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 files changed, 1 insertions(+), 0 deletions(-)
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 files changed, 1 insertions(+), 1 deletions(-)
36 What are the 7 digits of hex that git responded to the commit with?
39 We saw in part one of the tutorial that commits have names like this.
41 It turns out that every object in the git history is stored under a
42 40-digit hex name. That name is the SHA1 hash of the object’s contents;
43 among other things, this ensures that git will never store the same
44 data twice (since identical data is given an identical SHA1 name), and
45 that the contents of a git object will never change (since that would
46 change the object’s name as well). The 7 char hex strings here are
47 simply the abbreviation of such 40 character long strings.
48 Abbreviations can be used everywhere where the 40 character strings can
49 be used, so long as they are unambiguous.
50 It is expected that the content of the commit object you created while
52 following the example above generates a different SHA1 hash than the
53 one shown above because the commit object records the time when it was
54 created and the name of the person performing the commit.
55 We can ask git about this particular object with the cat-file command.
57 Don’t copy the 40 hex digits from this example but use those from your
58 own version. Note that you can shorten it to only a few characters to
59 save yourself typing all 40 hex digits:
60 $ git cat-file -t 54196cc2
62 commit
63 $ git cat-file commit 54196cc2
64 tree 92b8b694ffb1675e5975148e1121810081dbdffe
65 author J. Bruce Fields <bfields@puzzle.fieldses.org> 1143414668 -0500
66 committer J. Bruce Fields <bfields@puzzle.fieldses.org> 1143414668 -0500
67 initial commit
69 A tree can refer to one or more "blob" objects, each corresponding to a
72 file. In addition, a tree can also refer to other tree objects, thus
73 creating a directory hierarchy. You can examine the contents of any
74 tree using ls-tree (remember that a long enough initial portion of the
75 SHA1 will also work):
76 $ git ls-tree 92b8b694
78 100644 blob 3b18e512dba79e4c8300dd08aeb37f8e728b8dad file.txt
79 Thus we see that this tree has one file in it. The SHA1 hash is a
82 reference to that file’s data:
83 $ git cat-file -t 3b18e512
85 blob
86 A "blob" is just file data, which we can also examine with cat-file:
89 $ git cat-file blob 3b18e512
91 hello world
92 Note that this is the old file data; so the object that git named in
95 its response to the initial tree was a tree with a snapshot of the
96 directory state that was recorded by the first commit.
97 All of these objects are stored under their SHA1 names inside the git
99 directory:
100 $ find .git/objects/
102 .git/objects/
103 .git/objects/pack
104 .git/objects/info
105 .git/objects/3b
106 .git/objects/3b/18e512dba79e4c8300dd08aeb37f8e728b8dad
107 .git/objects/92
108 .git/objects/92/b8b694ffb1675e5975148e1121810081dbdffe
109 .git/objects/54
110 .git/objects/54/196cc2703dc165cbd373a65a4dcf22d50ae7f7
111 .git/objects/a0
112 .git/objects/a0/423896973644771497bdc03eb99d5281615b51
113 .git/objects/d0
114 .git/objects/d0/492b368b66bdabf2ac1fd8c92b39d3db916e59
115 .git/objects/c4
116 .git/objects/c4/d59f390b9cfd4318117afde11d601c1085f241
117 and the contents of these files is just the compressed data plus a
120 header identifying their length and their type. The type is either a
121 blob, a tree, a commit, or a tag.
122 The simplest commit to find is the HEAD commit, which we can find from
124 .git/HEAD:
125 $ cat .git/HEAD
127 ref: refs/heads/master
128 As you can see, this tells us which branch we’re currently on, and it
131 tells us this by naming a file under the .git directory, which itself
132 contains a SHA1 name referring to a commit object, which we can examine
133 with cat-file:
134 $ cat .git/refs/heads/master
136 c4d59f390b9cfd4318117afde11d601c1085f241
137 $ git cat-file -t c4d59f39
138 commit
139 $ git cat-file commit c4d59f39
140 tree d0492b368b66bdabf2ac1fd8c92b39d3db916e59
141 parent 54196cc2703dc165cbd373a65a4dcf22d50ae7f7
142 author J. Bruce Fields <bfields@puzzle.fieldses.org> 1143418702 -0500
143 committer J. Bruce Fields <bfields@puzzle.fieldses.org> 1143418702 -0500
144 add emphasis
146 The "tree" object here refers to the new state of the tree:
149 $ git ls-tree d0492b36
151 100644 blob a0423896973644771497bdc03eb99d5281615b51 file.txt
152 $ git cat-file blob a0423896
153 hello world!
154 and the "parent" object refers to the previous commit:
157 $ git cat-file commit 54196cc2
159 tree 92b8b694ffb1675e5975148e1121810081dbdffe
160 author J. Bruce Fields <bfields@puzzle.fieldses.org> 1143414668 -0500
161 committer J. Bruce Fields <bfields@puzzle.fieldses.org> 1143414668 -0500
162 initial commit
164 The tree object is the tree we examined first, and this commit is
167 unusual in that it lacks any parent.
168 Most commits have only one parent, but it is also common for a commit
170 to have multiple parents. In that case the commit represents a merge,
171 with the parent references pointing to the heads of the merged
172 branches.
173 Besides blobs, trees, and commits, the only remaining type of object is
175 a "tag", which we won’t discuss here; refer to git-tag(1) for details.
176 So now we know how git uses the object database to represent a
178 project’s history:
179 · "commit" objects refer to "tree" objects representing the snapshot
181 of a directory tree at a particular point in the history, and refer
182 to "parent" commits to show how they’re connected into the project
183 history.
184 · "tree" objects represent the state of a single directory,
186 associating directory names to "blob" objects containing file data
187 and "tree" objects containing subdirectory information.
188 · "blob" objects contain file data without any other structure.
190 · References to commit objects at the head of each branch are stored
192 in files under .git/refs/heads/.
193 · The name of the current branch is stored in .git/HEAD.
195 Note, by the way, that lots of commands take a tree as an argument. But
197 as we can see above, a tree can be referred to in many different ways—
198 by the SHA1 name for that tree, by the name of a commit that refers to
199 the tree, by the name of a branch whose head refers to that tree,
200 etc.--and most such commands can accept any of these names.
201 In command synopses, the word "tree-ish" is sometimes used to designate
203 such an argument.
204
206 The primary tool we’ve been using to create commits is git-commit -a,
207 which creates a commit including every change you’ve made to your
208 working tree. But what if you want to commit changes only to certain
209 files? Or only certain changes to certain files?
210 If we look at the way commits are created under the cover, we’ll see
212 that there are more flexible ways creating commits.
213 Continuing with our test-project, let’s modify file.txt again:
215 $ echo "hello world, again" >>file.txt
217 but this time instead of immediately making the commit, let’s take an
220 intermediate step, and ask for diffs along the way to keep track of
221 what’s happening:
222 $ git diff
224 --- a/file.txt
225 +++ b/file.txt
226 @@ -1 +1,2 @@
227 hello world!
228 +hello world, again
229 $ git add file.txt
230 $ git diff
231 The last diff is empty, but no new commits have been made, and the head
234 still doesn’t contain the new line:
235 $ git diff HEAD
237 diff --git a/file.txt b/file.txt
238 index a042389..513feba 100644
239 --- a/file.txt
240 +++ b/file.txt
241 @@ -1 +1,2 @@
242 hello world!
243 +hello world, again
244 So git diff is comparing against something other than the head. The
247 thing that it’s comparing against is actually the index file, which is
248 stored in .git/index in a binary format, but whose contents we can
249 examine with ls-files:
250 $ git ls-files --stage
252 100644 513feba2e53ebbd2532419ded848ba19de88ba00 0 file.txt
253 $ git cat-file -t 513feba2
254 blob
255 $ git cat-file blob 513feba2
256 hello world!
257 hello world, again
258 So what our git add did was store a new blob and then put a reference
261 to it in the index file. If we modify the file again, we’ll see that
262 the new modifications are reflected in the git diff output:
263 $ echo ´again?´ >>file.txt
265 $ git diff
266 index 513feba..ba3da7b 100644
267 --- a/file.txt
268 +++ b/file.txt
269 @@ -1,2 +1,3 @@
270 hello world!
271 hello world, again
272 +again?
273 With the right arguments, git diff can also show us the difference
276 between the working directory and the last commit, or between the index
277 and the last commit:
278 $ git diff HEAD
280 diff --git a/file.txt b/file.txt
281 index a042389..ba3da7b 100644
282 --- a/file.txt
283 +++ b/file.txt
284 @@ -1 +1,3 @@
285 hello world!
286 +hello world, again
287 +again?
288 $ git diff --cached
289 diff --git a/file.txt b/file.txt
290 index a042389..513feba 100644
291 --- a/file.txt
292 +++ b/file.txt
293 @@ -1 +1,2 @@
294 hello world!
295 +hello world, again
296 At any time, we can create a new commit using git commit (without the
299 "-a" option), and verify that the state committed only includes the
300 changes stored in the index file, not the additional change that is
301 still only in our working tree:
302 $ git commit -m "repeat"
304 $ git diff HEAD
305 diff --git a/file.txt b/file.txt
306 index 513feba..ba3da7b 100644
307 --- a/file.txt
308 +++ b/file.txt
309 @@ -1,2 +1,3 @@
310 hello world!
311 hello world, again
312 +again?
313 So by default git commit uses the index to create the commit, not the
316 working tree; the "-a" option to commit tells it to first update the
317 index with all changes in the working tree.
318 Finally, it’s worth looking at the effect of git add on the index file:
320 $ echo "goodbye, world" >closing.txt
322 $ git add closing.txt
323 The effect of the git add was to add one entry to the index file:
326 $ git ls-files --stage
328 100644 8b9743b20d4b15be3955fc8d5cd2b09cd2336138 0 closing.txt
329 100644 513feba2e53ebbd2532419ded848ba19de88ba00 0 file.txt
330 And, as you can see with cat-file, this new entry refers to the current
333 contents of the file:
334 $ git cat-file blob 8b9743b2
336 goodbye, world
337 The "status" command is a useful way to get a quick summary of the
340 situation:
341 $ git status
343 # On branch master
344 # Changes to be committed:
345 # (use "git reset HEAD <file>..." to unstage)
346 #
347 # new file: closing.txt
348 #
349 # Changed but not updated:
350 # (use "git add <file>..." to update what will be committed)
351 #
352 # modified: file.txt
353 #
354 Since the current state of closing.txt is cached in the index file, it
357 is listed as "Changes to be committed". Since file.txt has changes in
358 the working directory that aren’t reflected in the index, it is marked
359 "changed but not updated". At this point, running "git commit" would
360 create a commit that added closing.txt (with its new contents), but
361 that didn’t modify file.txt.
362 Also, note that a bare git diff shows the changes to file.txt, but not
364 the addition of closing.txt, because the version of closing.txt in the
365 index file is identical to the one in the working directory.
366 In addition to being the staging area for new commits, the index file
368 is also populated from the object database when checking out a branch,
369 and is used to hold the trees involved in a merge operation. See
370 gitcore-tutorial(7) and the relevant man pages for details.
371
373 At this point you should know everything necessary to read the man
374 pages for any of the git commands; one good place to start would be
375 with the commands mentioned in Everyday git[1]. You should be able to
376 find any unknown jargon in gitglossary(7).
377 The Git User’s Manual[2] provides a more comprehensive introduction to
379 git.
380 gitcvs-migration(7) explains how to import a CVS repository into git,
382 and shows how to use git in a CVS-like way.
383 For some interesting examples of git use, see the howtos[3].
385 For git developers, gitcore-tutorial(7) goes into detail on the
387 lower-level git mechanisms involved in, for example, creating a new
388 commit.
389
391 gittutorial(7), gitcvs-migration(7), gitcore-tutorial(7),
392 gitglossary(7), git-help(1), Everyday git[1], The Git User’s Manual[2]
393
395 Part of the git(1) suite.
396
398 1. Everyday git
399 file:///usr/share/doc/git-1.7.1/everyday.html
400 2. Git User’s Manual
402 file:///usr/share/doc/git-1.7.1/user-manual.html
403 3. howtos
405 file:///usr/share/doc/git-1.7.1/howto-index.html
406Git 1.7.1 08/16/2017 GITTUTORIAL-2(7)