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403 <body>
405 <h1>
407 </h1>
411 A tutorial introduction to git: part two
412 </p>
413 </div>
414 </div>
418 </div>
421 <div class="paragraph"><p>You should work through <a href="gittutorial.html">gittutorial(7)</a> before reading this tutorial.</p></div>
424 provide the reader with everything necessary to understand the rest
425 of the git documentation.</p></div>
426 </div>
429 <div class="paragraph"><p>Let’s start a new project and create a small amount of history:</p></div>
432 <pre><tt>$ mkdir test-project
433 $ cd test-project
434 $ git init
435 Initialized empty Git repository in .git/
436 $ echo 'hello world' > file.txt
437 $ git add .
438 $ git commit -a -m "initial commit"
439 [master (root-commit) 54196cc] initial commit
441 create mode 100644 file.txt
442 $ echo 'hello world!' >file.txt
443 $ git commit -a -m "add emphasis"
444 [master c4d59f3] add emphasis
446 </div></div>
447 <div class="paragraph"><p>What are the 7 digits of hex that git responded to the commit with?</p></div>
449 It turns out that every object in the git history is stored under
451 contents; among other things, this ensures that git will never store
452 the same data twice (since identical data is given an identical SHA1
453 name), and that the contents of a git object will never change (since
455 here are simply the abbreviation of such 40 character long strings.
456 Abbreviations can be used everywhere where the 40 character strings
457 can be used, so long as they are unambiguous.</p></div>
458 <div class="paragraph"><p>It is expected that the content of the commit object you created while
459 following the example above generates a different SHA1 hash than
460 the one shown above because the commit object records the time when
461 it was created and the name of the person performing the commit.</p></div>
462 <div class="paragraph"><p>We can ask git about this particular object with the <tt>cat-file</tt>
464 from your own version. Note that you can shorten it to only a few
469 commit
470 $ git cat-file commit 54196cc2
471 tree 92b8b694ffb1675e5975148e1121810081dbdffe
475 initial commit</tt></pre>
476 </div></div>
478 a file. In addition, a tree can also refer to other tree objects,
479 thus creating a directory hierarchy. You can examine the contents of
480 any tree using ls-tree (remember that a long enough initial portion
481 of the SHA1 will also work):</p></div>
486 </div></div>
492 blob</tt></pre>
493 </div></div>
494 <div class="paragraph"><p>A "blob" is just file data, which we can also examine with cat-file:</p></div>
498 hello world</tt></pre>
499 </div></div>
501 its response to the initial tree was a tree with a snapshot of the
502 directory state that was recorded by the first commit.</p></div>
504 directory:</p></div>
507 <pre><tt>$ find .git/objects/
508 .git/objects/
509 .git/objects/pack
510 .git/objects/info
511 .git/objects/3b
513 .git/objects/92
514 .git/objects/92/b8b694ffb1675e5975148e1121810081dbdffe
515 .git/objects/54
517 .git/objects/a0
518 .git/objects/a0/423896973644771497bdc03eb99d5281615b51
519 .git/objects/d0
520 .git/objects/d0/492b368b66bdabf2ac1fd8c92b39d3db916e59
521 .git/objects/c4
522 .git/objects/c4/d59f390b9cfd4318117afde11d601c1085f241</tt></pre>
523 </div></div>
525 header identifying their length and their type. The type is either a
526 blob, a tree, a commit, or a tag.</p></div>
528 from .git/HEAD:</p></div>
531 <pre><tt>$ cat .git/HEAD
532 ref: refs/heads/master</tt></pre>
533 </div></div>
534 <div class="paragraph"><p>As you can see, this tells us which branch we’re currently on, and it
535 tells us this by naming a file under the .git directory, which itself
536 contains a SHA1 name referring to a commit object, which we can
537 examine with cat-file:</p></div>
540 <pre><tt>$ cat .git/refs/heads/master
541 c4d59f390b9cfd4318117afde11d601c1085f241
542 $ git cat-file -t c4d59f39
543 commit
544 $ git cat-file commit c4d59f39
545 tree d0492b368b66bdabf2ac1fd8c92b39d3db916e59
546 parent 54196cc2703dc165cbd373a65a4dcf22d50ae7f7
550 add emphasis</tt></pre>
551 </div></div>
555 <pre><tt>$ git ls-tree d0492b36
556 100644 blob a0423896973644771497bdc03eb99d5281615b51 file.txt
557 $ git cat-file blob a0423896
558 hello world!</tt></pre>
559 </div></div>
564 tree 92b8b694ffb1675e5975148e1121810081dbdffe
568 initial commit</tt></pre>
569 </div></div>
571 unusual in that it lacks any parent.</p></div>
573 to have multiple parents. In that case the commit represents a
574 merge, with the parent references pointing to the heads of the merged
575 branches.</p></div>
578 for details.</p></div>
582 <li>
583 <p>
585 snapshot of a directory tree at a particular point in the
587 connected into the project history.
588 </p>
589 </li>
590 <li>
591 <p>
592 "tree" objects represent the state of a single directory,
593 associating directory names to "blob" objects containing file
594 data and "tree" objects containing subdirectory information.
595 </p>
596 </li>
597 <li>
598 <p>
599 "blob" objects contain file data without any other structure.
600 </p>
601 </li>
602 <li>
603 <p>
604 References to commit objects at the head of each branch are
605 stored in files under .git/refs/heads/.
606 </p>
607 </li>
608 <li>
609 <p>
610 The name of the current branch is stored in .git/HEAD.
611 </p>
612 </li>
613 </ul></div>
615 But as we can see above, a tree can be referred to in many different
616 ways—by the SHA1 name for that tree, by the name of a commit that
617 refers to the tree, by the name of a branch whose head refers to that
618 tree, etc.--and most such commands can accept any of these names.</p></div>
620 designate such an argument.</p></div>
621 </div>
624 <div class="paragraph"><p>The primary tool we’ve been using to create commits is <tt>git-commit
626 your working tree. But what if you want to commit changes only to
627 certain files? Or only certain changes to certain files?</p></div>
628 <div class="paragraph"><p>If we look at the way commits are created under the cover, we’ll see
629 that there are more flexible ways creating commits.</p></div>
630 <div class="paragraph"><p>Continuing with our test-project, let’s modify file.txt again:</p></div>
634 </div></div>
635 <div class="paragraph"><p>but this time instead of immediately making the commit, let’s take an
636 intermediate step, and ask for diffs along the way to keep track of
640 <pre><tt>$ git diff
641 --- a/file.txt
642 +++ b/file.txt
644 hello world!
645 +hello world, again
646 $ git add file.txt
647 $ git diff</tt></pre>
648 </div></div>
653 <pre><tt>$ git diff HEAD
654 diff --git a/file.txt b/file.txt
656 --- a/file.txt
657 +++ b/file.txt
659 hello world!
660 +hello world, again</tt></pre>
661 </div></div>
662 <div class="paragraph"><p>So <em>git diff</em> is comparing against something other than the head.
663 The thing that it’s comparing against is actually the index file,
664 which is stored in .git/index in a binary format, but whose contents
665 we can examine with ls-files:</p></div>
668 <pre><tt>$ git ls-files --stage
670 $ git cat-file -t 513feba2
671 blob
672 $ git cat-file blob 513feba2
673 hello world!
674 hello world, again</tt></pre>
675 </div></div>
677 a reference to it in the index file. If we modify the file again,
679 output:</p></div>
683 $ git diff
685 --- a/file.txt
686 +++ b/file.txt
688 hello world!
689 hello world, again
690 +again?</tt></pre>
691 </div></div>
692 <div class="paragraph"><p>With the right arguments, <em>git diff</em> can also show us the difference
693 between the working directory and the last commit, or between the
694 index and the last commit:</p></div>
697 <pre><tt>$ git diff HEAD
698 diff --git a/file.txt b/file.txt
699 index a042389..ba3da7b 100644
700 --- a/file.txt
701 +++ b/file.txt
703 hello world!
704 +hello world, again
705 +again?
706 $ git diff --cached
707 diff --git a/file.txt b/file.txt
709 --- a/file.txt
710 +++ b/file.txt
712 hello world!
713 +hello world, again</tt></pre>
714 </div></div>
715 <div class="paragraph"><p>At any time, we can create a new commit using <em>git commit</em> (without
716 the "-a" option), and verify that the state committed only includes the
717 changes stored in the index file, not the additional change that is
718 still only in our working tree:</p></div>
722 $ git diff HEAD
723 diff --git a/file.txt b/file.txt
725 --- a/file.txt
726 +++ b/file.txt
728 hello world!
729 hello world, again
730 +again?</tt></pre>
731 </div></div>
732 <div class="paragraph"><p>So by default <em>git commit</em> uses the index to create the commit, not
733 the working tree; the "-a" option to commit tells it to first update
734 the index with all changes in the working tree.</p></div>
735 <div class="paragraph"><p>Finally, it’s worth looking at the effect of <em>git add</em> on the index
736 file:</p></div>
740 $ git add closing.txt</tt></pre>
741 </div></div>
742 <div class="paragraph"><p>The effect of the <em>git add</em> was to add one entry to the index file:</p></div>
745 <pre><tt>$ git ls-files --stage
748 </div></div>
750 current contents of the file:</p></div>
754 goodbye, world</tt></pre>
755 </div></div>
757 situation:</p></div>
760 <pre><tt>$ git status
761 # On branch master
762 # Changes to be committed:
763 # (use "git reset HEAD <file>..." to unstage)
764 #
765 # new file: closing.txt
766 #
767 # Changes not staged for commit:
768 # (use "git add <file>..." to update what will be committed)
769 #
770 # modified: file.txt
771 #</tt></pre>
772 </div></div>
774 it is listed as "Changes to be committed". Since file.txt has
775 changes in the working directory that aren’t reflected in the index,
777 commit" would create a commit that added closing.txt (with its new
779 <div class="paragraph"><p>Also, note that a bare <tt>git diff</tt> shows the changes to file.txt, but
780 not the addition of closing.txt, because the version of closing.txt
781 in the index file is identical to the one in the working directory.</p></div>
783 is also populated from the object database when checking out a
784 branch, and is used to hold the trees involved in a merge operation.
786 pages for details.</p></div>
787 </div>
791 pages for any of the git commands; one good place to start would be
793 should be able to find any unknown jargon in <a href="gitglossary.html">gitglossary(7)</a>.</p></div>
794 <div class="paragraph"><p>The <a href="user-manual.html">Git User’s Manual</a> provides a more
795 comprehensive introduction to git.</p></div>
796 <div class="paragraph"><p><a href="gitcvs-migration.html">gitcvs-migration(7)</a> explains how to
797 import a CVS repository into git, and shows how to use git in a
798 CVS-like way.</p></div>
801 <div class="paragraph"><p>For git developers, <a href="gitcore-tutorial.html">gitcore-tutorial(7)</a> goes
802 into detail on the lower-level git mechanisms involved in, for
803 example, creating a new commit.</p></div>
804 </div>
814 </div>
818 </div>
822 </div>
823 </div>
824 </body>
825 </html>