules,
265 but it's actually exactly what you want in order to do a fast
266 merge. The different stages represent the "result tree" (stage 0, aka
267 "merged"), the original tree (stage 1, aka "orig"), and the two trees
268 you are trying to merge (stage 2 and 3 respectively).
269
270 The order of stages 1, 2 and 3 (hence the order of three
271 <tree-ish> command line arguments) are significant when you
272 start a 3-way merge with an index file that is already
273 populated. Here is an outline of how the algorithm works:
274
275 - if a file exists in identical format in all three trees, it will
276 automatically collapse to "merged" state by 'git-read-tree'.
277
278 - a file that has _any_ difference what-so-ever in the three trees
279 will 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
283 - 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. So
286 now the merge algorithm ends up being really simple:
287
288 * you walk the index in order, and ignore all entries of stage 0,
289 since they've already been done.
290
291 * if you find a "stage1", but no matching "stage2" or "stage3", you
292 know it's been removed from both trees (it only existed in the
293 original tree), and you remove that entry.
294
295 * if you find a matching "stage2" and "stage3" tree, you remove one
296 of them, and turn the other into a "stage0" entry. Remove any
297 matching "stage1" entry if it exists too. .. all the normal
298 trivial rules ..
299
300 You would normally use 'git-merge-index' with supplied
301 'git-merge-one-file' to do this last step. The script updates
302 the files in the working tree as it merges each path and at the
303 end of a successful merge.
304
305 When you start a 3-way merge with an index file that is already
306 populated, it is assumed that it represents the state of the
307 files in your work tree, and you can even have files with
308 changes unrecorded in the index file. It is further assumed
309 that this state is "derived" from the stage 2 tree. The 3-way
310 merge refuses to run if it finds an entry in the original index
311 file that does not match stage 2.
312
313 This is done to prevent you from losing your work-in-progress
314 changes, and mixing your random changes in an unrelated merge
315 commit. To illustrate, suppose you start from what has been
316 committed last to your repository:
317
318 ----------------
319 $ JC=`git rev-parse --verify "HEAD^0"`
320 $ git checkout-index -f -u -a $JC
321 ----------------
322
323 You do random edits, without running 'git-update-index'. And then
324 you notice that the tip of your "upstream" tree has advanced
325 since you pulled from him:
326
327 ----------------
328 $ git fetch git://.... linus
329 $ LT=`cat .git/FETCH_HEAD`
330 ----------------
331
332 Your work tree is still based on your HEAD ($JC), but you have
333 some edits since. Three-way merge makes sure that you have not
334 added or modified index entries since $JC, and if you haven't,
335 then does the right thing. So with the following sequence:
336
337 ----------------
338 $ git read-tree -m -u `git merge-base $JC $LT` $JC $LT
339 $ git merge-index git-merge-one-file -a
340 $ echo "Merge with Linus" | \
341 git commit-tree `git write-tree` -p $JC -p $LT
342 ----------------
343
344 what you would commit is a pure merge between $JC and $LT without
345 your work-in-progress changes, and your work tree would be
346 updated to the result of the merge.
347
348 However, if you have local changes in the working tree that
349 would be overwritten by this merge, 'git-read-tree' will refuse
350 to run to prevent your changes from being lost.
351
352 In other words, there is no need to worry about what exists only
353 in the working tree. When you have local changes in a part of
354 the project that is not involved in the merge, your changes do
355 not interfere with the merge, and are kept intact. When they
356 *do* interfere, the merge does not even start ('git-read-tree'
357 complains loudly and fails without modifying anything). In such
358 a case, you can simply continue doing what you were in the
359 middle of doing, and when your working tree is ready (i.e. you
360 have finished your work-in-progress), attempt the merge again.
361
362
363 SEE ALSO
364 --------
365 linkgit:git-write-tree[1]; linkgit:git-ls-files[1];
366 linkgit:gitignore[5]
367
368
369 Author
370 ------
371 Written by Linus Torvalds <torvalds@osdl.org>
372
373 Documentation
374 --------------
375 Documentation by David Greaves, Junio C Hamano and the git-list <git@vger.kernel.org>.
376
377 GIT
378 ---
379 Part of the linkgit:git[1] suite