read-tree: migrate to parse-options
[git/git.git] / Documentation / git-read-tree.txt
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1git-read-tree(1)
2================
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3
4NAME
5----
5f3aa197 6git-read-tree - Reads tree information into the index
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7
8
9SYNOPSIS
10--------
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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>]]
ccef66b5 15
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16
17DESCRIPTION
18-----------
5f3aa197 19Reads the tree information given by <tree-ish> into the index,
c1bdacf9 20but does not actually *update* any of the files it "caches". (see:
5162e697 21linkgit:git-checkout-index[1])
2cf565c5 22
5f3aa197 23Optionally, it can merge a tree into the index, perform a
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24fast-forward (i.e. 2-way) merge, or a 3-way merge, with the `-m`
25flag. When used with `-m`, the `-u` flag causes it to also update
ccef66b5 26the files in the work tree with the result of the merge.
2cf565c5 27
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28Trivial merges are done by 'git-read-tree' itself. Only conflicting paths
29will be in unmerged state when 'git-read-tree' returns.
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30
31OPTIONS
32-------
33-m::
3f41f5a9 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.
ccef66b5 38
2db0bfbc 39--reset::
3f41f5a9 40 Same as -m, except that unmerged entries are discarded
41 instead of failing.
2db0bfbc 42
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43-u::
44 After a successful merge, update the files in the work
45 tree with the result of the merge.
2cf565c5 46
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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
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56-v::
57 Show the progress of checking files out.
58
6da08783 59--trivial::
ba020ef5 60 Restrict three-way merge by 'git-read-tree' to happen
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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
afaa8d66 65--aggressive::
ba020ef5 66 Usually a three-way merge by 'git-read-tree' resolves
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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
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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
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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.
f4c6f2d3 100
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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
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113<tree-ish#>::
114 The id of the tree object(s) to be read/merged.
115
116
117Merging
118-------
ba020ef5 119If `-m` is specified, 'git-read-tree' can perform 3 kinds of
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120merge, a single tree merge if only 1 tree is given, a
121fast-forward merge with 2 trees, or a 3-way merge if 3 trees are
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122provided.
123
ccef66b5 124
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125Single Tree Merge
126~~~~~~~~~~~~~~~~~
ba020ef5 127If only 1 tree is specified, 'git-read-tree' operates as if the user did not
61f693bd 128specify `-m`, except that if the original index has an entry for a
2c6e4771 129given pathname, and the contents of the path matches with the tree
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130being read, the stat info from the index is used. (In other words, the
131index's stat()s take precedence over the merged tree's).
2cf565c5 132
b1889c36 133That means that if you do a `git read-tree -m <newtree>` followed by a
ba020ef5 134`git checkout-index -f -u -a`, the 'git-checkout-index' only checks out
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135the stuff that really changed.
136
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137This is used to avoid unnecessary false hits when 'git-diff-files' is
138run after 'git-read-tree'.
2cf565c5 139
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140
141Two Tree Merge
142~~~~~~~~~~~~~~
143
b1889c36 144Typically, this is invoked as `git read-tree -m $H $M`, where $H
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145is the head commit of the current repository, and $M is the head
146of a foreign tree, which is simply ahead of $H (i.e. we are in a
147fast forward situation).
148
ba020ef5 149When two trees are specified, the user is telling 'git-read-tree'
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150the following:
151
df8baa42 152 1. The current index and work tree is derived from $H, but
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153 the user may have local changes in them since $H;
154
df8baa42 155 2. The user wants to fast-forward to $M.
c8596009 156
b1889c36 157In this case, the `git read-tree -m $H $M` command makes sure
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158that no local change is lost as the result of this "merge".
159Here 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
5f3aa197 165 2 nothing exists nothing remove path from index
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166 3 nothing exists exists, use M if "initial checkout"
167 H == M keep index otherwise
168 exists fail
169 H != M
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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
5f3aa197 181 10 yes yes N/A exists nothing remove path from index
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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
5f3aa197 200In all "keep index" cases, the index entry stays as in the
c8596009 201original index file. If the entry were not up to date,
ba020ef5 202'git-read-tree' keeps the copy in the work tree intact when
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203operating under the -u flag.
204
ba020ef5 205When this form of 'git-read-tree' returns successfully, you can
c8596009 206see what "local changes" you made are carried forward by running
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207`git diff-index --cached $M`. Note that this does not
208necessarily match `git diff-index --cached $H` would have
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209produced before such a two tree merge. This is because of cases
21018 and 19 --- if you already had the changes in $M (e.g. maybe
b1889c36 211you picked it up via e-mail in a patch form), `git diff-index
61f693bd 212--cached $H` would have told you about the change before this
b1889c36 213merge, but it would not show in `git diff-index --cached $M`
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214output after two-tree merge.
215
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216Case #3 is slightly tricky and needs explanation. The result from this
217rule logically should be to remove the path if the user staged the removal
79fd4cc7 218of the path and then switching to a new branch. That however will prevent
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219the initial checkout from happening, so the rule is modified to use M (new
220tree) only when the contents of the index is empty. Otherwise the removal
221of the path is kept as long as $H and $M are the same.
c8596009 222
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2233-Way Merge
224~~~~~~~~~~~
225Each "index" entry has two bits worth of "stage" state. stage 0 is the
226normal one, and is the only one you'd see in any kind of normal use.
227
ba020ef5 228However, when you do 'git-read-tree' with three trees, the "stage"
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229starts out at 1.
230
231This means that you can do
232
61f693bd 233----------------
b1889c36 234$ git read-tree -m <tree1> <tree2> <tree3>
61f693bd 235----------------
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236
237and 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
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239<tree3> entries in "stage3". When performing a merge of another
240branch into the current branch, we use the common ancestor tree
241as <tree1>, the current branch head as <tree2>, and the other
242branch head as <tree3>.
2cf565c5 243
ba020ef5 244Furthermore, 'git-read-tree' has special-case logic that says: if you see
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245a 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
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249 difference - the same work has been done on our branch in
250 stage 2 and their branch in stage 3)
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251
252 - stage 1 and stage 2 are the same and stage 3 is different; take
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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)
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256
257 - stage 1 and stage 3 are the same and stage 2 is different take
bb6d7b89 258 stage 2 (we did something while they did nothing)
2cf565c5 259
ba020ef5 260The 'git-write-tree' command refuses to write a nonsensical tree, and it
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261will complain about unmerged entries if it sees a single entry that is not
262stage 0.
263
abda1ef5 264OK, this all sounds like a collection of totally nonsensical rules,
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265but it's actually exactly what you want in order to do a fast
266merge. The different stages represent the "result tree" (stage 0, aka
267"merged"), the original tree (stage 1, aka "orig"), and the two trees
268you are trying to merge (stage 2 and 3 respectively).
269
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270The order of stages 1, 2 and 3 (hence the order of three
271<tree-ish> command line arguments) are significant when you
272start a 3-way merge with an index file that is already
273populated. Here is an outline of how the algorithm works:
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274
275- if a file exists in identical format in all three trees, it will
ba020ef5 276 automatically collapse to "merged" state by 'git-read-tree'.
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277
278- a file that has _any_ difference what-so-ever in the three trees
2c6e4771 279 will stay as separate entries in the index. It's up to "porcelain
2cf565c5 280 policy" to determine how to remove the non-0 stages, and insert a
ccef66b5 281 merged version.
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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
abda1ef5 285 stages 1/2/3 (i.e., "unmerged entries") you can't write the result. So
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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
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300You would normally use 'git-merge-index' with supplied
301'git-merge-one-file' to do this last step. The script updates
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302the files in the working tree as it merges each path and at the
303end of a successful merge.
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304
305When you start a 3-way merge with an index file that is already
306populated, it is assumed that it represents the state of the
307files in your work tree, and you can even have files with
308changes unrecorded in the index file. It is further assumed
309that this state is "derived" from the stage 2 tree. The 3-way
310merge refuses to run if it finds an entry in the original index
311file that does not match stage 2.
312
313This is done to prevent you from losing your work-in-progress
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314changes, and mixing your random changes in an unrelated merge
315commit. To illustrate, suppose you start from what has been
37425065 316committed last to your repository:
ccef66b5 317
61f693bd 318----------------
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319$ JC=`git rev-parse --verify "HEAD^0"`
320$ git checkout-index -f -u -a $JC
61f693bd 321----------------
ccef66b5 322
ba020ef5 323You do random edits, without running 'git-update-index'. And then
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324you notice that the tip of your "upstream" tree has advanced
325since you pulled from him:
326
61f693bd 327----------------
b1889c36 328$ git fetch git://.... linus
bb6d7b89 329$ LT=`cat .git/FETCH_HEAD`
61f693bd 330----------------
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331
332Your work tree is still based on your HEAD ($JC), but you have
333some edits since. Three-way merge makes sure that you have not
5f3aa197 334added or modified index entries since $JC, and if you haven't,
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335then does the right thing. So with the following sequence:
336
61f693bd 337----------------
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338$ git read-tree -m -u `git merge-base $JC $LT` $JC $LT
339$ git merge-index git-merge-one-file -a
61f693bd 340$ echo "Merge with Linus" | \
b1889c36 341 git commit-tree `git write-tree` -p $JC -p $LT
61f693bd 342----------------
ccef66b5 343
61f693bd 344what you would commit is a pure merge between $JC and $LT without
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345your work-in-progress changes, and your work tree would be
346updated to the result of the merge.
347
bb6d7b89 348However, if you have local changes in the working tree that
ba020ef5 349would be overwritten by this merge, 'git-read-tree' will refuse
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350to run to prevent your changes from being lost.
351
352In other words, there is no need to worry about what exists only
353in the working tree. When you have local changes in a part of
354the project that is not involved in the merge, your changes do
355not interfere with the merge, and are kept intact. When they
ba020ef5 356*do* interfere, the merge does not even start ('git-read-tree'
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357complains loudly and fails without modifying anything). In such
358a case, you can simply continue doing what you were in the
359middle of doing, and when your working tree is ready (i.e. you
360have finished your work-in-progress), attempt the merge again.
361
2cf565c5 362
56ae8df5 363SEE ALSO
c1bdacf9 364--------
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365linkgit:git-write-tree[1]; linkgit:git-ls-files[1];
366linkgit:gitignore[5]
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367
368
369Author
370------
371Written by Linus Torvalds <torvalds@osdl.org>
372
373Documentation
374--------------
375Documentation by David Greaves, Junio C Hamano and the git-list <git@vger.kernel.org>.
376
377GIT
378---
9e1f0a85 379Part of the linkgit:git[1] suite