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8ac866a8 | 1 | //////////////////////////////////////////////////////////////// |
6ad6d3d3 | 2 | |
e83c5163 LT |
3 | GIT - the stupid content tracker |
4 | ||
8ac866a8 | 5 | //////////////////////////////////////////////////////////////// |
e83c5163 LT |
6 | "git" can mean anything, depending on your mood. |
7 | ||
8 | - random three-letter combination that is pronounceable, and not | |
9 | actually used by any common UNIX command. The fact that it is a | |
90c4851b | 10 | mispronunciation of "get" may or may not be relevant. |
e83c5163 LT |
11 | - stupid. contemptible and despicable. simple. Take your pick from the |
12 | dictionary of slang. | |
13 | - "global information tracker": you're in a good mood, and it actually | |
14 | works for you. Angels sing, and a light suddenly fills the room. | |
15 | - "goddamn idiotic truckload of sh*t": when it breaks | |
16 | ||
17 | This is a stupid (but extremely fast) directory content manager. It | |
18 | doesn't do a whole lot, but what it _does_ do is track directory | |
19 | contents efficiently. | |
20 | ||
21 | There are two object abstractions: the "object database", and the | |
6ad6d3d3 LT |
22 | "current directory cache" aka "index". |
23 | ||
8ac866a8 DG |
24 | The Object Database |
25 | ~~~~~~~~~~~~~~~~~~~ | |
e83c5163 LT |
26 | The object database is literally just a content-addressable collection |
27 | of objects. All objects are named by their content, which is | |
28 | approximated by the SHA1 hash of the object itself. Objects may refer | |
8ac866a8 DG |
29 | to other objects (by referencing their SHA1 hash), and so you can |
30 | build up a hierarchy of objects. | |
e83c5163 | 31 | |
6ad6d3d3 LT |
32 | All objects have a statically determined "type" aka "tag", which is |
33 | determined at object creation time, and which identifies the format of | |
7096a645 | 34 | the object (i.e. how it is used, and how it can refer to other |
c4584ae3 JH |
35 | objects). There are currently four different object types: "blob", |
36 | "tree", "commit" and "tag". | |
6ad6d3d3 LT |
37 | |
38 | A "blob" object cannot refer to any other object, and is, like the tag | |
39 | implies, a pure storage object containing some user data. It is used to | |
90c4851b | 40 | actually store the file data, i.e. a blob object is associated with some |
6ad6d3d3 LT |
41 | particular version of some file. |
42 | ||
43 | A "tree" object is an object that ties one or more "blob" objects into a | |
44 | directory structure. In addition, a tree object can refer to other tree | |
45 | objects, thus creating a directory hierarchy. | |
46 | ||
7096a645 | 47 | A "commit" object ties such directory hierarchies together into |
6ad6d3d3 LT |
48 | a DAG of revisions - each "commit" is associated with exactly one tree |
49 | (the directory hierarchy at the time of the commit). In addition, a | |
50 | "commit" refers to one or more "parent" commit objects that describe the | |
51 | history of how we arrived at that directory hierarchy. | |
52 | ||
53 | As a special case, a commit object with no parents is called the "root" | |
54 | object, and is the point of an initial project commit. Each project | |
55 | must have at least one root, and while you can tie several different | |
56 | root objects together into one project by creating a commit object which | |
57 | has two or more separate roots as its ultimate parents, that's probably | |
58 | just going to confuse people. So aim for the notion of "one root object | |
59 | per project", even if git itself does not enforce that. | |
60 | ||
8ac866a8 DG |
61 | A "tag" object symbolically identifies and can be used to sign other |
62 | objects. It contains the identifier and type of another object, a | |
63 | symbolic name (of course!) and, optionally, a signature. | |
64 | ||
2aef5bba DG |
65 | Regardless of object type, all objects share the following |
66 | characteristics: they are all deflated with zlib, and have a header | |
67 | that not only specifies their tag, but also provides size information | |
68 | about the data in the object. It's worth noting that the SHA1 hash | |
c4584ae3 JH |
69 | that is used to name the object is the hash of the original data. |
70 | (Historical note: in the dawn of the age of git the hash | |
2aef5bba | 71 | was the sha1 of the _compressed_ object) |
6ad6d3d3 LT |
72 | |
73 | As a result, the general consistency of an object can always be tested | |
e83c5163 LT |
74 | independently of the contents or the type of the object: all objects can |
75 | be validated by verifying that (a) their hashes match the content of the | |
76 | file and (b) the object successfully inflates to a stream of bytes that | |
77 | forms a sequence of <ascii tag without space> + <space> + <ascii decimal | |
78 | size> + <byte\0> + <binary object data>. | |
79 | ||
8ac866a8 DG |
80 | The structured objects can further have their structure and |
81 | connectivity to other objects verified. This is generally done with | |
7096a645 DG |
82 | the "git-fsck-cache" program, which generates a full dependency graph |
83 | of all objects, and verifies their internal consistency (in addition | |
84 | to just verifying their superficial consistency through the hash). | |
6ad6d3d3 LT |
85 | |
86 | The object types in some more detail: | |
87 | ||
8ac866a8 DG |
88 | Blob Object |
89 | ~~~~~~~~~~~ | |
90 | A "blob" object is nothing but a binary blob of data, and doesn't | |
91 | refer to anything else. There is no signature or any other | |
92 | verification of the data, so while the object is consistent (it _is_ | |
93 | indexed by its sha1 hash, so the data itself is certainly correct), it | |
94 | has absolutely no other attributes. No name associations, no | |
95 | permissions. It is purely a blob of data (i.e. normally "file | |
96 | contents"). | |
97 | ||
98 | In particular, since the blob is entirely defined by its data, if two | |
99 | files in a directory tree (or in multiple different versions of the | |
100 | repository) have the same contents, they will share the same blob | |
101 | object. The object is totally independent of it's location in the | |
102 | directory tree, and renaming a file does not change the object that | |
103 | file is associated with in any way. | |
104 | ||
7096a645 DG |
105 | A blob is created with link:git-write-blob.html[git-write-blob] and |
106 | it's data can be accessed by link:git-cat-file.html[git-cat-file] | |
107 | ||
8ac866a8 DG |
108 | Tree Object |
109 | ~~~~~~~~~~~ | |
110 | The next hierarchical object type is the "tree" object. A tree object | |
111 | is a list of mode/name/blob data, sorted by name. Alternatively, the | |
112 | mode data may specify a directory mode, in which case instead of | |
113 | naming a blob, that name is associated with another TREE object. | |
114 | ||
115 | Like the "blob" object, a tree object is uniquely determined by the | |
116 | set contents, and so two separate but identical trees will always | |
117 | share the exact same object. This is true at all levels, i.e. it's | |
118 | true for a "leaf" tree (which does not refer to any other trees, only | |
119 | blobs) as well as for a whole subdirectory. | |
120 | ||
121 | For that reason a "tree" object is just a pure data abstraction: it | |
122 | has no history, no signatures, no verification of validity, except | |
123 | that since the contents are again protected by the hash itself, we can | |
124 | trust that the tree is immutable and its contents never change. | |
125 | ||
126 | So you can trust the contents of a tree to be valid, the same way you | |
127 | can trust the contents of a blob, but you don't know where those | |
128 | contents _came_ from. | |
129 | ||
130 | Side note on trees: since a "tree" object is a sorted list of | |
131 | "filename+content", you can create a diff between two trees without | |
132 | actually having to unpack two trees. Just ignore all common parts, | |
133 | and your diff will look right. In other words, you can effectively | |
134 | (and efficiently) tell the difference between any two random trees by | |
135 | O(n) where "n" is the size of the difference, rather than the size of | |
136 | the tree. | |
137 | ||
138 | Side note 2 on trees: since the name of a "blob" depends entirely and | |
139 | exclusively on its contents (i.e. there are no names or permissions | |
140 | involved), you can see trivial renames or permission changes by | |
141 | noticing that the blob stayed the same. However, renames with data | |
142 | changes need a smarter "diff" implementation. | |
143 | ||
7096a645 DG |
144 | A tree is created with link:git-write-tree.html[git-write-tree] and |
145 | it's data can be accessed by link:git-ls-tree.html[git-ls-tree] | |
8ac866a8 | 146 | |
7096a645 DG |
147 | Commit Object |
148 | ~~~~~~~~~~~~~ | |
149 | The "commit" object is an object that introduces the notion of | |
8ac866a8 DG |
150 | history into the picture. In contrast to the other objects, it |
151 | doesn't just describe the physical state of a tree, it describes how | |
152 | we got there, and why. | |
153 | ||
7096a645 DG |
154 | A "commit" is defined by the tree-object that it results in, the |
155 | parent commits (zero, one or more) that led up to that point, and a | |
156 | comment on what happened. Again, a commit is not trusted per se: | |
8ac866a8 DG |
157 | the contents are well-defined and "safe" due to the cryptographically |
158 | strong signatures at all levels, but there is no reason to believe | |
159 | that the tree is "good" or that the merge information makes sense. | |
160 | The parents do not have to actually have any relationship with the | |
161 | result, for example. | |
162 | ||
7096a645 DG |
163 | Note on commits: unlike real SCM's, commits do not contain |
164 | rename information or file mode chane information. All of that is | |
8ac866a8 DG |
165 | implicit in the trees involved (the result tree, and the result trees |
166 | of the parents), and describing that makes no sense in this idiotic | |
167 | file manager. | |
168 | ||
7096a645 DG |
169 | A commit is created with link:git-commit-tree.html[git-commit-tree] and |
170 | it's data can be accessed by link:git-cat-file.html[git-cat-file] | |
171 | ||
172 | Trust | |
173 | ~~~~~ | |
174 | An aside on the notion of "trust". Trust is really outside the scope | |
175 | of "git", but it's worth noting a few things. First off, since | |
176 | everything is hashed with SHA1, you _can_ trust that an object is | |
177 | intact and has not been messed with by external sources. So the name | |
178 | of an object uniquely identifies a known state - just not a state that | |
179 | you may want to trust. | |
8ac866a8 | 180 | |
7096a645 | 181 | Furthermore, since the SHA1 signature of a commit refers to the |
8ac866a8 | 182 | SHA1 signatures of the tree it is associated with and the signatures |
7096a645 | 183 | of the parent, a single named commit specifies uniquely a whole set |
8ac866a8 | 184 | of history, with full contents. You can't later fake any step of the |
7096a645 | 185 | way once you have the name of a commit. |
8ac866a8 DG |
186 | |
187 | So to introduce some real trust in the system, the only thing you need | |
188 | to do is to digitally sign just _one_ special note, which includes the | |
7096a645 DG |
189 | name of a top-level commit. Your digital signature shows others |
190 | that you trust that commit, and the immutability of the history of | |
191 | commits tells others that they can trust the whole history. | |
8ac866a8 DG |
192 | |
193 | In other words, you can easily validate a whole archive by just | |
194 | sending out a single email that tells the people the name (SHA1 hash) | |
7096a645 | 195 | of the top commit, and digitally sign that email using something |
8ac866a8 DG |
196 | like GPG/PGP. |
197 | ||
7096a645 | 198 | To assist in this, git also provides the tag object... |
8ac866a8 | 199 | |
7096a645 DG |
200 | Tag Object |
201 | ~~~~~~~~~~ | |
202 | Git provides the "tag" object to simplify creating, managing and | |
203 | exchanging symbolic and signed tokens. The "tag" object at its | |
204 | simplest simply symbolically identifies another object by containing | |
205 | the sha1, type and symbolic name. | |
8ac866a8 | 206 | |
7096a645 DG |
207 | However it can optionally contain additional signature information |
208 | (which git doesn't care about as long as there's less than 8k of | |
209 | it). This can then be verified externally to git. | |
8ac866a8 | 210 | |
7096a645 DG |
211 | Note that despite the tag features, "git" itself only handles content |
212 | integrity; the trust framework (and signature provision and | |
213 | verification) has to come from outside. | |
8ac866a8 | 214 | |
7096a645 DG |
215 | A tag is created with link:git-mktag.html[git-mktag] and |
216 | it's data can be accessed by link:git-cat-file.html[git-cat-file] | |
8ac866a8 | 217 | |
2aef5bba | 218 | |
8ac866a8 DG |
219 | The "index" aka "Current Directory Cache" |
220 | ----------------------------------------- | |
6ad6d3d3 LT |
221 | The index is a simple binary file, which contains an efficient |
222 | representation of a virtual directory content at some random time. It | |
223 | does so by a simple array that associates a set of names, dates, | |
224 | permissions and content (aka "blob") objects together. The cache is | |
225 | always kept ordered by name, and names are unique (with a few very | |
226 | specific rules) at any point in time, but the cache has no long-term | |
8ac866a8 | 227 | meaning, and can be partially updated at any time. |
6ad6d3d3 LT |
228 | |
229 | In particular, the index certainly does not need to be consistent with | |
230 | the current directory contents (in fact, most operations will depend on | |
231 | different ways to make the index _not_ be consistent with the directory | |
232 | hierarchy), but it has three very important attributes: | |
e83c5163 | 233 | |
8ac866a8 DG |
234 | '(a) it can re-generate the full state it caches (not just the |
235 | directory structure: it contains pointers to the "blob" objects so | |
236 | that it can regenerate the data too)' | |
e83c5163 | 237 | |
8ac866a8 DG |
238 | As a special case, there is a clear and unambiguous one-way mapping |
239 | from a current directory cache to a "tree object", which can be | |
240 | efficiently created from just the current directory cache without | |
241 | actually looking at any other data. So a directory cache at any one | |
242 | time uniquely specifies one and only one "tree" object (but has | |
243 | additional data to make it easy to match up that tree object with what | |
244 | has happened in the directory) | |
e83c5163 | 245 | |
8ac866a8 DG |
246 | '(b) it has efficient methods for finding inconsistencies between that |
247 | cached state ("tree object waiting to be instantiated") and the | |
248 | current state.' | |
e83c5163 | 249 | |
8ac866a8 DG |
250 | '(c) it can additionally efficiently represent information about merge |
251 | conflicts between different tree objects, allowing each pathname to be | |
252 | associated with sufficient information about the trees involved that | |
253 | you can create a three-way merge between them.' | |
6ad6d3d3 LT |
254 | |
255 | Those are the three ONLY things that the directory cache does. It's a | |
e83c5163 LT |
256 | cache, and the normal operation is to re-generate it completely from a |
257 | known tree object, or update/compare it with a live tree that is being | |
6ad6d3d3 LT |
258 | developed. If you blow the directory cache away entirely, you generally |
259 | haven't lost any information as long as you have the name of the tree | |
260 | that it described. | |
261 | ||
262 | At the same time, the directory index is at the same time also the | |
263 | staging area for creating new trees, and creating a new tree always | |
264 | involves a controlled modification of the index file. In particular, | |
265 | the index file can have the representation of an intermediate tree that | |
266 | has not yet been instantiated. So the index can be thought of as a | |
267 | write-back cache, which can contain dirty information that has not yet | |
8ac866a8 | 268 | been written back to the backing store. |
6ad6d3d3 LT |
269 | |
270 | ||
271 | ||
8ac866a8 DG |
272 | The Workflow |
273 | ------------ | |
6ad6d3d3 | 274 | Generally, all "git" operations work on the index file. Some operations |
8ac866a8 | 275 | work *purely* on the index file (showing the current state of the |
6ad6d3d3 LT |
276 | index), but most operations move data to and from the index file. Either |
277 | from the database or from the working directory. Thus there are four | |
278 | main combinations: | |
279 | ||
8ac866a8 DG |
280 | 1) working directory -> index |
281 | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ | |
6ad6d3d3 | 282 | |
8ac866a8 | 283 | You update the index with information from the working directory with |
7096a645 DG |
284 | the link:git-update-cache.html[git-update-cache] command. You |
285 | generally update the index information by just specifying the filename | |
286 | you want to update, like so: | |
6ad6d3d3 | 287 | |
7096a645 | 288 | git-update-cache filename |
6ad6d3d3 | 289 | |
8ac866a8 DG |
290 | but to avoid common mistakes with filename globbing etc, the command |
291 | will not normally add totally new entries or remove old entries, | |
292 | i.e. it will normally just update existing cache entries. | |
6ad6d3d3 | 293 | |
8ac866a8 DG |
294 | To tell git that yes, you really do realize that certain files no |
295 | longer exist in the archive, or that new files should be added, you | |
296 | should use the "--remove" and "--add" flags respectively. | |
6ad6d3d3 | 297 | |
8ac866a8 DG |
298 | NOTE! A "--remove" flag does _not_ mean that subsequent filenames will |
299 | necessarily be removed: if the files still exist in your directory | |
300 | structure, the index will be updated with their new status, not | |
301 | removed. The only thing "--remove" means is that update-cache will be | |
302 | considering a removed file to be a valid thing, and if the file really | |
303 | does not exist any more, it will update the index accordingly. | |
6ad6d3d3 | 304 | |
7096a645 | 305 | As a special case, you can also do "git-update-cache --refresh", which |
8ac866a8 DG |
306 | will refresh the "stat" information of each index to match the current |
307 | stat information. It will _not_ update the object status itself, and | |
308 | it will only update the fields that are used to quickly test whether | |
309 | an object still matches its old backing store object. | |
6ad6d3d3 | 310 | |
8ac866a8 DG |
311 | 2) index -> object database |
312 | ~~~~~~~~~~~~~~~~~~~~~~~~~~~ | |
6ad6d3d3 | 313 | |
8ac866a8 | 314 | You write your current index file to a "tree" object with the program |
6ad6d3d3 | 315 | |
7096a645 | 316 | git-write-tree |
6ad6d3d3 | 317 | |
8ac866a8 DG |
318 | that doesn't come with any options - it will just write out the |
319 | current index into the set of tree objects that describe that state, | |
320 | and it will return the name of the resulting top-level tree. You can | |
321 | use that tree to re-generate the index at any time by going in the | |
322 | other direction: | |
6ad6d3d3 | 323 | |
8ac866a8 DG |
324 | 3) object database -> index |
325 | ~~~~~~~~~~~~~~~~~~~~~~~~~~~ | |
6ad6d3d3 | 326 | |
8ac866a8 DG |
327 | You read a "tree" file from the object database, and use that to |
328 | populate (and overwrite - don't do this if your index contains any | |
329 | unsaved state that you might want to restore later!) your current | |
330 | index. Normal operation is just | |
6ad6d3d3 | 331 | |
7096a645 | 332 | git-read-tree <sha1 of tree> |
6ad6d3d3 | 333 | |
8ac866a8 DG |
334 | and your index file will now be equivalent to the tree that you saved |
335 | earlier. However, that is only your _index_ file: your working | |
336 | directory contents have not been modified. | |
6ad6d3d3 | 337 | |
8ac866a8 DG |
338 | 4) index -> working directory |
339 | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ | |
6ad6d3d3 | 340 | |
8ac866a8 DG |
341 | You update your working directory from the index by "checking out" |
342 | files. This is not a very common operation, since normally you'd just | |
343 | keep your files updated, and rather than write to your working | |
344 | directory, you'd tell the index files about the changes in your | |
7096a645 | 345 | working directory (i.e. "git-update-cache"). |
6ad6d3d3 | 346 | |
8ac866a8 DG |
347 | However, if you decide to jump to a new version, or check out somebody |
348 | else's version, or just restore a previous tree, you'd populate your | |
349 | index file with read-tree, and then you need to check out the result | |
350 | with | |
7096a645 | 351 | git-checkout-cache filename |
6ad6d3d3 | 352 | |
8ac866a8 | 353 | or, if you want to check out all of the index, use "-a". |
6ad6d3d3 | 354 | |
7096a645 DG |
355 | NOTE! git-checkout-cache normally refuses to overwrite old files, so |
356 | if you have an old version of the tree already checked out, you will | |
357 | need to use the "-f" flag (_before_ the "-a" flag or the filename) to | |
8ac866a8 | 358 | _force_ the checkout. |
6ad6d3d3 LT |
359 | |
360 | ||
8ac866a8 DG |
361 | Finally, there are a few odds and ends which are not purely moving |
362 | from one representation to the other: | |
6ad6d3d3 | 363 | |
8ac866a8 DG |
364 | 5) Tying it all together |
365 | ~~~~~~~~~~~~~~~~~~~~~~~~ | |
7096a645 DG |
366 | To commit a tree you have instantiated with "git-write-tree", you'd |
367 | create a "commit" object that refers to that tree and the history | |
368 | behind it - most notably the "parent" commits that preceded it in | |
369 | history. | |
6ad6d3d3 | 370 | |
8ac866a8 DG |
371 | Normally a "commit" has one parent: the previous state of the tree |
372 | before a certain change was made. However, sometimes it can have two | |
373 | or more parent commits, in which case we call it a "merge", due to the | |
374 | fact that such a commit brings together ("merges") two or more | |
375 | previous states represented by other commits. | |
6ad6d3d3 | 376 | |
8ac866a8 DG |
377 | In other words, while a "tree" represents a particular directory state |
378 | of a working directory, a "commit" represents that state in "time", | |
379 | and explains how we got there. | |
6ad6d3d3 | 380 | |
8ac866a8 DG |
381 | You create a commit object by giving it the tree that describes the |
382 | state at the time of the commit, and a list of parents: | |
6ad6d3d3 | 383 | |
7096a645 | 384 | git-commit-tree <tree> -p <parent> [-p <parent2> ..] |
6ad6d3d3 | 385 | |
8ac866a8 DG |
386 | and then giving the reason for the commit on stdin (either through |
387 | redirection from a pipe or file, or by just typing it at the tty). | |
6ad6d3d3 | 388 | |
7096a645 DG |
389 | git-commit-tree will return the name of the object that represents |
390 | that commit, and you should save it away for later use. Normally, | |
391 | you'd commit a new "HEAD" state, and while git doesn't care where you | |
392 | save the note about that state, in practice we tend to just write the | |
8ac866a8 DG |
393 | result to the file ".git/HEAD", so that we can always see what the |
394 | last committed state was. | |
6ad6d3d3 | 395 | |
8ac866a8 DG |
396 | 6) Examining the data |
397 | ~~~~~~~~~~~~~~~~~~~~~ | |
6ad6d3d3 | 398 | |
8ac866a8 DG |
399 | You can examine the data represented in the object database and the |
400 | index with various helper tools. For every object, you can use | |
7096a645 DG |
401 | link:git-cat-file.html[git-cat-file] to examine details about the |
402 | object: | |
6ad6d3d3 | 403 | |
7096a645 | 404 | git-cat-file -t <objectname> |
6ad6d3d3 | 405 | |
8ac866a8 DG |
406 | shows the type of the object, and once you have the type (which is |
407 | usually implicit in where you find the object), you can use | |
6ad6d3d3 | 408 | |
7096a645 | 409 | git-cat-file blob|tree|commit <objectname> |
6ad6d3d3 | 410 | |
8ac866a8 | 411 | to show its contents. NOTE! Trees have binary content, and as a result |
7096a645 DG |
412 | there is a special helper for showing that content, called |
413 | "git-ls-tree", which turns the binary content into a more easily | |
414 | readable form. | |
6ad6d3d3 | 415 | |
8ac866a8 DG |
416 | It's especially instructive to look at "commit" objects, since those |
417 | tend to be small and fairly self-explanatory. In particular, if you | |
418 | follow the convention of having the top commit name in ".git/HEAD", | |
419 | you can do | |
6ad6d3d3 | 420 | |
7096a645 | 421 | git-cat-file commit $(cat .git/HEAD) |
6ad6d3d3 | 422 | |
8ac866a8 | 423 | to see what the top commit was. |
6ad6d3d3 | 424 | |
8ac866a8 DG |
425 | 7) Merging multiple trees |
426 | ~~~~~~~~~~~~~~~~~~~~~~~~~ | |
6ad6d3d3 | 427 | |
8ac866a8 DG |
428 | Git helps you do a three-way merge, which you can expand to n-way by |
429 | repeating the merge procedure arbitrary times until you finally | |
430 | "commit" the state. The normal situation is that you'd only do one | |
431 | three-way merge (two parents), and commit it, but if you like to, you | |
432 | can do multiple parents in one go. | |
6ad6d3d3 | 433 | |
8ac866a8 DG |
434 | To do a three-way merge, you need the two sets of "commit" objects |
435 | that you want to merge, use those to find the closest common parent (a | |
436 | third "commit" object), and then use those commit objects to find the | |
437 | state of the directory ("tree" object) at these points. | |
6ad6d3d3 | 438 | |
8ac866a8 DG |
439 | To get the "base" for the merge, you first look up the common parent |
440 | of two commits with | |
6ad6d3d3 | 441 | |
7096a645 | 442 | git-merge-base <commit1> <commit2> |
6ad6d3d3 | 443 | |
8ac866a8 DG |
444 | which will return you the commit they are both based on. You should |
445 | now look up the "tree" objects of those commits, which you can easily | |
446 | do with (for example) | |
6ad6d3d3 | 447 | |
7096a645 | 448 | git-cat-file commit <commitname> | head -1 |
6ad6d3d3 | 449 | |
8ac866a8 DG |
450 | since the tree object information is always the first line in a commit |
451 | object. | |
452 | ||
453 | Once you know the three trees you are going to merge (the one | |
454 | "original" tree, aka the common case, and the two "result" trees, aka | |
455 | the branches you want to merge), you do a "merge" read into the | |
456 | index. This will throw away your old index contents, so you should | |
457 | make sure that you've committed those - in fact you would normally | |
458 | always do a merge against your last commit (which should thus match | |
459 | what you have in your current index anyway). | |
6ad6d3d3 | 460 | |
8ac866a8 | 461 | To do the merge, do |
6ad6d3d3 | 462 | |
7096a645 | 463 | git-read-tree -m <origtree> <target1tree> <target2tree> |
6ad6d3d3 | 464 | |
8ac866a8 | 465 | which will do all trivial merge operations for you directly in the |
7096a645 DG |
466 | index file, and you can just write the result out with |
467 | "git-write-tree". | |
6ad6d3d3 | 468 | |
8ac866a8 DG |
469 | NOTE! Because the merge is done in the index file, and not in your |
470 | working directory, your working directory will no longer match your | |
7096a645 DG |
471 | index. You can use "git-checkout-cache -f -a" to make the effect of |
472 | the merge be seen in your working directory. | |
6ad6d3d3 | 473 | |
8ac866a8 DG |
474 | NOTE2! Sadly, many merges aren't trivial. If there are files that have |
475 | been added.moved or removed, or if both branches have modified the | |
476 | same file, you will be left with an index tree that contains "merge | |
477 | entries" in it. Such an index tree can _NOT_ be written out to a tree | |
478 | object, and you will have to resolve any such merge clashes using | |
479 | other tools before you can write out the result. | |
6ad6d3d3 | 480 | |
6ad6d3d3 | 481 | |
8ac866a8 | 482 | [ fixme: talk about resolving merges here ] |