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] before reading this tutorial. 16 17The goal of this tutorial is to introduce two fundamental pieces of 18git's architecture--the object database and the index file--and to 19provide the reader with everything necessary to understand the rest 20of the git documentation. 21 22The git object database 23----------------------- 24 25Let's start a new project and create a small amount of history: 26 27------------------------------------------------ 28$ mkdir test-project 29$ cd test-project 30$ git init 31Initialized empty Git repository in .git/ 32$ echo 'hello world' > file.txt 33$ git add . 34$ git commit -a -m "initial commit" 35Created initial commit 54196cc2703dc165cbd373a65a4dcf22d50ae7f7 36 create mode 100644 file.txt 37$ echo 'hello world!' >file.txt 38$ git commit -a -m "add emphasis" 39Created commit c4d59f390b9cfd4318117afde11d601c1085f241 40------------------------------------------------ 41 42What are the 40 digits of hex that git responded to the commit with? 43 44We saw in part one of the tutorial that commits have names like this. 45It turns out that every object in the git history is stored under 46such a 40-digit hex name. That name is the SHA1 hash of the object's 47contents; among other things, this ensures that git will never store 48the same data twice (since identical data is given an identical SHA1 49name), and that the contents of a git object will never change (since 50that would change the object's name as well). 51 52It is expected that the content of the commit object you created while 53following the example above generates a different SHA1 hash than 54the one shown above because the commit object records the time when 55it was created and the name of the person performing the commit. 56 57We can ask git about this particular object with the `cat-file` 58command. Don't copy the 40 hex digits from this example but use those 59from your own version. Note that you can shorten it to only a few 60characters to save yourself typing all 40 hex digits: 61 62------------------------------------------------ 63$ git cat-file -t 54196cc2 64commit 65$ git cat-file commit 54196cc2 66tree 92b8b694ffb1675e5975148e1121810081dbdffe 67author J. Bruce Fields <bfields@puzzle.fieldses.org> 1143414668 -0500 68committer J. Bruce Fields <bfields@puzzle.fieldses.org> 1143414668 -0500 69 70initial commit 71------------------------------------------------ 72 73A tree can refer to one or more "blob" objects, each corresponding to 74a file. In addition, a tree can also refer to other tree objects, 75thus creating a directory hierarchy. You can examine the contents of 76any tree using ls-tree (remember that a long enough initial portion 77of the SHA1 will also work): 78 79------------------------------------------------ 80$ git ls-tree 92b8b694 81100644 blob 3b18e512dba79e4c8300dd08aeb37f8e728b8dad file.txt 82------------------------------------------------ 83 84Thus we see that this tree has one file in it. The SHA1 hash is a 85reference to that file's data: 86 87------------------------------------------------ 88$ git cat-file -t 3b18e512 89blob 90------------------------------------------------ 91 92A "blob" is just file data, which we can also examine with cat-file: 93 94------------------------------------------------ 95$ git cat-file blob 3b18e512 96hello world 97------------------------------------------------ 98 99Note that this is the old file data; so the object that git named in 100its response to the initial tree was a tree with a snapshot of the 101directory state that was recorded by the first commit. 102 103All of these objects are stored under their SHA1 names inside the git 104directory: 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 125and the contents of these files is just the compressed data plus a 126header identifying their length and their type. The type is either a 127blob, a tree, a commit, or a tag. 128 129The simplest commit to find is the HEAD commit, which we can find 130from .git/HEAD: 131 132------------------------------------------------ 133$ cat .git/HEAD 134ref: refs/heads/master 135------------------------------------------------ 136 137As you can see, this tells us which branch we're currently on, and it 138tells us this by naming a file under the .git directory, which itself 139contains a SHA1 name referring to a commit object, which we can 140examine with cat-file: 141 142------------------------------------------------ 143$ cat .git/refs/heads/master 144c4d59f390b9cfd4318117afde11d601c1085f241 145$ git cat-file -t c4d59f39 146commit 147$ git cat-file commit c4d59f39 148tree d0492b368b66bdabf2ac1fd8c92b39d3db916e59 149parent 54196cc2703dc165cbd373a65a4dcf22d50ae7f7 150author J. Bruce Fields <bfields@puzzle.fieldses.org> 1143418702 -0500 151committer J. Bruce Fields <bfields@puzzle.fieldses.org> 1143418702 -0500 152 153add emphasis 154------------------------------------------------ 155 156The "tree" object here refers to the new state of the tree: 157 158------------------------------------------------ 159$ git ls-tree d0492b36 160100644 blob a0423896973644771497bdc03eb99d5281615b51 file.txt 161$ git cat-file blob a0423896 162hello world! 163------------------------------------------------ 164 165and the "parent" object refers to the previous commit: 166 167------------------------------------------------ 168$ git cat-file commit 54196cc2 169tree 92b8b694ffb1675e5975148e1121810081dbdffe 170author J. Bruce Fields <bfields@puzzle.fieldses.org> 1143414668 -0500 171committer J. Bruce Fields <bfields@puzzle.fieldses.org> 1143414668 -0500 172 173initial commit 174------------------------------------------------ 175 176The tree object is the tree we examined first, and this commit is 177unusual in that it lacks any parent. 178 179Most commits have only one parent, but it is also common for a commit 180to have multiple parents. In that case the commit represents a 181merge, with the parent references pointing to the heads of the merged 182branches. 183 184Besides blobs, trees, and commits, the only remaining type of object 185is a "tag", which we won't discuss here; refer to linkgit:git-tag[1] 186for details. 187 188So now we know how git uses the object database to represent a 189project'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 203Note, by the way, that lots of commands take a tree as an argument. 204But as we can see above, a tree can be referred to in many different 205ways--by the SHA1 name for that tree, by the name of a commit that 206refers to the tree, by the name of a branch whose head refers to that 207tree, etc.--and most such commands can accept any of these names. 208 209In command synopses, the word "tree-ish" is sometimes used to 210designate such an argument. 211 212The index file 213-------------- 214 215The primary tool we've been using to create commits is `git-commit 216-a`, which creates a commit including every change you've made to 217your working tree. But what if you want to commit changes only to 218certain files? Or only certain changes to certain files? 219 220If we look at the way commits are created under the cover, we'll see 221that there are more flexible ways creating commits. 222 223Continuing with our test-project, let's modify file.txt again: 224 225------------------------------------------------ 226$ echo "hello world, again" >>file.txt 227------------------------------------------------ 228 229but this time instead of immediately making the commit, let's take an 230intermediate step, and ask for diffs along the way to keep track of 231what's happening: 232 233------------------------------------------------ 234$ git diff 235--- a/file.txt 236+++ b/file.txt 237@@ -1 +1,2 @@ 238 hello world! 239+hello world, again 240$ git add file.txt 241$ git diff 242------------------------------------------------ 243 244The last diff is empty, but no new commits have been made, and the 245head still doesn't contain the new line: 246 247------------------------------------------------ 248$ git diff HEAD 249diff --git a/file.txt b/file.txt 250index a042389..513feba 100644 251--- a/file.txt 252+++ b/file.txt 253@@ -1 +1,2 @@ 254 hello world! 255+hello world, again 256------------------------------------------------ 257 258So 'git-diff' is comparing against something other than the head. 259The thing that it's comparing against is actually the index file, 260which is stored in .git/index in a binary format, but whose contents 261we can examine with ls-files: 262 263------------------------------------------------ 264$ git ls-files --stage 265100644 513feba2e53ebbd2532419ded848ba19de88ba00 0 file.txt 266$ git cat-file -t 513feba2 267blob 268$ git cat-file blob 513feba2 269hello world! 270hello world, again 271------------------------------------------------ 272 273So what our 'git-add' did was store a new blob and then put 274a reference to it in the index file. If we modify the file again, 275we'll see that the new modifications are reflected in the 'git-diff' 276output: 277 278------------------------------------------------ 279$ echo 'again?' >>file.txt 280$ git diff 281index 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 290With the right arguments, 'git-diff' can also show us the difference 291between the working directory and the last commit, or between the 292index and the last commit: 293 294------------------------------------------------ 295$ git diff HEAD 296diff --git a/file.txt b/file.txt 297index 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 305diff --git a/file.txt b/file.txt 306index a042389..513feba 100644 307--- a/file.txt 308+++ b/file.txt 309@@ -1 +1,2 @@ 310 hello world! 311+hello world, again 312------------------------------------------------ 313 314At any time, we can create a new commit using 'git-commit' (without 315the "-a" option), and verify that the state committed only includes the 316changes stored in the index file, not the additional change that is 317still only in our working tree: 318 319------------------------------------------------ 320$ git commit -m "repeat" 321$ git diff HEAD 322diff --git a/file.txt b/file.txt 323index 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 332So by default 'git-commit' uses the index to create the commit, not 333the working tree; the "-a" option to commit tells it to first update 334the index with all changes in the working tree. 335 336Finally, it's worth looking at the effect of 'git-add' on the index 337file: 338 339------------------------------------------------ 340$ echo "goodbye, world" >closing.txt 341$ git add closing.txt 342------------------------------------------------ 343 344The effect of the 'git-add' was to add one entry to the index file: 345 346------------------------------------------------ 347$ git ls-files --stage 348100644 8b9743b20d4b15be3955fc8d5cd2b09cd2336138 0 closing.txt 349100644 513feba2e53ebbd2532419ded848ba19de88ba00 0 file.txt 350------------------------------------------------ 351 352And, as you can see with cat-file, this new entry refers to the 353current contents of the file: 354 355------------------------------------------------ 356$ git cat-file blob 8b9743b2 357goodbye, world 358------------------------------------------------ 359 360The "status" command is a useful way to get a quick summary of the 361situation: 362 363------------------------------------------------ 364$ git status 365# On branch master 366# Changes to be committed: 367# (use "git reset HEAD <file>..." to unstage) 368# 369# new file: closing.txt 370# 371# Changed but not updated: 372# (use "git add <file>..." to update what will be committed) 373# 374# modified: file.txt 375# 376------------------------------------------------ 377 378Since the current state of closing.txt is cached in the index file, 379it is listed as "Changes to be committed". Since file.txt has 380changes in the working directory that aren't reflected in the index, 381it is marked "changed but not updated". At this point, running "git 382commit" would create a commit that added closing.txt (with its new 383contents), but that didn't modify file.txt. 384 385Also, note that a bare `git diff` shows the changes to file.txt, but 386not the addition of closing.txt, because the version of closing.txt 387in the index file is identical to the one in the working directory. 388 389In addition to being the staging area for new commits, the index file 390is also populated from the object database when checking out a 391branch, and is used to hold the trees involved in a merge operation. 392See linkgit:gitcore-tutorial[7] and the relevant man 393pages for details. 394 395What next? 396---------- 397 398At this point you should know everything necessary to read the man 399pages for any of the git commands; one good place to start would be 400with the commands mentioned in link:everyday.html[Everyday git]. You 401should be able to find any unknown jargon in linkgit:gitglossary[7]. 402 403The link:user-manual.html[Git User's Manual] provides a more 404comprehensive introduction to git. 405 406linkgit:gitcvs-migration[7] explains how to 407import a CVS repository into git, and shows how to use git in a 408CVS-like way. 409 410For some interesting examples of git use, see the 411link:howto-index.html[howtos]. 412 413For git developers, linkgit:gitcore-tutorial[7] goes 414into detail on the lower-level git mechanisms involved in, for 415example, creating a new commit. 416 417SEE ALSO 418-------- 419linkgit:gittutorial[7], 420linkgit:gitcvs-migration[7], 421linkgit:gitcore-tutorial[7], 422linkgit:gitglossary[7], 423link:everyday.html[Everyday git], 424link:user-manual.html[The Git User's Manual] 425 426GIT 427--- 428Part of the linkgit:git[1] suite.