Do a cross-project merge of Paul Mackerras' gitk visualizer
[git/git.git] / Documentation / git-read-tree.txt
1 git-read-tree(1)
2 ================
3 v0.1, May 2005
4
5 NAME
6 ----
7 git-read-tree - Reads tree information into the directory cache
8
9
10 SYNOPSIS
11 --------
12 'git-read-tree' (<tree-ish> | [-m [-u]] <tree-ish1> [<tree-ish2> [<tree-ish3>]])
13
14
15 DESCRIPTION
16 -----------
17 Reads the tree information given by <tree-ish> into the directory cache,
18 but does not actually *update* any of the files it "caches". (see:
19 git-checkout-cache)
20
21 Optionally, it can merge a tree into the cache, perform a
22 fast-forward (i.e. 2-way) merge, or a 3-way merge, with the -m
23 flag. When used with -m, the -u flag causes it to also update
24 the files in the work tree with the result of the merge.
25
26 Trivial merges are done by "git-read-tree" itself. Only conflicting paths
27 will be in unmerged state when "git-read-tree" returns.
28
29 OPTIONS
30 -------
31 -m::
32 Perform a merge, not just a read.
33
34 -u::
35 After a successful merge, update the files in the work
36 tree with the result of the merge.
37
38 <tree-ish#>::
39 The id of the tree object(s) to be read/merged.
40
41
42 Merging
43 -------
44 If '-m' is specified, "git-read-tree" can performs 3 kinds of
45 merge, a single tree merge if only 1 tree is given, a
46 fast-forward merge with 2 trees, or a 3-way merge if 3 trees are
47 provided.
48
49
50 Single Tree Merge
51 ~~~~~~~~~~~~~~~~~
52 If only 1 tree is specified, git-read-tree operates as if the user did not
53 specify '-m', except that if the original cache has an entry for a
54 given pathname; and the contents of the path matches with the tree
55 being read, the stat info from the cache is used. (In other words, the
56 cache's stat()s take precedence over the merged tree's)
57
58 That means that if you do a "git-read-tree -m <newtree>" followed by a
59 "git-checkout-cache -f -u -a", the "git-checkout-cache" only checks out
60 the stuff that really changed.
61
62 This is used to avoid unnecessary false hits when "git-diff-files" is
63 run after git-read-tree.
64
65
66 Two Tree Merge
67 ~~~~~~~~~~~~~~
68
69 Typically, this is invoked as "git-read-tree -m $H $M", where $H
70 is the head commit of the current repository, and $M is the head
71 of a foreign tree, which is simply ahead of $H (i.e. we are in a
72 fast forward situation).
73
74 When two trees are specified, the user is telling git-read-tree
75 the following:
76
77 (1) The current index and work tree is derived from $H, but
78 the user may have local changes in them since $H;
79
80 (2) The user wants to fast-forward to $M.
81
82 In this case, the "git-read-tree -m $H $M" command makes sure
83 that no local change is lost as the result of this "merge".
84 Here are the "carry forward" rules:
85
86 I (index) H M Result
87 -------------------------------------------------------
88 0 nothing nothing nothing (does not happen)
89 1 nothing nothing exists use M
90 2 nothing exists nothing remove path from cache
91 3 nothing exists exists use M
92
93 clean I==H I==M
94 ------------------
95 4 yes N/A N/A nothing nothing keep index
96 5 no N/A N/A nothing nothing keep index
97
98 6 yes N/A yes nothing exists keep index
99 7 no N/A yes nothing exists keep index
100 8 yes N/A no nothing exists fail
101 9 no N/A no nothing exists fail
102
103 10 yes yes N/A exists nothing remove path from cache
104 11 no yes N/A exists nothing fail
105 12 yes no N/A exists nothing fail
106 13 no no N/A exists nothing fail
107
108 clean (H=M)
109 ------
110 14 yes exists exists keep index
111 15 no exists exists keep index
112
113 clean I==H I==M (H!=M)
114 ------------------
115 16 yes no no exists exists fail
116 17 no no no exists exists fail
117 18 yes no yes exists exists keep index
118 19 no no yes exists exists keep index
119 20 yes yes no exists exists use M
120 21 no yes no exists exists fail
121
122 In all "keep index" cases, the cache entry stays as in the
123 original index file. If the entry were not up to date,
124 git-read-tree keeps the copy in the work tree intact when
125 operating under the -u flag.
126
127 When this form of git-read-tree returns successfully, you can
128 see what "local changes" you made are carried forward by running
129 "git-diff-cache --cached $M". Note that this does not
130 necessarily match "git-diff-cache --cached $H" would have
131 produced before such a two tree merge. This is because of cases
132 18 and 19 --- if you already had the changes in $M (e.g. maybe
133 you picked it up via e-mail in a patch form), "git-diff-cache
134 --cached $H" would have told you about the change before this
135 merge, but it would not show in "git-diff-cache --cached $M"
136 output after two-tree merge.
137
138
139 3-Way Merge
140 ~~~~~~~~~~~
141 Each "index" entry has two bits worth of "stage" state. stage 0 is the
142 normal one, and is the only one you'd see in any kind of normal use.
143
144 However, when you do "git-read-tree" with three trees, the "stage"
145 starts out at 1.
146
147 This means that you can do
148
149 git-read-tree -m <tree1> <tree2> <tree3>
150
151 and you will end up with an index with all of the <tree1> entries in
152 "stage1", all of the <tree2> entries in "stage2" and all of the
153 <tree3> entries in "stage3".
154
155 Furthermore, "git-read-tree" has special-case logic that says: if you see
156 a file that matches in all respects in the following states, it
157 "collapses" back to "stage0":
158
159 - stage 2 and 3 are the same; take one or the other (it makes no
160 difference - the same work has been done on stage 2 and 3)
161
162 - stage 1 and stage 2 are the same and stage 3 is different; take
163 stage 3 (some work has been done on stage 3)
164
165 - stage 1 and stage 3 are the same and stage 2 is different take
166 stage 2 (some work has been done on stage 2)
167
168 The "git-write-tree" command refuses to write a nonsensical tree, and it
169 will complain about unmerged entries if it sees a single entry that is not
170 stage 0.
171
172 Ok, this all sounds like a collection of totally nonsensical rules,
173 but it's actually exactly what you want in order to do a fast
174 merge. The different stages represent the "result tree" (stage 0, aka
175 "merged"), the original tree (stage 1, aka "orig"), and the two trees
176 you are trying to merge (stage 2 and 3 respectively).
177
178 The order of stages 1, 2 and 3 (hence the order of three
179 <tree-ish> command line arguments) are significant when you
180 start a 3-way merge with an index file that is already
181 populated. Here is an outline of how the algorithm works:
182
183 - if a file exists in identical format in all three trees, it will
184 automatically collapse to "merged" state by git-read-tree.
185
186 - a file that has _any_ difference what-so-ever in the three trees
187 will stay as separate entries in the index. It's up to "script
188 policy" to determine how to remove the non-0 stages, and insert a
189 merged version.
190
191 - the index file saves and restores with all this information, so you
192 can merge things incrementally, but as long as it has entries in
193 stages 1/2/3 (ie "unmerged entries") you can't write the result. So
194 now the merge algorithm ends up being really simple:
195
196 * you walk the index in order, and ignore all entries of stage 0,
197 since they've already been done.
198
199 * if you find a "stage1", but no matching "stage2" or "stage3", you
200 know it's been removed from both trees (it only existed in the
201 original tree), and you remove that entry.
202
203 * if you find a matching "stage2" and "stage3" tree, you remove one
204 of them, and turn the other into a "stage0" entry. Remove any
205 matching "stage1" entry if it exists too. .. all the normal
206 trivial rules ..
207
208 You would normally use "git-merge-cache" with supplied
209 "git-merge-one-file-script" to do this last step. The script
210 does not touch the files in the work tree, and the entire merge
211 happens in the index file. In other words, there is no need to
212 worry about what is in the working directory, since it is never
213 shown and never used.
214
215 When you start a 3-way merge with an index file that is already
216 populated, it is assumed that it represents the state of the
217 files in your work tree, and you can even have files with
218 changes unrecorded in the index file. It is further assumed
219 that this state is "derived" from the stage 2 tree. The 3-way
220 merge refuses to run if it finds an entry in the original index
221 file that does not match stage 2.
222
223 This is done to prevent you from losing your work-in-progress
224 changes. To illustrate, suppose you start from what has been
225 commited last to your repository:
226
227 $ JC=`cat .git/HEAD`
228 $ git-checkout-cache -f -u -a $JC
229
230 You do random edits, without running git-update-cache. And then
231 you notice that the tip of your "upstream" tree has advanced
232 since you pulled from him:
233
234 $ git-fetch-script rsync://.... linus
235 $ LT=`cat .git/MERGE_HEAD`
236
237 Your work tree is still based on your HEAD ($JC), but you have
238 some edits since. Three-way merge makes sure that you have not
239 added or modified cache entries since $JC, and if you haven't,
240 then does the right thing. So with the following sequence:
241
242 $ git-read-tree -m -u `git-merge-base $JC $LT` $JC $LT
243 $ git-merge-cache git-merge-one-file-script -a
244 $ echo "Merge with Linus" | \
245 git-commit-tree `git-write-tree` -p $JC -p $LT
246
247 what you would commit is a pure merge between $JC and LT without
248 your work-in-progress changes, and your work tree would be
249 updated to the result of the merge.
250
251
252 See Also
253 --------
254 link:git-write-tree.html[git-write-tree]; link:git-ls-files.html[git-ls-files]
255
256
257 Author
258 ------
259 Written by Linus Torvalds <torvalds@osdl.org>
260
261 Documentation
262 --------------
263 Documentation by David Greaves, Junio C Hamano and the git-list <git@vger.kernel.org>.
264
265 GIT
266 ---
267 Part of the link:git.html[git] suite
268