Merge master.kernel.org:/pub/scm/gitk/gitk
[git/git.git] / Documentation / tutorial.txt
1 A short git tutorial
2 ====================
3 May 2005
4
5
6 Introduction
7 ------------
8
9 This is trying to be a short tutorial on setting up and using a git
10 archive, mainly because being hands-on and using explicit examples is
11 often the best way of explaining what is going on.
12
13 In normal life, most people wouldn't use the "core" git programs
14 directly, but rather script around them to make them more palatable.
15 Understanding the core git stuff may help some people get those scripts
16 done, though, and it may also be instructive in helping people
17 understand what it is that the higher-level helper scripts are actually
18 doing.
19
20 The core git is often called "plumbing", with the prettier user
21 interfaces on top of it called "porcelain". You may not want to use the
22 plumbing directly very often, but it can be good to know what the
23 plumbing does for when the porcelain isn't flushing...
24
25
26 Creating a git archive
27 ----------------------
28
29 Creating a new git archive couldn't be easier: all git archives start
30 out empty, and the only thing you need to do is find yourself a
31 subdirectory that you want to use as a working tree - either an empty
32 one for a totally new project, or an existing working tree that you want
33 to import into git.
34
35 For our first example, we're going to start a totally new archive from
36 scratch, with no pre-existing files, and we'll call it "git-tutorial".
37 To start up, create a subdirectory for it, change into that
38 subdirectory, and initialize the git infrastructure with "git-init-db":
39
40 mkdir git-tutorial
41 cd git-tutorial
42 git-init-db
43
44 to which git will reply
45
46 defaulting to local storage area
47
48 which is just git's way of saying that you haven't been doing anything
49 strange, and that it will have created a local .git directory setup for
50 your new project. You will now have a ".git" directory, and you can
51 inspect that with "ls". For your new empty project, ls should show you
52 three entries:
53
54 - a symlink called HEAD, pointing to "refs/heads/master"
55
56 Don't worry about the fact that the file that the HEAD link points to
57 doesn't even exist yet - you haven't created the commit that will
58 start your HEAD development branch yet.
59
60 - a subdirectory called "objects", which will contain all the git SHA1
61 objects of your project. You should never have any real reason to
62 look at the objects directly, but you might want to know that these
63 objects are what contains all the real _data_ in your repository.
64
65 - a subdirectory called "refs", which contains references to objects.
66
67 In particular, the "refs" subdirectory will contain two other
68 subdirectories, named "heads" and "tags" respectively. They do
69 exactly what their names imply: they contain references to any number
70 of different "heads" of development (aka "branches"), and to any
71 "tags" that you have created to name specific versions of your
72 repository.
73
74 One note: the special "master" head is the default branch, which is
75 why the .git/HEAD file was created as a symlink to it even if it
76 doesn't yet exist. Basically, the HEAD link is supposed to always
77 point to the branch you are working on right now, and you always
78 start out expecting to work on the "master" branch.
79
80 However, this is only a convention, and you can name your branches
81 anything you want, and don't have to ever even _have_ a "master"
82 branch. A number of the git tools will assume that .git/HEAD is
83 valid, though.
84
85 [ Implementation note: an "object" is identified by its 160-bit SHA1
86 hash, aka "name", and a reference to an object is always the 40-byte
87 hex representation of that SHA1 name. The files in the "refs"
88 subdirectory are expected to contain these hex references (usually
89 with a final '\n' at the end), and you should thus expect to see a
90 number of 41-byte files containing these references in this refs
91 subdirectories when you actually start populating your tree ]
92
93 You have now created your first git archive. Of course, since it's
94 empty, that's not very useful, so let's start populating it with data.
95
96
97 Populating a git archive
98 ------------------------
99
100 We'll keep this simple and stupid, so we'll start off with populating a
101 few trivial files just to get a feel for it.
102
103 Start off with just creating any random files that you want to maintain
104 in your git archive. We'll start off with a few bad examples, just to
105 get a feel for how this works:
106
107 echo "Hello World" > a
108 echo "Silly example" > b
109
110 you have now created two files in your working directory, but to
111 actually check in your hard work, you will have to go through two steps:
112
113 - fill in the "cache" aka "index" file with the information about your
114 working directory state
115
116 - commit that index file as an object.
117
118 The first step is trivial: when you want to tell git about any changes
119 to your working directory, you use the "git-update-cache" program. That
120 program normally just takes a list of filenames you want to update, but
121 to avoid trivial mistakes, it refuses to add new entries to the cache
122 (or remove existing ones) unless you explicitly tell it that you're
123 adding a new entry with the "--add" flag (or removing an entry with the
124 "--remove") flag.
125
126 So to populate the index with the two files you just created, you can do
127
128 git-update-cache --add a b
129
130 and you have now told git to track those two files.
131
132 In fact, as you did that, if you now look into your object directory,
133 you'll notice that git will have added two new objects to the object
134 store. If you did exactly the steps above, you should now be able to do
135
136 ls .git/objects/??/*
137
138 and see two files:
139
140 .git/objects/55/7db03de997c86a4a028e1ebd3a1ceb225be238
141 .git/objects/f2/4c74a2e500f5ee1332c86b94199f52b1d1d962
142
143 which correspond with the object with SHA1 names of 557db... and f24c7..
144 respectively.
145
146 If you want to, you can use "git-cat-file" to look at those objects, but
147 you'll have to use the object name, not the filename of the object:
148
149 git-cat-file -t 557db03de997c86a4a028e1ebd3a1ceb225be238
150
151 where the "-t" tells git-cat-file to tell you what the "type" of the
152 object is. Git will tell you that you have a "blob" object (ie just a
153 regular file), and you can see the contents with
154
155 git-cat-file "blob" 557db03de997c86a4a028e1ebd3a1ceb225be238
156
157 which will print out "Hello World". The object 557db... is nothing
158 more than the contents of your file "a".
159
160 [ Digression: don't confuse that object with the file "a" itself. The
161 object is literally just those specific _contents_ of the file, and
162 however much you later change the contents in file "a", the object we
163 just looked at will never change. Objects are immutable. ]
164
165 Anyway, as we mentioned previously, you normally never actually take a
166 look at the objects themselves, and typing long 40-character hex SHA1
167 names is not something you'd normally want to do. The above digression
168 was just to show that "git-update-cache" did something magical, and
169 actually saved away the contents of your files into the git content
170 store.
171
172 Updating the cache did something else too: it created a ".git/index"
173 file. This is the index that describes your current working tree, and
174 something you should be very aware of. Again, you normally never worry
175 about the index file itself, but you should be aware of the fact that
176 you have not actually really "checked in" your files into git so far,
177 you've only _told_ git about them.
178
179 However, since git knows about them, you can now start using some of the
180 most basic git commands to manipulate the files or look at their status.
181
182 In particular, let's not even check in the two files into git yet, we'll
183 start off by adding another line to "a" first:
184
185 echo "It's a new day for git" >> a
186
187 and you can now, since you told git about the previous state of "a", ask
188 git what has changed in the tree compared to your old index, using the
189 "git-diff-files" command:
190
191 git-diff-files
192
193 oops. That wasn't very readable. It just spit out its own internal
194 version of a "diff", but that internal version really just tells you
195 that it has noticed that "a" has been modified, and that the old object
196 contents it had have been replaced with something else.
197
198 To make it readable, we can tell git-diff-files to output the
199 differences as a patch, using the "-p" flag:
200
201 git-diff-files -p
202
203 which will spit out
204
205 diff --git a/a b/a
206 --- a/a
207 +++ b/a
208 @@ -1 +1,2 @@
209 Hello World
210 +It's a new day for git
211
212 ie the diff of the change we caused by adding another line to "a".
213
214 In other words, git-diff-files always shows us the difference between
215 what is recorded in the index, and what is currently in the working
216 tree. That's very useful.
217
218
219 Committing git state
220 --------------------
221
222 Now, we want to go to the next stage in git, which is to take the files
223 that git knows about in the index, and commit them as a real tree. We do
224 that in two phases: creating a "tree" object, and committing that "tree"
225 object as a "commit" object together with an explanation of what the
226 tree was all about, along with information of how we came to that state.
227
228 Creating a tree object is trivial, and is done with "git-write-tree".
229 There are no options or other input: git-write-tree will take the
230 current index state, and write an object that describes that whole
231 index. In other words, we're now tying together all the different
232 filenames with their contents (and their permissions), and we're
233 creating the equivalent of a git "directory" object:
234
235 git-write-tree
236
237 and this will just output the name of the resulting tree, in this case
238 (if you have does exactly as I've described) it should be
239
240 3ede4ed7e895432c0a247f09d71a76db53bd0fa4
241
242 which is another incomprehensible object name. Again, if you want to,
243 you can use "git-cat-file -t 3ede4.." to see that this time the object
244 is not a "blob" object, but a "tree" object (you can also use
245 git-cat-file to actually output the raw object contents, but you'll see
246 mainly a binary mess, so that's less interesting).
247
248 However - normally you'd never use "git-write-tree" on its own, because
249 normally you always commit a tree into a commit object using the
250 "git-commit-tree" command. In fact, it's easier to not actually use
251 git-write-tree on its own at all, but to just pass its result in as an
252 argument to "git-commit-tree".
253
254 "git-commit-tree" normally takes several arguments - it wants to know
255 what the _parent_ of a commit was, but since this is the first commit
256 ever in this new archive, and it has no parents, we only need to pass in
257 the tree ID. However, git-commit-tree also wants to get a commit message
258 on its standard input, and it will write out the resulting ID for the
259 commit to its standard output.
260
261 And this is where we start using the .git/HEAD file. The HEAD file is
262 supposed to contain the reference to the top-of-tree, and since that's
263 exactly what git-commit-tree spits out, we can do this all with a simple
264 shell pipeline:
265
266 echo "Initial commit" | git-commit-tree $(git-write-tree) > .git/HEAD
267
268 which will say:
269
270 Committing initial tree 3ede4ed7e895432c0a247f09d71a76db53bd0fa4
271
272 just to warn you about the fact that it created a totally new commit
273 that is not related to anything else. Normally you do this only _once_
274 for a project ever, and all later commits will be parented on top of an
275 earlier commit, and you'll never see this "Committing initial tree"
276 message ever again.
277
278
279 Making a change
280 ---------------
281
282 Remember how we did the "git-update-cache" on file "a" and then we
283 changed "a" afterward, and could compare the new state of "a" with the
284 state we saved in the index file?
285
286 Further, remember how I said that "git-write-tree" writes the contents
287 of the _index_ file to the tree, and thus what we just committed was in
288 fact the _original_ contents of the file "a", not the new ones. We did
289 that on purpose, to show the difference between the index state, and the
290 state in the working directory, and how they don't have to match, even
291 when we commit things.
292
293 As before, if we do "git-diff-files -p" in our git-tutorial project,
294 we'll still see the same difference we saw last time: the index file
295 hasn't changed by the act of committing anything. However, now that we
296 have committed something, we can also learn to use a new command:
297 "git-diff-cache".
298
299 Unlike "git-diff-files", which showed the difference between the index
300 file and the working directory, "git-diff-cache" shows the differences
301 between a committed _tree_ and either the the index file or the working
302 directory. In other words, git-diff-cache wants a tree to be diffed
303 against, and before we did the commit, we couldn't do that, because we
304 didn't have anything to diff against.
305
306 But now we can do
307
308 git-diff-cache -p HEAD
309
310 (where "-p" has the same meaning as it did in git-diff-files), and it
311 will show us the same difference, but for a totally different reason.
312 Now we're comparing the working directory not against the index file,
313 but against the tree we just wrote. It just so happens that those two
314 are obviously the same, so we get the same result.
315
316 In other words, "git-diff-cache" normally compares a tree against the
317 working directory, but when given the "--cached" flag, it is told to
318 instead compare against just the index cache contents, and ignore the
319 current working directory state entirely. Since we just wrote the index
320 file to HEAD, doing "git-diff-cache --cached -p HEAD" should thus return
321 an empty set of differences, and that's exactly what it does.
322
323 [ Digression: "git-diff-cache" really always uses the index for its
324 comparisons, and saying that it compares a tree against the working
325 directory is thus not strictly accurate. In particular, the list of
326 files to compare (the "meta-data") _always_ comes from the index file,
327 regardless of whether the --cached flag is used or not. The --cached
328 flag really only determines whether the file _contents_ to be compared
329 come from the working directory or not.
330
331 This is not hard to understand, as soon as you realize that git simply
332 never knows (or cares) about files that it is not told about
333 explicitly. Git will never go _looking_ for files to compare, it
334 expects you to tell it what the files are, and that's what the index
335 is there for. ]
336
337 However, our next step is to commit the _change_ we did, and again, to
338 understand what's going on, keep in mind the difference between "working
339 directory contents", "index file" and "committed tree". We have changes
340 in the working directory that we want to commit, and we always have to
341 work through the index file, so the first thing we need to do is to
342 update the index cache:
343
344 git-update-cache a
345
346 (note how we didn't need the "--add" flag this time, since git knew
347 about the file already).
348
349 Note what happens to the different git-diff-xxx versions here. After
350 we've updated "a" in the index, "git-diff-files -p" now shows no
351 differences, but "git-diff-cache -p HEAD" still _does_ show that the
352 current state is different from the state we committed. In fact, now
353 "git-diff-cache" shows the same difference whether we use the "--cached"
354 flag or not, since now the index is coherent with the working directory.
355
356 Now, since we've updated "a" in the index, we can commit the new
357 version. We could do it by writing the tree by hand, and committing the
358 tree (this time we'd have to use the "-p HEAD" flag to tell commit that
359 the HEAD was the _parent_ of the new commit, and that this wasn't an
360 initial commit any more), but the fact is, git has a simple helper
361 script for doing all of the non-initial commits that does all of this
362 for you, and starts up an editor to let you write your commit message
363 yourself, so let's just use that:
364
365 git commit
366
367 Write whatever message you want, and all the lines that start with '#'
368 will be pruned out, and the rest will be used as the commit message for
369 the change. If you decide you don't want to commit anything after all at
370 this point (you can continue to edit things and update the cache), you
371 can just leave an empty message. Otherwise git-commit-script will commit
372 the change for you.
373
374 You've now made your first real git commit. And if you're interested in
375 looking at what git-commit-script really does, feel free to investigate:
376 it's a few very simple shell scripts to generate the helpful (?) commit
377 message headers, and a few one-liners that actually do the commit itself.
378
379
380 Checking it out
381 ---------------
382
383 While creating changes is useful, it's even more useful if you can tell
384 later what changed. The most useful command for this is another of the
385 "diff" family, namely "git-diff-tree".
386
387 git-diff-tree can be given two arbitrary trees, and it will tell you the
388 differences between them. Perhaps even more commonly, though, you can
389 give it just a single commit object, and it will figure out the parent
390 of that commit itself, and show the difference directly. Thus, to get
391 the same diff that we've already seen several times, we can now do
392
393 git-diff-tree -p HEAD
394
395 (again, "-p" means to show the difference as a human-readable patch),
396 and it will show what the last commit (in HEAD) actually changed.
397
398 More interestingly, you can also give git-diff-tree the "-v" flag, which
399 tells it to also show the commit message and author and date of the
400 commit, and you can tell it to show a whole series of diffs.
401 Alternatively, you can tell it to be "silent", and not show the diffs at
402 all, but just show the actual commit message.
403
404 In fact, together with the "git-rev-list" program (which generates a
405 list of revisions), git-diff-tree ends up being a veritable fount of
406 changes. A trivial (but very useful) script called "git-whatchanged" is
407 included with git which does exactly this, and shows a log of recent
408 activity.
409
410 To see the whole history of our pitiful little git-tutorial project, you
411 can do
412
413 git log
414
415 which shows just the log messages, or if we want to see the log together
416 with the associated patches use the more complex (and much more
417 powerful)
418
419 git-whatchanged -p --root
420
421 and you will see exactly what has changed in the repository over its
422 short history.
423
424 [ Side note: the "--root" flag is a flag to git-diff-tree to tell it to
425 show the initial aka "root" commit too. Normally you'd probably not
426 want to see the initial import diff, but since the tutorial project
427 was started from scratch and is so small, we use it to make the result
428 a bit more interesting ]
429
430 With that, you should now be having some inkling of what git does, and
431 can explore on your own.
432
433
434 Copying archives
435 -----------------
436
437 Git archives are normally totally self-sufficient, and it's worth noting
438 that unlike CVS, for example, there is no separate notion of
439 "repository" and "working tree". A git repository normally _is_ the
440 working tree, with the local git information hidden in the ".git"
441 subdirectory. There is nothing else. What you see is what you got.
442
443 [ Side note: you can tell git to split the git internal information from
444 the directory that it tracks, but we'll ignore that for now: it's not
445 how normal projects work, and it's really only meant for special uses.
446 So the mental model of "the git information is always tied directly to
447 the working directory that it describes" may not be technically 100%
448 accurate, but it's a good model for all normal use ]
449
450 This has two implications:
451
452 - if you grow bored with the tutorial archive you created (or you've
453 made a mistake and want to start all over), you can just do simple
454
455 rm -rf git-tutorial
456
457 and it will be gone. There's no external repository, and there's no
458 history outside of the project you created.
459
460 - if you want to move or duplicate a git archive, you can do so. There
461 is no "git clone" command: if you want to create a copy of your
462 archive (with all the full history that went along with it), you can
463 do so with a regular "cp -a git-tutorial new-git-tutorial".
464
465 Note that when you've moved or copied a git archive, your git index
466 file (which caches various information, notably some of the "stat"
467 information for the files involved) will likely need to be refreshed.
468 So after you do a "cp -a" to create a new copy, you'll want to do
469
470 git-update-cache --refresh
471
472 to make sure that the index file is up-to-date in the new one.
473
474 Note that the second point is true even across machines. You can
475 duplicate a remote git archive with _any_ regular copy mechanism, be it
476 "scp", "rsync" or "wget".
477
478 When copying a remote repository, you'll want to at a minimum update the
479 index cache when you do this, and especially with other peoples
480 repositories you often want to make sure that the index cache is in some
481 known state (you don't know _what_ they've done and not yet checked in),
482 so usually you'll precede the "git-update-cache" with a
483
484 git-read-tree --reset HEAD
485 git-update-cache --refresh
486
487 which will force a total index re-build from the tree pointed to by HEAD
488 (it resets the index contents to HEAD, and then the git-update-cache
489 makes sure to match up all index entries with the checked-out files).
490
491 The above can also be written as simply
492
493 git reset
494
495 and in fact a lot of the common git command combinations can be scripted
496 with the "git xyz" interfaces, and you can learn things by just looking
497 at what the git-*-script scripts do ("git reset" is the above two lines
498 implemented in "git-reset-script", but some things like "git status" and
499 "git commit" are slightly more complex scripts around the basic git
500 commands).
501
502 NOTE! Many (most?) public remote repositories will not contain any of
503 the checked out files or even an index file, and will _only_ contain the
504 actual core git files. Such a repository usually doesn't even have the
505 ".git" subdirectory, but has all the git files directly in the
506 repository.
507
508 To create your own local live copy of such a "raw" git repository, you'd
509 first create your own subdirectory for the project, and then copy the
510 raw repository contents into the ".git" directory. For example, to
511 create your own copy of the git repository, you'd do the following
512
513 mkdir my-git
514 cd my-git
515 rsync -rL rsync://rsync.kernel.org/pub/scm/linux/kernel/git/torvalds/git.git/ .git
516
517 followed by
518
519 git-read-tree HEAD
520
521 to populate the index. However, now you have populated the index, and
522 you have all the git internal files, but you will notice that you don't
523 actually have any of the _working_directory_ files to work on. To get
524 those, you'd check them out with
525
526 git-checkout-cache -u -a
527
528 where the "-u" flag means that you want the checkout to keep the index
529 up-to-date (so that you don't have to refresh it afterward), and the
530 "-a" file means "check out all files" (if you have a stale copy or an
531 older version of a checked out tree you may also need to add the "-f"
532 file first, to tell git-checkout-cache to _force_ overwriting of any old
533 files).
534
535 You have now successfully copied somebody else's (mine) remote
536 repository, and checked it out.
537
538 [ to be continued.. cvs2git, tagging versions, branches, merging.. ]