1A tutorial introduction to git: part two 2======================================== 3 4You should work through link:tutorial.html[A tutorial introduction to 5git] before reading this tutorial. 6 7The goal of this tutorial is to introduce two fundamental pieces of 8git's architecture--the object database and the index file--and to 9provide the reader with everything necessary to understand the rest 10of the git documentation. 11 12The git object database 13----------------------- 14 15Let's start a new project and create a small amount of history: 16 17------------------------------------------------ 18$ mkdir test-project 19$ cd test-project 20$ git init-db 21defaulting to local storage area 22$ echo 'hello world' > file.txt 23$ git add . 24$ git commit -a -m "initial commit" 25Committing initial tree 92b8b694ffb1675e5975148e1121810081dbdffe 26 create mode 100644 file.txt 27$ echo 'hello world!' >file.txt 28$ git commit -a -m "add emphasis" 29------------------------------------------------ 30 31What are the 40 digits of hex that git responded to the first commit 32with? 33 34We saw in part one of the tutorial that commits have names like this. 35It turns out that every object in the git history is stored under 36such a 40-digit hex name. That name is the SHA1 hash of the object's 37contents; among other things, this ensures that git will never store 38the same data twice (since identical data is given an identical SHA1 39name), and that the contents of a git object will never change (since 40that would change the object's name as well). 41 42We can ask git about this particular object with the cat-file 43command--just cut-and-paste from the reply to the initial commit, to 44save yourself typing all 40 hex digits: 45 46------------------------------------------------ 47$ git cat-file -t 92b8b694ffb1675e5975148e1121810081dbdffe 48tree 49------------------------------------------------ 50 51A tree can refer to one or more "blob" objects, each corresponding to 52a file. In addition, a tree can also refer to other tree objects, 53thus creating a directory hierarchy. You can examine the contents of 54any tree using ls-tree (remember that a long enough initial portion 55of the SHA1 will also work): 56 57------------------------------------------------ 58$ git ls-tree 92b8b694 59100644 blob 3b18e512dba79e4c8300dd08aeb37f8e728b8dad file.txt 60------------------------------------------------ 61 62Thus we see that this tree has one file in it. The SHA1 hash is a 63reference to that file's data: 64 65------------------------------------------------ 66$ git cat-file -t 3b18e512 67blob 68------------------------------------------------ 69 70A "blob" is just file data, which we can also examine with cat-file: 71 72------------------------------------------------ 73$ git cat-file blob 3b18e512 74hello world 75------------------------------------------------ 76 77Note that this is the old file data; so the object that git named in 78its response to the initial tree was a tree with a snapshot of the 79directory state that was recorded by the first commit. 80 81All of these objects are stored under their SHA1 names inside the git 82directory: 83 84------------------------------------------------ 85$ find .git/objects/ 86.git/objects/ 87.git/objects/pack 88.git/objects/info 89.git/objects/3b 90.git/objects/3b/18e512dba79e4c8300dd08aeb37f8e728b8dad 91.git/objects/92 92.git/objects/92/b8b694ffb1675e5975148e1121810081dbdffe 93.git/objects/54 94.git/objects/54/196cc2703dc165cbd373a65a4dcf22d50ae7f7 95.git/objects/a0 96.git/objects/a0/423896973644771497bdc03eb99d5281615b51 97.git/objects/d0 98.git/objects/d0/492b368b66bdabf2ac1fd8c92b39d3db916e59 99.git/objects/c4 100.git/objects/c4/d59f390b9cfd4318117afde11d601c1085f241 101------------------------------------------------ 102 103and the contents of these files is just the compressed data plus a 104header identifying their length and their type. The type is either a 105blob, a tree, a commit, or a tag. We've seen a blob and a tree now, 106so next we should look at a commit. 107 108The simplest commit to find is the HEAD commit, which we can find 109from .git/HEAD: 110 111------------------------------------------------ 112$ cat .git/HEAD 113ref: refs/heads/master 114------------------------------------------------ 115 116As you can see, this tells us which branch we're currently on, and it 117tells us this by naming a file under the .git directory, which itself 118contains a SHA1 name referring to a commit object, which we can 119examine with cat-file: 120 121------------------------------------------------ 122$ cat .git/refs/heads/master 123c4d59f390b9cfd4318117afde11d601c1085f241 124$ git cat-file -t c4d59f39 125commit 126$ git cat-file commit c4d59f39 127tree d0492b368b66bdabf2ac1fd8c92b39d3db916e59 128parent 54196cc2703dc165cbd373a65a4dcf22d50ae7f7 129author J. Bruce Fields <bfields@puzzle.fieldses.org> 1143418702 -0500 130committer J. Bruce Fields <bfields@puzzle.fieldses.org> 1143418702 -0500 131 132add emphasis 133------------------------------------------------ 134 135The "tree" object here refers to the new state of the tree: 136 137------------------------------------------------ 138$ git ls-tree d0492b36 139100644 blob a0423896973644771497bdc03eb99d5281615b51 file.txt 140$ git cat-file blob a0423896 141hello world! 142------------------------------------------------ 143 144and the "parent" object refers to the previous commit: 145 146------------------------------------------------ 147$ git-cat-file commit 54196cc2 148tree 92b8b694ffb1675e5975148e1121810081dbdffe 149author J. Bruce Fields <bfields@puzzle.fieldses.org> 1143414668 -0500 150committer J. Bruce Fields <bfields@puzzle.fieldses.org> 1143414668 -0500 151 152initial commit 153------------------------------------------------ 154 155The tree object is the tree we examined first, and this commit is 156unusual in that it lacks any parent. 157 158Most commits have only one parent, but it is also common for a commit 159to have multiple parents. In that case the commit represents a 160merge, with the parent references pointing to the heads of the merged 161branches. 162 163Besides blobs, trees, and commits, the only remaining type of object 164is a "tag", which we won't discuss here; refer to gitlink:git-tag[1] 165for details. 166 167So now we know how git uses the object database to represent a 168project's history: 169 170 * "commit" objects refer to "tree" objects representing the 171 snapshot of a directory tree at a particular point in the 172 history, and refer to "parent" commits to show how they're 173 connected into the project history. 174 * "tree" objects represent the state of a single directory, 175 associating directory names to "blob" objects containing file 176 data and "tree" objects containing subdirectory information. 177 * "blob" objects contain file data without any other structure. 178 * References to commit objects at the head of each branch are 179 stored in files under .git/refs/heads/. 180 * The name of the current branch is stored in .git/HEAD. 181 182Note, by the way, that lots of commands take a tree as an argument. 183But as we can see above, a tree can be referred to in many different 184ways--by the SHA1 name for that tree, by the name of a commit that 185refers to the tree, by the name of a branch whose head refers to that 186tree, etc.--and most such commands can accept any of these names. 187 188In command synopses, the word "tree-ish" is sometimes used to 189designate such an argument. 190 191The index file 192-------------- 193 194The primary tool we've been using to create commits is "git commit 195-a", which creates a commit including every change you've made to 196your working tree. But what if you want to commit changes only to 197certain files? Or only certain changes to certain files? 198 199If we look at the way commits are created under the cover, we'll see 200that there are more flexible ways creating commits. 201 202Continuing with our test-project, let's modify file.txt again: 203 204------------------------------------------------ 205$ echo "hello world, again" >>file.txt 206------------------------------------------------ 207 208but this time instead of immediately making the commit, let's take an 209intermediate step, and ask for diffs along the way to keep track of 210what's happening: 211 212------------------------------------------------ 213$ git diff 214--- a/file.txt 215+++ b/file.txt 216@@ -1 +1,2 @@ 217 hello world! 218+hello world, again 219$ git update-index file.txt 220$ git diff 221------------------------------------------------ 222 223The last diff is empty, but no new commits have been made, and the 224head still doesn't contain the new line: 225 226------------------------------------------------ 227$ git-diff HEAD 228diff --git a/file.txt b/file.txt 229index a042389..513feba 100644 230--- a/file.txt 231+++ b/file.txt 232@@ -1 +1,2 @@ 233 hello world! 234+hello world, again 235------------------------------------------------ 236 237So "git diff" is comparing against something other than the head. 238The thing that it's comparing against is actually the index file, 239which is stored in .git/index in a binary format, but whose contents 240we can examine with ls-files: 241 242------------------------------------------------ 243$ git ls-files --stage 244100644 513feba2e53ebbd2532419ded848ba19de88ba00 0 file.txt 245$ git cat-file -t 513feba2 246blob 247$ git cat-file blob 513feba2 248hello world! 249hello world, again 250------------------------------------------------ 251 252So what our "git update-index" did was store a new blob and then put 253a reference to it in the index file. If we modify the file again, 254we'll see that the new modifications are reflected in the "git-diff" 255output: 256 257------------------------------------------------ 258$ echo 'again?' >>file.txt 259$ git diff 260index 513feba..ba3da7b 100644 261--- a/file.txt 262+++ b/file.txt 263@@ -1,2 +1,3 @@ 264 hello world! 265 hello world, again 266+again? 267------------------------------------------------ 268 269With the right arguments, git diff can also show us the difference 270between the working directory and the last commit, or between the 271index and the last commit: 272 273------------------------------------------------ 274$ git diff HEAD 275diff --git a/file.txt b/file.txt 276index a042389..ba3da7b 100644 277--- a/file.txt 278+++ b/file.txt 279@@ -1 +1,3 @@ 280 hello world! 281+hello world, again 282+again? 283$ git diff --cached 284diff --git a/file.txt b/file.txt 285index a042389..513feba 100644 286--- a/file.txt 287+++ b/file.txt 288@@ -1 +1,2 @@ 289 hello world! 290+hello world, again 291------------------------------------------------ 292 293At any time, we can create a new commit using "git commit" (without 294the -a option), and verify that the state committed only includes the 295changes stored in the index file, not the additional change that is 296still only in our working tree: 297 298------------------------------------------------ 299$ git commit -m "repeat" 300$ git diff HEAD 301diff --git a/file.txt b/file.txt 302index 513feba..ba3da7b 100644 303--- a/file.txt 304+++ b/file.txt 305@@ -1,2 +1,3 @@ 306 hello world! 307 hello world, again 308+again? 309------------------------------------------------ 310 311So by default "git commit" uses the index to create the commit, not 312the working tree; the -a option to commit tells it to first update 313the index with all changes in the working tree. 314 315Finally, it's worth looking at the effect of "git add" on the index 316file: 317 318------------------------------------------------ 319$ echo "goodbye, world" >closing.txt 320$ git add closing.txt 321------------------------------------------------ 322 323The effect of the "git add" was to add one entry to the index file: 324 325------------------------------------------------ 326$ git ls-files --stage 327100644 8b9743b20d4b15be3955fc8d5cd2b09cd2336138 0 closing.txt 328100644 513feba2e53ebbd2532419ded848ba19de88ba00 0 file.txt 329------------------------------------------------ 330 331And, as you can see with cat-file, this new entry refers to the 332current contents of the file: 333 334------------------------------------------------ 335$ git cat-file blob a6b11f7a 336goodbye, word 337------------------------------------------------ 338 339The "status" command is a useful way to get a quick summary of the 340situation: 341 342------------------------------------------------ 343$ git status 344# 345# Updated but not checked in: 346# (will commit) 347# 348# new file: closing.txt 349# 350# 351# Changed but not updated: 352# (use git-update-index to mark for commit) 353# 354# modified: file.txt 355# 356------------------------------------------------ 357 358Since the current state of closing.txt is cached in the index file, 359it is listed as "updated but not checked in". Since file.txt has 360changes in the working directory that aren't reflected in the index, 361it is marked "changed but not updated". At this point, running "git 362commit" would create a commit that added closing.txt (with its new 363contents), but that didn't modify file.txt. 364 365Also, note that a bare "git diff" shows the changes to file.txt, but 366not the addition of closing.txt, because the version of closing.txt 367in the index file is identical to the one in the working directory. 368 369In addition to being the staging area for new commits, the index file 370is also populated from the object database when checking out a 371branch, and is used to hold the trees involved in a merge operation. 372See the link:core-tutorial.html[core tutorial] and the relevant man 373pages for details. 374 375What next? 376---------- 377 378At this point you should know everything necessary to read the man 379pages for any of the git commands; one good place to start would be 380with the commands mentioned in link:everyday.html[Everyday git]. You 381should be able to find any unknown jargon in the 382link:glossary.html[Glossary]. 383 384The link:cvs-migration.html[CVS migration] document explains how to 385import a CVS repository into git, and shows how to use git in a 386CVS-like way. 387 388For some interesting examples of git use, see the 389link:howto-index.html[howtos]. 390 391For git developers, the link:core-tutorial.html[Core tutorial] goes 392into detail on the lower-level git mechanisms involved in, for 393example, creating a new commit.