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1 | gittutorial-2(7) |
2 | ================ | |
e31952da | 3 | |
b27a23e3 CC |
4 | NAME |
5 | ---- | |
6 | gittutorial-2 - A tutorial introduction to git: part two | |
7 | ||
8 | SYNOPSIS | |
9 | -------- | |
10 | git * | |
11 | ||
12 | DESCRIPTION | |
13 | ----------- | |
14 | ||
6998e4db | 15 | You should work through linkgit:gittutorial[7] before reading this tutorial. |
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16 | |
17 | The goal of this tutorial is to introduce two fundamental pieces of | |
18 | git's architecture--the object database and the index file--and to | |
19 | provide the reader with everything necessary to understand the rest | |
20 | of the git documentation. | |
21 | ||
22 | The git object database | |
23 | ----------------------- | |
24 | ||
25 | Let's start a new project and create a small amount of history: | |
26 | ||
27 | ------------------------------------------------ | |
28 | $ mkdir test-project | |
29 | $ cd test-project | |
515377ea | 30 | $ git init |
ef0a89a6 | 31 | Initialized empty Git repository in .git/ |
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32 | $ echo 'hello world' > file.txt |
33 | $ git add . | |
34 | $ git commit -a -m "initial commit" | |
ebd124c6 | 35 | Created initial commit 54196cc2703dc165cbd373a65a4dcf22d50ae7f7 |
61f5cb7f | 36 | create mode 100644 file.txt |
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37 | $ echo 'hello world!' >file.txt |
38 | $ git commit -a -m "add emphasis" | |
ebd124c6 | 39 | Created commit c4d59f390b9cfd4318117afde11d601c1085f241 |
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40 | ------------------------------------------------ |
41 | ||
ebd124c6 | 42 | What are the 40 digits of hex that git responded to the commit with? |
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43 | |
44 | We saw in part one of the tutorial that commits have names like this. | |
45 | It turns out that every object in the git history is stored under | |
46 | such a 40-digit hex name. That name is the SHA1 hash of the object's | |
47 | contents; among other things, this ensures that git will never store | |
48 | the same data twice (since identical data is given an identical SHA1 | |
49 | name), and that the contents of a git object will never change (since | |
50 | that would change the object's name as well). | |
51 | ||
ebd124c6 NP |
52 | It is expected that the content of the commit object you created while |
53 | following the example above generates a different SHA1 hash than | |
54 | the one shown above because the commit object records the time when | |
55 | it was created and the name of the person performing the commit. | |
56 | ||
e31952da | 57 | We can ask git about this particular object with the cat-file |
ebd124c6 NP |
58 | command. Don't copy the 40 hex digits from this example but use those |
59 | from your own version. Note that you can shorten it to only a few | |
60 | characters to save yourself typing all 40 hex digits: | |
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61 | |
62 | ------------------------------------------------ | |
d54467b8 | 63 | $ git cat-file -t 54196cc2 |
ebd124c6 | 64 | commit |
d54467b8 | 65 | $ git cat-file commit 54196cc2 |
ebd124c6 NP |
66 | tree 92b8b694ffb1675e5975148e1121810081dbdffe |
67 | author J. Bruce Fields <bfields@puzzle.fieldses.org> 1143414668 -0500 | |
68 | committer J. Bruce Fields <bfields@puzzle.fieldses.org> 1143414668 -0500 | |
69 | ||
70 | initial commit | |
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71 | ------------------------------------------------ |
72 | ||
73 | A tree can refer to one or more "blob" objects, each corresponding to | |
74 | a file. In addition, a tree can also refer to other tree objects, | |
abda1ef5 | 75 | thus creating a directory hierarchy. You can examine the contents of |
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76 | any tree using ls-tree (remember that a long enough initial portion |
77 | of the SHA1 will also work): | |
78 | ||
79 | ------------------------------------------------ | |
80 | $ git ls-tree 92b8b694 | |
81 | 100644 blob 3b18e512dba79e4c8300dd08aeb37f8e728b8dad file.txt | |
82 | ------------------------------------------------ | |
83 | ||
84 | Thus we see that this tree has one file in it. The SHA1 hash is a | |
85 | reference to that file's data: | |
86 | ||
87 | ------------------------------------------------ | |
88 | $ git cat-file -t 3b18e512 | |
89 | blob | |
90 | ------------------------------------------------ | |
91 | ||
92 | A "blob" is just file data, which we can also examine with cat-file: | |
93 | ||
94 | ------------------------------------------------ | |
95 | $ git cat-file blob 3b18e512 | |
96 | hello world | |
97 | ------------------------------------------------ | |
98 | ||
99 | Note that this is the old file data; so the object that git named in | |
100 | its response to the initial tree was a tree with a snapshot of the | |
101 | directory state that was recorded by the first commit. | |
102 | ||
103 | All of these objects are stored under their SHA1 names inside the git | |
104 | directory: | |
105 | ||
106 | ------------------------------------------------ | |
107 | $ find .git/objects/ | |
108 | .git/objects/ | |
109 | .git/objects/pack | |
110 | .git/objects/info | |
111 | .git/objects/3b | |
112 | .git/objects/3b/18e512dba79e4c8300dd08aeb37f8e728b8dad | |
113 | .git/objects/92 | |
114 | .git/objects/92/b8b694ffb1675e5975148e1121810081dbdffe | |
115 | .git/objects/54 | |
116 | .git/objects/54/196cc2703dc165cbd373a65a4dcf22d50ae7f7 | |
117 | .git/objects/a0 | |
118 | .git/objects/a0/423896973644771497bdc03eb99d5281615b51 | |
119 | .git/objects/d0 | |
120 | .git/objects/d0/492b368b66bdabf2ac1fd8c92b39d3db916e59 | |
121 | .git/objects/c4 | |
122 | .git/objects/c4/d59f390b9cfd4318117afde11d601c1085f241 | |
123 | ------------------------------------------------ | |
124 | ||
125 | and the contents of these files is just the compressed data plus a | |
126 | header identifying their length and their type. The type is either a | |
ebd124c6 | 127 | blob, a tree, a commit, or a tag. |
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128 | |
129 | The simplest commit to find is the HEAD commit, which we can find | |
130 | from .git/HEAD: | |
131 | ||
132 | ------------------------------------------------ | |
133 | $ cat .git/HEAD | |
134 | ref: refs/heads/master | |
135 | ------------------------------------------------ | |
136 | ||
137 | As you can see, this tells us which branch we're currently on, and it | |
138 | tells us this by naming a file under the .git directory, which itself | |
139 | contains a SHA1 name referring to a commit object, which we can | |
140 | examine with cat-file: | |
141 | ||
142 | ------------------------------------------------ | |
143 | $ cat .git/refs/heads/master | |
144 | c4d59f390b9cfd4318117afde11d601c1085f241 | |
145 | $ git cat-file -t c4d59f39 | |
146 | commit | |
147 | $ git cat-file commit c4d59f39 | |
148 | tree d0492b368b66bdabf2ac1fd8c92b39d3db916e59 | |
149 | parent 54196cc2703dc165cbd373a65a4dcf22d50ae7f7 | |
150 | author J. Bruce Fields <bfields@puzzle.fieldses.org> 1143418702 -0500 | |
151 | committer J. Bruce Fields <bfields@puzzle.fieldses.org> 1143418702 -0500 | |
152 | ||
153 | add emphasis | |
154 | ------------------------------------------------ | |
155 | ||
156 | The "tree" object here refers to the new state of the tree: | |
157 | ||
158 | ------------------------------------------------ | |
159 | $ git ls-tree d0492b36 | |
160 | 100644 blob a0423896973644771497bdc03eb99d5281615b51 file.txt | |
2befe6fe | 161 | $ git cat-file blob a0423896 |
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162 | hello world! |
163 | ------------------------------------------------ | |
164 | ||
165 | and the "parent" object refers to the previous commit: | |
166 | ||
167 | ------------------------------------------------ | |
d54467b8 | 168 | $ git cat-file commit 54196cc2 |
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169 | tree 92b8b694ffb1675e5975148e1121810081dbdffe |
170 | author J. Bruce Fields <bfields@puzzle.fieldses.org> 1143414668 -0500 | |
171 | committer J. Bruce Fields <bfields@puzzle.fieldses.org> 1143414668 -0500 | |
172 | ||
173 | initial commit | |
174 | ------------------------------------------------ | |
175 | ||
176 | The tree object is the tree we examined first, and this commit is | |
177 | unusual in that it lacks any parent. | |
178 | ||
179 | Most commits have only one parent, but it is also common for a commit | |
180 | to have multiple parents. In that case the commit represents a | |
181 | merge, with the parent references pointing to the heads of the merged | |
182 | branches. | |
183 | ||
184 | Besides blobs, trees, and commits, the only remaining type of object | |
5162e697 | 185 | is a "tag", which we won't discuss here; refer to linkgit:git-tag[1] |
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186 | for details. |
187 | ||
188 | So now we know how git uses the object database to represent a | |
189 | project's history: | |
190 | ||
191 | * "commit" objects refer to "tree" objects representing the | |
192 | snapshot of a directory tree at a particular point in the | |
193 | history, and refer to "parent" commits to show how they're | |
194 | connected into the project history. | |
195 | * "tree" objects represent the state of a single directory, | |
196 | associating directory names to "blob" objects containing file | |
197 | data and "tree" objects containing subdirectory information. | |
198 | * "blob" objects contain file data without any other structure. | |
199 | * References to commit objects at the head of each branch are | |
200 | stored in files under .git/refs/heads/. | |
201 | * The name of the current branch is stored in .git/HEAD. | |
202 | ||
203 | Note, by the way, that lots of commands take a tree as an argument. | |
204 | But as we can see above, a tree can be referred to in many different | |
205 | ways--by the SHA1 name for that tree, by the name of a commit that | |
206 | refers to the tree, by the name of a branch whose head refers to that | |
207 | tree, etc.--and most such commands can accept any of these names. | |
208 | ||
209 | In command synopses, the word "tree-ish" is sometimes used to | |
210 | designate such an argument. | |
211 | ||
212 | The index file | |
213 | -------------- | |
214 | ||
b1889c36 | 215 | The primary tool we've been using to create commits is "git-commit |
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216 | -a", which creates a commit including every change you've made to |
217 | your working tree. But what if you want to commit changes only to | |
218 | certain files? Or only certain changes to certain files? | |
219 | ||
220 | If we look at the way commits are created under the cover, we'll see | |
221 | that there are more flexible ways creating commits. | |
222 | ||
223 | Continuing with our test-project, let's modify file.txt again: | |
224 | ||
225 | ------------------------------------------------ | |
226 | $ echo "hello world, again" >>file.txt | |
227 | ------------------------------------------------ | |
228 | ||
229 | but this time instead of immediately making the commit, let's take an | |
230 | intermediate step, and ask for diffs along the way to keep track of | |
231 | what's happening: | |
232 | ||
233 | ------------------------------------------------ | |
234 | $ git diff | |
235 | --- a/file.txt | |
236 | +++ b/file.txt | |
237 | @@ -1 +1,2 @@ | |
238 | hello world! | |
d5e3d60c | 239 | +hello world, again |
d7f078b8 | 240 | $ git add file.txt |
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241 | $ git diff |
242 | ------------------------------------------------ | |
243 | ||
244 | The last diff is empty, but no new commits have been made, and the | |
245 | head still doesn't contain the new line: | |
246 | ||
247 | ------------------------------------------------ | |
d54467b8 | 248 | $ git diff HEAD |
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249 | diff --git a/file.txt b/file.txt |
250 | index a042389..513feba 100644 | |
251 | --- a/file.txt | |
252 | +++ b/file.txt | |
253 | @@ -1 +1,2 @@ | |
254 | hello world! | |
d5e3d60c | 255 | +hello world, again |
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256 | ------------------------------------------------ |
257 | ||
b1889c36 | 258 | So "git-diff" is comparing against something other than the head. |
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259 | The thing that it's comparing against is actually the index file, |
260 | which is stored in .git/index in a binary format, but whose contents | |
261 | we can examine with ls-files: | |
262 | ||
263 | ------------------------------------------------ | |
264 | $ git ls-files --stage | |
265 | 100644 513feba2e53ebbd2532419ded848ba19de88ba00 0 file.txt | |
266 | $ git cat-file -t 513feba2 | |
267 | blob | |
268 | $ git cat-file blob 513feba2 | |
1d17c25c | 269 | hello world! |
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270 | hello world, again |
271 | ------------------------------------------------ | |
272 | ||
b1889c36 | 273 | So what our "git-add" did was store a new blob and then put |
e31952da | 274 | a reference to it in the index file. If we modify the file again, |
b1889c36 | 275 | we'll see that the new modifications are reflected in the "git-diff" |
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276 | output: |
277 | ||
278 | ------------------------------------------------ | |
279 | $ echo 'again?' >>file.txt | |
280 | $ git diff | |
281 | index 513feba..ba3da7b 100644 | |
282 | --- a/file.txt | |
283 | +++ b/file.txt | |
284 | @@ -1,2 +1,3 @@ | |
285 | hello world! | |
286 | hello world, again | |
287 | +again? | |
288 | ------------------------------------------------ | |
289 | ||
b1889c36 | 290 | With the right arguments, git-diff can also show us the difference |
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291 | between the working directory and the last commit, or between the |
292 | index and the last commit: | |
293 | ||
294 | ------------------------------------------------ | |
295 | $ git diff HEAD | |
296 | diff --git a/file.txt b/file.txt | |
297 | index a042389..ba3da7b 100644 | |
298 | --- a/file.txt | |
299 | +++ b/file.txt | |
300 | @@ -1 +1,3 @@ | |
301 | hello world! | |
302 | +hello world, again | |
303 | +again? | |
304 | $ git diff --cached | |
305 | diff --git a/file.txt b/file.txt | |
306 | index a042389..513feba 100644 | |
307 | --- a/file.txt | |
308 | +++ b/file.txt | |
309 | @@ -1 +1,2 @@ | |
310 | hello world! | |
311 | +hello world, again | |
312 | ------------------------------------------------ | |
313 | ||
b1889c36 | 314 | At any time, we can create a new commit using "git-commit" (without |
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315 | the -a option), and verify that the state committed only includes the |
316 | changes stored in the index file, not the additional change that is | |
317 | still only in our working tree: | |
318 | ||
319 | ------------------------------------------------ | |
320 | $ git commit -m "repeat" | |
321 | $ git diff HEAD | |
322 | diff --git a/file.txt b/file.txt | |
323 | index 513feba..ba3da7b 100644 | |
324 | --- a/file.txt | |
325 | +++ b/file.txt | |
326 | @@ -1,2 +1,3 @@ | |
327 | hello world! | |
328 | hello world, again | |
329 | +again? | |
330 | ------------------------------------------------ | |
331 | ||
b1889c36 | 332 | So by default "git-commit" uses the index to create the commit, not |
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333 | the working tree; the -a option to commit tells it to first update |
334 | the index with all changes in the working tree. | |
335 | ||
b1889c36 | 336 | Finally, it's worth looking at the effect of "git-add" on the index |
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337 | file: |
338 | ||
339 | ------------------------------------------------ | |
340 | $ echo "goodbye, world" >closing.txt | |
341 | $ git add closing.txt | |
342 | ------------------------------------------------ | |
343 | ||
b1889c36 | 344 | The effect of the "git-add" was to add one entry to the index file: |
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345 | |
346 | ------------------------------------------------ | |
347 | $ git ls-files --stage | |
348 | 100644 8b9743b20d4b15be3955fc8d5cd2b09cd2336138 0 closing.txt | |
349 | 100644 513feba2e53ebbd2532419ded848ba19de88ba00 0 file.txt | |
350 | ------------------------------------------------ | |
351 | ||
352 | And, as you can see with cat-file, this new entry refers to the | |
353 | current contents of the file: | |
354 | ||
355 | ------------------------------------------------ | |
1bb91460 JH |
356 | $ git cat-file blob 8b9743b2 |
357 | goodbye, world | |
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358 | ------------------------------------------------ |
359 | ||
360 | The "status" command is a useful way to get a quick summary of the | |
361 | situation: | |
362 | ||
363 | ------------------------------------------------ | |
364 | $ git status | |
bf3478de JH |
365 | # On branch master |
366 | # Changes to be committed: | |
367 | # (use "git reset HEAD <file>..." to unstage) | |
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368 | # |
369 | # new file: closing.txt | |
370 | # | |
bf3478de JH |
371 | # Changed but not updated: |
372 | # (use "git add <file>..." to update what will be committed) | |
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373 | # |
374 | # modified: file.txt | |
375 | # | |
376 | ------------------------------------------------ | |
377 | ||
378 | Since the current state of closing.txt is cached in the index file, | |
bf3478de | 379 | it is listed as "Changes to be committed". Since file.txt has |
e31952da | 380 | changes in the working directory that aren't reflected in the index, |
bf3478de | 381 | it is marked "changed but not updated". At this point, running "git |
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382 | commit" would create a commit that added closing.txt (with its new |
383 | contents), but that didn't modify file.txt. | |
384 | ||
385 | Also, note that a bare "git diff" shows the changes to file.txt, but | |
386 | not the addition of closing.txt, because the version of closing.txt | |
387 | in the index file is identical to the one in the working directory. | |
388 | ||
389 | In addition to being the staging area for new commits, the index file | |
390 | is also populated from the object database when checking out a | |
391 | branch, and is used to hold the trees involved in a merge operation. | |
6998e4db | 392 | See linkgit:gitcore-tutorial[7] and the relevant man |
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393 | pages for details. |
394 | ||
395 | What next? | |
396 | ---------- | |
397 | ||
398 | At this point you should know everything necessary to read the man | |
399 | pages for any of the git commands; one good place to start would be | |
884e3134 | 400 | with the commands mentioned in link:everyday.html[Everyday git]. You |
6998e4db | 401 | should be able to find any unknown jargon in linkgit:gitglossary[7]. |
e31952da | 402 | |
cd50aba9 BF |
403 | The link:user-manual.html[Git User's Manual] provides a more |
404 | comprehensive introduction to git. | |
405 | ||
6998e4db | 406 | linkgit:gitcvs-migration[7] explains how to |
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407 | import a CVS repository into git, and shows how to use git in a |
408 | CVS-like way. | |
409 | ||
410 | For some interesting examples of git use, see the | |
411 | link:howto-index.html[howtos]. | |
412 | ||
6998e4db | 413 | For git developers, linkgit:gitcore-tutorial[7] goes |
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414 | into detail on the lower-level git mechanisms involved in, for |
415 | example, creating a new commit. | |
b27a23e3 CC |
416 | |
417 | SEE ALSO | |
418 | -------- | |
419 | linkgit:gittutorial[7], | |
420 | linkgit:gitcvs-migration[7], | |
497c8331 CC |
421 | linkgit:gitcore-tutorial[7], |
422 | linkgit:gitglossary[7], | |
b27a23e3 CC |
423 | link:everyday.html[Everyday git], |
424 | link:user-manual.html[The Git User's Manual] | |
425 | ||
426 | GIT | |
427 | --- | |
9e1f0a85 | 428 | Part of the linkgit:git[1] suite. |