1GITTUTORIAL-2(7) Git Manual GITTUTORIAL-2(7)
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6 gittutorial-2 - A tutorial introduction to Git: part two
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9 git *
10
12 You should work through gittutorial(7) before reading this tutorial.
13 The goal of this tutorial is to introduce two fundamental pieces of
15 Git’s architecture—the object database and the index file—and to
16 provide the reader with everything necessary to understand the rest of
17 the Git documentation.
18
20 Let’s start a new project and create a small amount of history:
21 $ mkdir test-project
23 $ cd test-project
24 $ git init
25 Initialized empty Git repository in .git/
26 $ echo 'hello world' > file.txt
27 $ git add .
28 $ git commit -a -m "initial commit"
29 [master (root-commit) 54196cc] initial commit
30 1 file changed, 1 insertion(+)
31 create mode 100644 file.txt
32 $ echo 'hello world!' >file.txt
33 $ git commit -a -m "add emphasis"
34 [master c4d59f3] add emphasis
35 1 file changed, 1 insertion(+), 1 deletion(-)
36 What are the 7 digits of hex that Git responded to the commit with?
38 We saw in part one of the tutorial that commits have names like this.
40 It turns out that every object in the Git history is stored under a
41 40-digit hex name. That name is the SHA-1 hash of the object’s
42 contents; among other things, this ensures that Git will never store
43 the same data twice (since identical data is given an identical SHA-1
44 name), and that the contents of a Git object will never change (since
45 that would change the object’s name as well). The 7 char hex strings
46 here are simply the abbreviation of such 40 character long strings.
47 Abbreviations can be used everywhere where the 40 character strings can
48 be used, so long as they are unambiguous.
49 It is expected that the content of the commit object you created while
51 following the example above generates a different SHA-1 hash than the
52 one shown above because the commit object records the time when it was
53 created and the name of the person performing the commit.
54 We can ask Git about this particular object with the cat-file command.
56 Don’t copy the 40 hex digits from this example but use those from your
57 own version. Note that you can shorten it to only a few characters to
58 save yourself typing all 40 hex digits:
59 $ git cat-file -t 54196cc2
61 commit
62 $ git cat-file commit 54196cc2
63 tree 92b8b694ffb1675e5975148e1121810081dbdffe
64 author J. Bruce Fields <bfields@puzzle.fieldses.org> 1143414668 -0500
65 committer J. Bruce Fields <bfields@puzzle.fieldses.org> 1143414668 -0500
66 initial commit
68 A tree can refer to one or more "blob" objects, each corresponding to a
70 file. In addition, a tree can also refer to other tree objects, thus
71 creating a directory hierarchy. You can examine the contents of any
72 tree using ls-tree (remember that a long enough initial portion of the
73 SHA-1 will also work):
74 $ git ls-tree 92b8b694
76 100644 blob 3b18e512dba79e4c8300dd08aeb37f8e728b8dad file.txt
77 Thus we see that this tree has one file in it. The SHA-1 hash is a
79 reference to that file’s data:
80 $ git cat-file -t 3b18e512
82 blob
83 A "blob" is just file data, which we can also examine with cat-file:
85 $ git cat-file blob 3b18e512
87 hello world
88 Note that this is the old file data; so the object that Git named in
90 its response to the initial tree was a tree with a snapshot of the
91 directory state that was recorded by the first commit.
92 All of these objects are stored under their SHA-1 names inside the Git
94 directory:
95 $ find .git/objects/
97 .git/objects/
98 .git/objects/pack
99 .git/objects/info
100 .git/objects/3b
101 .git/objects/3b/18e512dba79e4c8300dd08aeb37f8e728b8dad
102 .git/objects/92
103 .git/objects/92/b8b694ffb1675e5975148e1121810081dbdffe
104 .git/objects/54
105 .git/objects/54/196cc2703dc165cbd373a65a4dcf22d50ae7f7
106 .git/objects/a0
107 .git/objects/a0/423896973644771497bdc03eb99d5281615b51
108 .git/objects/d0
109 .git/objects/d0/492b368b66bdabf2ac1fd8c92b39d3db916e59
110 .git/objects/c4
111 .git/objects/c4/d59f390b9cfd4318117afde11d601c1085f241
112 and the contents of these files is just the compressed data plus a
114 header identifying their length and their type. The type is either a
115 blob, a tree, a commit, or a tag.
116 The simplest commit to find is the HEAD commit, which we can find from
118 .git/HEAD:
119 $ cat .git/HEAD
121 ref: refs/heads/master
122 As you can see, this tells us which branch we’re currently on, and it
124 tells us this by naming a file under the .git directory, which itself
125 contains a SHA-1 name referring to a commit object, which we can
126 examine with cat-file:
127 $ cat .git/refs/heads/master
129 c4d59f390b9cfd4318117afde11d601c1085f241
130 $ git cat-file -t c4d59f39
131 commit
132 $ git cat-file commit c4d59f39
133 tree d0492b368b66bdabf2ac1fd8c92b39d3db916e59
134 parent 54196cc2703dc165cbd373a65a4dcf22d50ae7f7
135 author J. Bruce Fields <bfields@puzzle.fieldses.org> 1143418702 -0500
136 committer J. Bruce Fields <bfields@puzzle.fieldses.org> 1143418702 -0500
137 add emphasis
139 The "tree" object here refers to the new state of the tree:
141 $ git ls-tree d0492b36
143 100644 blob a0423896973644771497bdc03eb99d5281615b51 file.txt
144 $ git cat-file blob a0423896
145 hello world!
146 and the "parent" object refers to the previous commit:
148 $ git cat-file commit 54196cc2
150 tree 92b8b694ffb1675e5975148e1121810081dbdffe
151 author J. Bruce Fields <bfields@puzzle.fieldses.org> 1143414668 -0500
152 committer J. Bruce Fields <bfields@puzzle.fieldses.org> 1143414668 -0500
153 initial commit
155 The tree object is the tree we examined first, and this commit is
157 unusual in that it lacks any parent.
158 Most commits have only one parent, but it is also common for a commit
160 to have multiple parents. In that case the commit represents a merge,
161 with the parent references pointing to the heads of the merged
162 branches.
163 Besides blobs, trees, and commits, the only remaining type of object is
165 a "tag", which we won’t discuss here; refer to git-tag(1) for details.
166 So now we know how Git uses the object database to represent a
168 project’s history:
169 · "commit" objects refer to "tree" objects representing the snapshot
171 of a directory tree at a particular point in the history, and refer
172 to "parent" commits to show how they’re connected into the project
173 history.
174 · "tree" objects represent the state of a single directory,
176 associating directory names to "blob" objects containing file data
177 and "tree" objects containing subdirectory information.
178 · "blob" objects contain file data without any other structure.
180 · References to commit objects at the head of each branch are stored
182 in files under .git/refs/heads/.
183 · The name of the current branch is stored in .git/HEAD.
185 Note, by the way, that lots of commands take a tree as an argument. But
187 as we can see above, a tree can be referred to in many different ways—
188 by the SHA-1 name for that tree, by the name of a commit that refers to
189 the tree, by the name of a branch whose head refers to that tree,
190 etc.--and most such commands can accept any of these names.
191 In command synopses, the word "tree-ish" is sometimes used to designate
193 such an argument.
194
196 The primary tool we’ve been using to create commits is git-commit -a,
197 which creates a commit including every change you’ve made to your
198 working tree. But what if you want to commit changes only to certain
199 files? Or only certain changes to certain files?
200 If we look at the way commits are created under the cover, we’ll see
202 that there are more flexible ways creating commits.
203 Continuing with our test-project, let’s modify file.txt again:
205 $ echo "hello world, again" >>file.txt
207 but this time instead of immediately making the commit, let’s take an
209 intermediate step, and ask for diffs along the way to keep track of
210 what’s happening:
211 $ git diff
213 --- a/file.txt
214 +++ b/file.txt
215 @@ -1 +1,2 @@
216 hello world!
217 +hello world, again
218 $ git add file.txt
219 $ git diff
220 The last diff is empty, but no new commits have been made, and the head
222 still doesn’t contain the new line:
223 $ git diff HEAD
225 diff --git a/file.txt b/file.txt
226 index a042389..513feba 100644
227 --- a/file.txt
228 +++ b/file.txt
229 @@ -1 +1,2 @@
230 hello world!
231 +hello world, again
232 So git diff is comparing against something other than the head. The
234 thing that it’s comparing against is actually the index file, which is
235 stored in .git/index in a binary format, but whose contents we can
236 examine with ls-files:
237 $ git ls-files --stage
239 100644 513feba2e53ebbd2532419ded848ba19de88ba00 0 file.txt
240 $ git cat-file -t 513feba2
241 blob
242 $ git cat-file blob 513feba2
243 hello world!
244 hello world, again
245 So what our git add did was store a new blob and then put a reference
247 to it in the index file. If we modify the file again, we’ll see that
248 the new modifications are reflected in the git diff output:
249 $ echo 'again?' >>file.txt
251 $ git diff
252 index 513feba..ba3da7b 100644
253 --- a/file.txt
254 +++ b/file.txt
255 @@ -1,2 +1,3 @@
256 hello world!
257 hello world, again
258 +again?
259 With the right arguments, git diff can also show us the difference
261 between the working directory and the last commit, or between the index
262 and the last commit:
263 $ git diff HEAD
265 diff --git a/file.txt b/file.txt
266 index a042389..ba3da7b 100644
267 --- a/file.txt
268 +++ b/file.txt
269 @@ -1 +1,3 @@
270 hello world!
271 +hello world, again
272 +again?
273 $ git diff --cached
274 diff --git a/file.txt b/file.txt
275 index a042389..513feba 100644
276 --- a/file.txt
277 +++ b/file.txt
278 @@ -1 +1,2 @@
279 hello world!
280 +hello world, again
281 At any time, we can create a new commit using git commit (without the
283 "-a" option), and verify that the state committed only includes the
284 changes stored in the index file, not the additional change that is
285 still only in our working tree:
286 $ git commit -m "repeat"
288 $ git diff HEAD
289 diff --git a/file.txt b/file.txt
290 index 513feba..ba3da7b 100644
291 --- a/file.txt
292 +++ b/file.txt
293 @@ -1,2 +1,3 @@
294 hello world!
295 hello world, again
296 +again?
297 So by default git commit uses the index to create the commit, not the
299 working tree; the "-a" option to commit tells it to first update the
300 index with all changes in the working tree.
301 Finally, it’s worth looking at the effect of git add on the index file:
303 $ echo "goodbye, world" >closing.txt
305 $ git add closing.txt
306 The effect of the git add was to add one entry to the index file:
308 $ git ls-files --stage
310 100644 8b9743b20d4b15be3955fc8d5cd2b09cd2336138 0 closing.txt
311 100644 513feba2e53ebbd2532419ded848ba19de88ba00 0 file.txt
312 And, as you can see with cat-file, this new entry refers to the current
314 contents of the file:
315 $ git cat-file blob 8b9743b2
317 goodbye, world
318 The "status" command is a useful way to get a quick summary of the
320 situation:
321 $ git status
323 On branch master
324 Changes to be committed:
325 (use "git restore --staged <file>..." to unstage)
326 new file: closing.txt
328 Changes not staged for commit:
330 (use "git add <file>..." to update what will be committed)
331 (use "git restore <file>..." to discard changes in working directory)
332 modified: file.txt
334 Since the current state of closing.txt is cached in the index file, it
336 is listed as "Changes to be committed". Since file.txt has changes in
337 the working directory that aren’t reflected in the index, it is marked
338 "changed but not updated". At this point, running "git commit" would
339 create a commit that added closing.txt (with its new contents), but
340 that didn’t modify file.txt.
341 Also, note that a bare git diff shows the changes to file.txt, but not
343 the addition of closing.txt, because the version of closing.txt in the
344 index file is identical to the one in the working directory.
345 In addition to being the staging area for new commits, the index file
347 is also populated from the object database when checking out a branch,
348 and is used to hold the trees involved in a merge operation. See
349 gitcore-tutorial(7) and the relevant man pages for details.
350
352 At this point you should know everything necessary to read the man
353 pages for any of the git commands; one good place to start would be
354 with the commands mentioned in giteveryday(7). You should be able to
355 find any unknown jargon in gitglossary(7).
356 The Git User’s Manual[1] provides a more comprehensive introduction to
358 Git.
359 gitcvs-migration(7) explains how to import a CVS repository into Git,
361 and shows how to use Git in a CVS-like way.
362 For some interesting examples of Git use, see the howtos[2].
364 For Git developers, gitcore-tutorial(7) goes into detail on the
366 lower-level Git mechanisms involved in, for example, creating a new
367 commit.
368
370 gittutorial(7), gitcvs-migration(7), gitcore-tutorial(7),
371 gitglossary(7), git-help(1), giteveryday(7), The Git User’s Manual[1]
372
374 Part of the git(1) suite
375
377 1. Git User’s Manual
378 file:///usr/share/doc/git/user-manual.html
379 2. howtos
381 file:///usr/share/doc/git/howto-index.html
382Git 2.30.2 2021-03-08 GITTUTORIAL-2(7)