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