1gittutorial-2(7) 2================ 3 4NAME 5---- 6gittutorial-2 - A tutorial introduction to git: part two 7 8SYNOPSIS 9-------- 10git * 11 12DESCRIPTION 13----------- 14 15You should work through linkgit:gittutorial[7][A tutorial introduction to 16git] before reading this tutorial. 17 18The goal of this tutorial is to introduce two fundamental pieces of 19git's architecture--the object database and the index file--and to 20provide the reader with everything necessary to understand the rest 21of the git documentation. 22 23The git object database 24----------------------- 25 26Let's start a new project and create a small amount of history: 27 28------------------------------------------------ 29$ mkdir test-project 30$ cd test-project 31$ git init 32Initialized empty Git repository in .git/ 33$ echo 'hello world' > file.txt 34$ git add . 35$ git commit -a -m "initial commit" 36Created initial commit 54196cc2703dc165cbd373a65a4dcf22d50ae7f7 37 create mode 100644 file.txt 38$ echo 'hello world!' >file.txt 39$ git commit -a -m "add emphasis" 40Created commit c4d59f390b9cfd4318117afde11d601c1085f241 41------------------------------------------------ 42 43What are the 40 digits of hex that git responded to the commit with? 44 45We saw in part one of the tutorial that commits have names like this. 46It turns out that every object in the git history is stored under 47such a 40-digit hex name. That name is the SHA1 hash of the object's 48contents; among other things, this ensures that git will never store 49the same data twice (since identical data is given an identical SHA1 50name), and that the contents of a git object will never change (since 51that would change the object's name as well). 52 53It is expected that the content of the commit object you created while 54following the example above generates a different SHA1 hash than 55the one shown above because the commit object records the time when 56it was created and the name of the person performing the commit. 57 58We can ask git about this particular object with the cat-file 59command. Don't copy the 40 hex digits from this example but use those 60from your own version. Note that you can shorten it to only a few 61characters to save yourself typing all 40 hex digits: 62 63------------------------------------------------ 64$ git cat-file -t 54196cc2 65commit 66$ git cat-file commit 54196cc2 67tree 92b8b694ffb1675e5975148e1121810081dbdffe 68author J. Bruce Fields <bfields@puzzle.fieldses.org> 1143414668 -0500 69committer J. Bruce Fields <bfields@puzzle.fieldses.org> 1143414668 -0500 70 71initial commit 72------------------------------------------------ 73 74A tree can refer to one or more "blob" objects, each corresponding to 75a file. In addition, a tree can also refer to other tree objects, 76thus creating a directory hierarchy. You can examine the contents of 77any tree using ls-tree (remember that a long enough initial portion 78of the SHA1 will also work): 79 80------------------------------------------------ 81$ git ls-tree 92b8b694 82100644 blob 3b18e512dba79e4c8300dd08aeb37f8e728b8dad file.txt 83------------------------------------------------ 84 85Thus we see that this tree has one file in it. The SHA1 hash is a 86reference to that file's data: 87 88------------------------------------------------ 89$ git cat-file -t 3b18e512 90blob 91------------------------------------------------ 92 93A "blob" is just file data, which we can also examine with cat-file: 94 95------------------------------------------------ 96$ git cat-file blob 3b18e512 97hello world 98------------------------------------------------ 99 100Note that this is the old file data; so the object that git named in 101its response to the initial tree was a tree with a snapshot of the 102directory state that was recorded by the first commit. 103 104All of these objects are stored under their SHA1 names inside the git 105directory: 106 107------------------------------------------------ 108$ find .git/objects/ 109.git/objects/ 110.git/objects/pack 111.git/objects/info 112.git/objects/3b 113.git/objects/3b/18e512dba79e4c8300dd08aeb37f8e728b8dad 114.git/objects/92 115.git/objects/92/b8b694ffb1675e5975148e1121810081dbdffe 116.git/objects/54 117.git/objects/54/196cc2703dc165cbd373a65a4dcf22d50ae7f7 118.git/objects/a0 119.git/objects/a0/423896973644771497bdc03eb99d5281615b51 120.git/objects/d0 121.git/objects/d0/492b368b66bdabf2ac1fd8c92b39d3db916e59 122.git/objects/c4 123.git/objects/c4/d59f390b9cfd4318117afde11d601c1085f241 124------------------------------------------------ 125 126and the contents of these files is just the compressed data plus a 127header identifying their length and their type. The type is either a 128blob, a tree, a commit, or a tag. 129 130The simplest commit to find is the HEAD commit, which we can find 131from .git/HEAD: 132 133------------------------------------------------ 134$ cat .git/HEAD 135ref: refs/heads/master 136------------------------------------------------ 137 138As you can see, this tells us which branch we're currently on, and it 139tells us this by naming a file under the .git directory, which itself 140contains a SHA1 name referring to a commit object, which we can 141examine with cat-file: 142 143------------------------------------------------ 144$ cat .git/refs/heads/master 145c4d59f390b9cfd4318117afde11d601c1085f241 146$ git cat-file -t c4d59f39 147commit 148$ git cat-file commit c4d59f39 149tree d0492b368b66bdabf2ac1fd8c92b39d3db916e59 150parent 54196cc2703dc165cbd373a65a4dcf22d50ae7f7 151author J. Bruce Fields <bfields@puzzle.fieldses.org> 1143418702 -0500 152committer J. Bruce Fields <bfields@puzzle.fieldses.org> 1143418702 -0500 153 154add emphasis 155------------------------------------------------ 156 157The "tree" object here refers to the new state of the tree: 158 159------------------------------------------------ 160$ git ls-tree d0492b36 161100644 blob a0423896973644771497bdc03eb99d5281615b51 file.txt 162$ git cat-file blob a0423896 163hello world! 164------------------------------------------------ 165 166and the "parent" object refers to the previous commit: 167 168------------------------------------------------ 169$ git cat-file commit 54196cc2 170tree 92b8b694ffb1675e5975148e1121810081dbdffe 171author J. Bruce Fields <bfields@puzzle.fieldses.org> 1143414668 -0500 172committer J. Bruce Fields <bfields@puzzle.fieldses.org> 1143414668 -0500 173 174initial commit 175------------------------------------------------ 176 177The tree object is the tree we examined first, and this commit is 178unusual in that it lacks any parent. 179 180Most commits have only one parent, but it is also common for a commit 181to have multiple parents. In that case the commit represents a 182merge, with the parent references pointing to the heads of the merged 183branches. 184 185Besides blobs, trees, and commits, the only remaining type of object 186is a "tag", which we won't discuss here; refer to linkgit:git-tag[1] 187for details. 188 189So now we know how git uses the object database to represent a 190project's history: 191 192 * "commit" objects refer to "tree" objects representing the 193 snapshot of a directory tree at a particular point in the 194 history, and refer to "parent" commits to show how they're 195 connected into the project history. 196 * "tree" objects represent the state of a single directory, 197 associating directory names to "blob" objects containing file 198 data and "tree" objects containing subdirectory information. 199 * "blob" objects contain file data without any other structure. 200 * References to commit objects at the head of each branch are 201 stored in files under .git/refs/heads/. 202 * The name of the current branch is stored in .git/HEAD. 203 204Note, by the way, that lots of commands take a tree as an argument. 205But as we can see above, a tree can be referred to in many different 206ways--by the SHA1 name for that tree, by the name of a commit that 207refers to the tree, by the name of a branch whose head refers to that 208tree, etc.--and most such commands can accept any of these names. 209 210In command synopses, the word "tree-ish" is sometimes used to 211designate such an argument. 212 213The index file 214-------------- 215 216The primary tool we've been using to create commits is "git commit 217-a", which creates a commit including every change you've made to 218your working tree. But what if you want to commit changes only to 219certain files? Or only certain changes to certain files? 220 221If we look at the way commits are created under the cover, we'll see 222that there are more flexible ways creating commits. 223 224Continuing with our test-project, let's modify file.txt again: 225 226------------------------------------------------ 227$ echo "hello world, again" >>file.txt 228------------------------------------------------ 229 230but this time instead of immediately making the commit, let's take an 231intermediate step, and ask for diffs along the way to keep track of 232what's happening: 233 234------------------------------------------------ 235$ git diff 236--- a/file.txt 237+++ b/file.txt 238@@ -1 +1,2 @@ 239 hello world! 240+hello world, again 241$ git add file.txt 242$ git diff 243------------------------------------------------ 244 245The last diff is empty, but no new commits have been made, and the 246head still doesn't contain the new line: 247 248------------------------------------------------ 249$ git diff HEAD 250diff --git a/file.txt b/file.txt 251index a042389..513feba 100644 252--- a/file.txt 253+++ b/file.txt 254@@ -1 +1,2 @@ 255 hello world! 256+hello world, again 257------------------------------------------------ 258 259So "git diff" is comparing against something other than the head. 260The thing that it's comparing against is actually the index file, 261which is stored in .git/index in a binary format, but whose contents 262we can examine with ls-files: 263 264------------------------------------------------ 265$ git ls-files --stage 266100644 513feba2e53ebbd2532419ded848ba19de88ba00 0 file.txt 267$ git cat-file -t 513feba2 268blob 269$ git cat-file blob 513feba2 270hello world! 271hello world, again 272------------------------------------------------ 273 274So what our "git add" did was store a new blob and then put 275a reference to it in the index file. If we modify the file again, 276we'll see that the new modifications are reflected in the "git diff" 277output: 278 279------------------------------------------------ 280$ echo 'again?' >>file.txt 281$ git diff 282index 513feba..ba3da7b 100644 283--- a/file.txt 284+++ b/file.txt 285@@ -1,2 +1,3 @@ 286 hello world! 287 hello world, again 288+again? 289------------------------------------------------ 290 291With the right arguments, git diff can also show us the difference 292between the working directory and the last commit, or between the 293index and the last commit: 294 295------------------------------------------------ 296$ git diff HEAD 297diff --git a/file.txt b/file.txt 298index a042389..ba3da7b 100644 299--- a/file.txt 300+++ b/file.txt 301@@ -1 +1,3 @@ 302 hello world! 303+hello world, again 304+again? 305$ git diff --cached 306diff --git a/file.txt b/file.txt 307index a042389..513feba 100644 308--- a/file.txt 309+++ b/file.txt 310@@ -1 +1,2 @@ 311 hello world! 312+hello world, again 313------------------------------------------------ 314 315At any time, we can create a new commit using "git commit" (without 316the -a option), and verify that the state committed only includes the 317changes stored in the index file, not the additional change that is 318still only in our working tree: 319 320------------------------------------------------ 321$ git commit -m "repeat" 322$ git diff HEAD 323diff --git a/file.txt b/file.txt 324index 513feba..ba3da7b 100644 325--- a/file.txt 326+++ b/file.txt 327@@ -1,2 +1,3 @@ 328 hello world! 329 hello world, again 330+again? 331------------------------------------------------ 332 333So by default "git commit" uses the index to create the commit, not 334the working tree; the -a option to commit tells it to first update 335the index with all changes in the working tree. 336 337Finally, it's worth looking at the effect of "git add" on the index 338file: 339 340------------------------------------------------ 341$ echo "goodbye, world" >closing.txt 342$ git add closing.txt 343------------------------------------------------ 344 345The effect of the "git add" was to add one entry to the index file: 346 347------------------------------------------------ 348$ git ls-files --stage 349100644 8b9743b20d4b15be3955fc8d5cd2b09cd2336138 0 closing.txt 350100644 513feba2e53ebbd2532419ded848ba19de88ba00 0 file.txt 351------------------------------------------------ 352 353And, as you can see with cat-file, this new entry refers to the 354current contents of the file: 355 356------------------------------------------------ 357$ git cat-file blob 8b9743b2 358goodbye, world 359------------------------------------------------ 360 361The "status" command is a useful way to get a quick summary of the 362situation: 363 364------------------------------------------------ 365$ git status 366# On branch master 367# Changes to be committed: 368# (use "git reset HEAD <file>..." to unstage) 369# 370# new file: closing.txt 371# 372# Changed but not updated: 373# (use "git add <file>..." to update what will be committed) 374# 375# modified: file.txt 376# 377------------------------------------------------ 378 379Since the current state of closing.txt is cached in the index file, 380it is listed as "Changes to be committed". Since file.txt has 381changes in the working directory that aren't reflected in the index, 382it is marked "changed but not updated". At this point, running "git 383commit" would create a commit that added closing.txt (with its new 384contents), but that didn't modify file.txt. 385 386Also, note that a bare "git diff" shows the changes to file.txt, but 387not the addition of closing.txt, because the version of closing.txt 388in the index file is identical to the one in the working directory. 389 390In addition to being the staging area for new commits, the index file 391is also populated from the object database when checking out a 392branch, and is used to hold the trees involved in a merge operation. 393See the linkgit:gitcore-tutorial[7][core tutorial] and the relevant man 394pages for details. 395 396What next? 397---------- 398 399At this point you should know everything necessary to read the man 400pages for any of the git commands; one good place to start would be 401with the commands mentioned in link:everyday.html[Everyday git]. You 402should be able to find any unknown jargon in the 403linkgit:gitglossary[7][Glossary]. 404 405The link:user-manual.html[Git User's Manual] provides a more 406comprehensive introduction to git. 407 408The linkgit:gitcvs-migration[7][CVS migration] document explains how to 409import a CVS repository into git, and shows how to use git in a 410CVS-like way. 411 412For some interesting examples of git use, see the 413link:howto-index.html[howtos]. 414 415For git developers, the linkgit:gitcore-tutorial[7][Core tutorial] goes 416into detail on the lower-level git mechanisms involved in, for 417example, creating a new commit. 418 419SEE ALSO 420-------- 421linkgit:gittutorial[7], 422linkgit:gitcvs-migration[7], 423linkgit:gitcore-tutorial[7], 424linkgit:gitglossary[7], 425link:everyday.html[Everyday git], 426link:user-manual.html[The Git User's Manual] 427 428GIT 429--- 430Part of the linkgit:git[1] suite.