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403 <body>
405 <h1>
407 </h1>
411 A tutorial introduction to git: part two
412 </p>
413 </div>
414 </div>
420 </div></div>
421 </div>
424 <div class="paragraph"><p>You should work through <a href="gittutorial.html">gittutorial(7)</a> before reading this tutorial.</p></div>
427 provide the reader with everything necessary to understand the rest
428 of the git documentation.</p></div>
429 </div>
432 <div class="paragraph"><p>Let’s start a new project and create a small amount of history:</p></div>
435 <pre><tt>$ mkdir test-project
436 $ cd test-project
437 $ git init
438 Initialized empty Git repository in .git/
439 $ echo 'hello world' > file.txt
440 $ git add .
441 $ git commit -a -m "initial commit"
442 [master (root-commit) 54196cc] initial commit
444 create mode 100644 file.txt
445 $ echo 'hello world!' >file.txt
446 $ git commit -a -m "add emphasis"
447 [master c4d59f3] add emphasis
449 </div></div>
450 <div class="paragraph"><p>What are the 7 digits of hex that git responded to the commit with?</p></div>
452 It turns out that every object in the git history is stored under
454 contents; among other things, this ensures that git will never store
455 the same data twice (since identical data is given an identical SHA1
456 name), and that the contents of a git object will never change (since
458 here are simply the abbreviation of such 40 character long strings.
459 Abbreviations can be used everywhere where the 40 character strings
460 can be used, so long as they are unambiguous.</p></div>
461 <div class="paragraph"><p>It is expected that the content of the commit object you created while
462 following the example above generates a different SHA1 hash than
463 the one shown above because the commit object records the time when
464 it was created and the name of the person performing the commit.</p></div>
465 <div class="paragraph"><p>We can ask git about this particular object with the <tt>cat-file</tt>
467 from your own version. Note that you can shorten it to only a few
472 commit
473 $ git cat-file commit 54196cc2
474 tree 92b8b694ffb1675e5975148e1121810081dbdffe
478 initial commit</tt></pre>
479 </div></div>
481 a file. In addition, a tree can also refer to other tree objects,
482 thus creating a directory hierarchy. You can examine the contents of
483 any tree using ls-tree (remember that a long enough initial portion
484 of the SHA1 will also work):</p></div>
489 </div></div>
495 blob</tt></pre>
496 </div></div>
497 <div class="paragraph"><p>A "blob" is just file data, which we can also examine with cat-file:</p></div>
501 hello world</tt></pre>
502 </div></div>
504 its response to the initial tree was a tree with a snapshot of the
505 directory state that was recorded by the first commit.</p></div>
507 directory:</p></div>
510 <pre><tt>$ find .git/objects/
511 .git/objects/
512 .git/objects/pack
513 .git/objects/info
514 .git/objects/3b
516 .git/objects/92
517 .git/objects/92/b8b694ffb1675e5975148e1121810081dbdffe
518 .git/objects/54
520 .git/objects/a0
521 .git/objects/a0/423896973644771497bdc03eb99d5281615b51
522 .git/objects/d0
523 .git/objects/d0/492b368b66bdabf2ac1fd8c92b39d3db916e59
524 .git/objects/c4
525 .git/objects/c4/d59f390b9cfd4318117afde11d601c1085f241</tt></pre>
526 </div></div>
528 header identifying their length and their type. The type is either a
529 blob, a tree, a commit, or a tag.</p></div>
531 from .git/HEAD:</p></div>
534 <pre><tt>$ cat .git/HEAD
535 ref: refs/heads/master</tt></pre>
536 </div></div>
537 <div class="paragraph"><p>As you can see, this tells us which branch we’re currently on, and it
538 tells us this by naming a file under the .git directory, which itself
539 contains a SHA1 name referring to a commit object, which we can
540 examine with cat-file:</p></div>
543 <pre><tt>$ cat .git/refs/heads/master
544 c4d59f390b9cfd4318117afde11d601c1085f241
545 $ git cat-file -t c4d59f39
546 commit
547 $ git cat-file commit c4d59f39
548 tree d0492b368b66bdabf2ac1fd8c92b39d3db916e59
549 parent 54196cc2703dc165cbd373a65a4dcf22d50ae7f7
553 add emphasis</tt></pre>
554 </div></div>
558 <pre><tt>$ git ls-tree d0492b36
559 100644 blob a0423896973644771497bdc03eb99d5281615b51 file.txt
560 $ git cat-file blob a0423896
561 hello world!</tt></pre>
562 </div></div>
567 tree 92b8b694ffb1675e5975148e1121810081dbdffe
571 initial commit</tt></pre>
572 </div></div>
574 unusual in that it lacks any parent.</p></div>
576 to have multiple parents. In that case the commit represents a
577 merge, with the parent references pointing to the heads of the merged
578 branches.</p></div>
581 for details.</p></div>
585 <li>
586 <p>
588 snapshot of a directory tree at a particular point in the
590 connected into the project history.
591 </p>
592 </li>
593 <li>
594 <p>
595 "tree" objects represent the state of a single directory,
596 associating directory names to "blob" objects containing file
597 data and "tree" objects containing subdirectory information.
598 </p>
599 </li>
600 <li>
601 <p>
602 "blob" objects contain file data without any other structure.
603 </p>
604 </li>
605 <li>
606 <p>
607 References to commit objects at the head of each branch are
608 stored in files under .git/refs/heads/.
609 </p>
610 </li>
611 <li>
612 <p>
613 The name of the current branch is stored in .git/HEAD.
614 </p>
615 </li>
616 </ul></div>
618 But as we can see above, a tree can be referred to in many different
619 ways—by the SHA1 name for that tree, by the name of a commit that
620 refers to the tree, by the name of a branch whose head refers to that
621 tree, etc.--and most such commands can accept any of these names.</p></div>
623 designate such an argument.</p></div>
624 </div>
627 <div class="paragraph"><p>The primary tool we’ve been using to create commits is <tt>git-commit
629 your working tree. But what if you want to commit changes only to
630 certain files? Or only certain changes to certain files?</p></div>
631 <div class="paragraph"><p>If we look at the way commits are created under the cover, we’ll see
632 that there are more flexible ways creating commits.</p></div>
633 <div class="paragraph"><p>Continuing with our test-project, let’s modify file.txt again:</p></div>
637 </div></div>
638 <div class="paragraph"><p>but this time instead of immediately making the commit, let’s take an
639 intermediate step, and ask for diffs along the way to keep track of
643 <pre><tt>$ git diff
644 --- a/file.txt
645 +++ b/file.txt
647 hello world!
648 +hello world, again
649 $ git add file.txt
650 $ git diff</tt></pre>
651 </div></div>
656 <pre><tt>$ git diff HEAD
657 diff --git a/file.txt b/file.txt
659 --- a/file.txt
660 +++ b/file.txt
662 hello world!
663 +hello world, again</tt></pre>
664 </div></div>
665 <div class="paragraph"><p>So <em>git diff</em> is comparing against something other than the head.
666 The thing that it’s comparing against is actually the index file,
667 which is stored in .git/index in a binary format, but whose contents
668 we can examine with ls-files:</p></div>
671 <pre><tt>$ git ls-files --stage
673 $ git cat-file -t 513feba2
674 blob
675 $ git cat-file blob 513feba2
676 hello world!
677 hello world, again</tt></pre>
678 </div></div>
680 a reference to it in the index file. If we modify the file again,
682 output:</p></div>
686 $ git diff
688 --- a/file.txt
689 +++ b/file.txt
691 hello world!
692 hello world, again
693 +again?</tt></pre>
694 </div></div>
695 <div class="paragraph"><p>With the right arguments, <em>git diff</em> can also show us the difference
696 between the working directory and the last commit, or between the
697 index and the last commit:</p></div>
700 <pre><tt>$ git diff HEAD
701 diff --git a/file.txt b/file.txt
702 index a042389..ba3da7b 100644
703 --- a/file.txt
704 +++ b/file.txt
706 hello world!
707 +hello world, again
708 +again?
709 $ git diff --cached
710 diff --git a/file.txt b/file.txt
712 --- a/file.txt
713 +++ b/file.txt
715 hello world!
716 +hello world, again</tt></pre>
717 </div></div>
718 <div class="paragraph"><p>At any time, we can create a new commit using <em>git commit</em> (without
719 the "-a" option), and verify that the state committed only includes the
720 changes stored in the index file, not the additional change that is
721 still only in our working tree:</p></div>
725 $ git diff HEAD
726 diff --git a/file.txt b/file.txt
728 --- a/file.txt
729 +++ b/file.txt
731 hello world!
732 hello world, again
733 +again?</tt></pre>
734 </div></div>
735 <div class="paragraph"><p>So by default <em>git commit</em> uses the index to create the commit, not
736 the working tree; the "-a" option to commit tells it to first update
737 the index with all changes in the working tree.</p></div>
738 <div class="paragraph"><p>Finally, it’s worth looking at the effect of <em>git add</em> on the index
739 file:</p></div>
743 $ git add closing.txt</tt></pre>
744 </div></div>
745 <div class="paragraph"><p>The effect of the <em>git add</em> was to add one entry to the index file:</p></div>
748 <pre><tt>$ git ls-files --stage
751 </div></div>
753 current contents of the file:</p></div>
757 goodbye, world</tt></pre>
758 </div></div>
760 situation:</p></div>
763 <pre><tt>$ git status
764 # On branch master
765 # Changes to be committed:
766 # (use "git reset HEAD <file>..." to unstage)
767 #
768 # new file: closing.txt
769 #
770 # Changes not staged for commit:
771 # (use "git add <file>..." to update what will be committed)
772 #
773 # modified: file.txt
774 #</tt></pre>
775 </div></div>
777 it is listed as "Changes to be committed". Since file.txt has
778 changes in the working directory that aren’t reflected in the index,
780 commit" would create a commit that added closing.txt (with its new
782 <div class="paragraph"><p>Also, note that a bare <tt>git diff</tt> shows the changes to file.txt, but
783 not the addition of closing.txt, because the version of closing.txt
784 in the index file is identical to the one in the working directory.</p></div>
786 is also populated from the object database when checking out a
787 branch, and is used to hold the trees involved in a merge operation.
789 pages for details.</p></div>
790 </div>
794 pages for any of the git commands; one good place to start would be
796 should be able to find any unknown jargon in <a href="gitglossary.html">gitglossary(7)</a>.</p></div>
797 <div class="paragraph"><p>The <a href="user-manual.html">Git User’s Manual</a> provides a more
798 comprehensive introduction to git.</p></div>
799 <div class="paragraph"><p><a href="gitcvs-migration.html">gitcvs-migration(7)</a> explains how to
800 import a CVS repository into git, and shows how to use git in a
801 CVS-like way.</p></div>
804 <div class="paragraph"><p>For git developers, <a href="gitcore-tutorial.html">gitcore-tutorial(7)</a> goes
805 into detail on the lower-level git mechanisms involved in, for
806 example, creating a new commit.</p></div>
807 </div>
817 </div>
821 </div>
825 </div>
826 </div>
827 </body>
828 </html>