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Diffstat (limited to 'Documentation')
| -rw-r--r-- | Documentation/Makefile | 1 | ||||
| -rw-r--r-- | Documentation/tutorial-2.txt | 391 | ||||
| -rw-r--r-- | Documentation/tutorial.txt | 28 |
3 files changed, 414 insertions, 6 deletions
diff --git a/Documentation/Makefile b/Documentation/Makefile index c1af22ce04..2a08f592d9 100644 --- a/Documentation/Makefile +++ b/Documentation/Makefile @@ -7,6 +7,7 @@ MAN7_TXT=git.txt DOC_HTML=$(patsubst %.txt,%.html,$(MAN1_TXT) $(MAN7_TXT)) ARTICLES = tutorial +ARTICLES += tutorial-2 ARTICLES += core-tutorial ARTICLES += cvs-migration ARTICLES += diffcore diff --git a/Documentation/tutorial-2.txt b/Documentation/tutorial-2.txt new file mode 100644 index 0000000000..a3d45ee569 --- /dev/null +++ b/Documentation/tutorial-2.txt @@ -0,0 +1,391 @@ +A tutorial introduction to git: part two +======================================== + +You should work through link:tutorial.html[A tutorial introduction to +git] before reading this tutorial. + +The goal of this tutorial is to introduce two fundamental pieces of +git's architecture--the object database and the index file--and to +provide the reader with everything necessary to understand the rest +of the git documentation. + +The git object database +----------------------- + +Let's start a new project and create a small amount of history: + +------------------------------------------------ +$ mkdir test-project +$ cd test-project +$ git init-db +defaulting to local storage area +$ echo 'hello world' > file.txt +$ git add . +$ git commit -a -m "initial commit" +Committing initial tree 92b8b694ffb1675e5975148e1121810081dbdffe +$ echo 'hello world!' >file.txt +$ git commit -a -m "add emphasis" +------------------------------------------------ + +What are the 40 digits of hex that git responded to the first commit +with? + +We saw in part one of the tutorial that commits have names like this. +It turns out that every object in the git history is stored under +such a 40-digit hex name. That name is the SHA1 hash of the object's +contents; among other things, this ensures that git will never store +the same data twice (since identical data is given an identical SHA1 +name), and that the contents of a git object will never change (since +that would change the object's name as well). + +We can ask git about this particular object with the cat-file +command--just cut-and-paste from the reply to the initial commit, to +save yourself typing all 40 hex digits: + +------------------------------------------------ +$ git cat-file -t 92b8b694ffb1675e5975148e1121810081dbdffe +tree +------------------------------------------------ + +A tree can refer to one or more "blob" objects, each corresponding to +a file. In addition, a tree can also refer to other tree objects, +thus creating a directory heirarchy. You can examine the contents of +any tree using ls-tree (remember that a long enough initial portion +of the SHA1 will also work): + +------------------------------------------------ +$ git ls-tree 92b8b694 +100644 blob 3b18e512dba79e4c8300dd08aeb37f8e728b8dad file.txt +------------------------------------------------ + +Thus we see that this tree has one file in it. The SHA1 hash is a +reference to that file's data: + +------------------------------------------------ +$ git cat-file -t 3b18e512 +blob +------------------------------------------------ + +A "blob" is just file data, which we can also examine with cat-file: + +------------------------------------------------ +$ git cat-file blob 3b18e512 +hello world +------------------------------------------------ + +Note that this is the old file data; so the object that git named in +its response to the initial tree was a tree with a snapshot of the +directory state that was recorded by the first commit. + +All of these objects are stored under their SHA1 names inside the git +directory: + +------------------------------------------------ +$ find .git/objects/ +.git/objects/ +.git/objects/pack +.git/objects/info +.git/objects/3b +.git/objects/3b/18e512dba79e4c8300dd08aeb37f8e728b8dad +.git/objects/92 +.git/objects/92/b8b694ffb1675e5975148e1121810081dbdffe +.git/objects/54 +.git/objects/54/196cc2703dc165cbd373a65a4dcf22d50ae7f7 +.git/objects/a0 +.git/objects/a0/423896973644771497bdc03eb99d5281615b51 +.git/objects/d0 +.git/objects/d0/492b368b66bdabf2ac1fd8c92b39d3db916e59 +.git/objects/c4 +.git/objects/c4/d59f390b9cfd4318117afde11d601c1085f241 +------------------------------------------------ + +and the contents of these files is just the compressed data plus a +header identifying their length and their type. The type is either a +blob, a tree, a commit, or a tag. We've seen a blob and a tree now, +so next we should look at a commit. + +The simplest commit to find is the HEAD commit, which we can find +from .git/HEAD: + +------------------------------------------------ +$ cat .git/HEAD +ref: refs/heads/master +------------------------------------------------ + +As you can see, this tells us which branch we're currently on, and it +tells us this by naming a file under the .git directory, which itself +contains a SHA1 name referring to a commit object, which we can +examine with cat-file: + +------------------------------------------------ +$ cat .git/refs/heads/master +c4d59f390b9cfd4318117afde11d601c1085f241 +$ git cat-file -t c4d59f39 +commit +$ git cat-file commit c4d59f39 +tree d0492b368b66bdabf2ac1fd8c92b39d3db916e59 +parent 54196cc2703dc165cbd373a65a4dcf22d50ae7f7 +author J. Bruce Fields <bfields@puzzle.fieldses.org> 1143418702 -0500 +committer J. Bruce Fields <bfields@puzzle.fieldses.org> 1143418702 -0500 + +add emphasis +------------------------------------------------ + +The "tree" object here refers to the new state of the tree: + +------------------------------------------------ +$ git ls-tree d0492b36 +100644 blob a0423896973644771497bdc03eb99d5281615b51 file.txt +$ git cat-file commit a0423896 +hello world! +------------------------------------------------ + +and the "parent" object refers to the previous commit: + +------------------------------------------------ +$ git-cat-file commit 54196cc2 +tree 92b8b694ffb1675e5975148e1121810081dbdffe +author J. Bruce Fields <bfields@puzzle.fieldses.org> 1143414668 -0500 +committer J. Bruce Fields <bfields@puzzle.fieldses.org> 1143414668 -0500 + +initial commit +------------------------------------------------ + +The tree object is the tree we examined first, and this commit is +unusual in that it lacks any parent. + +Most commits have only one parent, but it is also common for a commit +to have multiple parents. In that case the commit represents a +merge, with the parent references pointing to the heads of the merged +branches. + +Besides blobs, trees, and commits, the only remaining type of object +is a "tag", which we won't discuss here; refer to gitlink:git-tag[1] +for details. + +So now we know how git uses the object database to represent a +project's history: + + * "commit" objects refer to "tree" objects representing the + snapshot of a directory tree at a particular point in the + history, and refer to "parent" commits to show how they're + connected into the project history. + * "tree" objects represent the state of a single directory, + associating directory names to "blob" objects containing file + data and "tree" objects containing subdirectory information. + * "blob" objects contain file data without any other structure. + * References to commit objects at the head of each branch are + stored in files under .git/refs/heads/. + * The name of the current branch is stored in .git/HEAD. + +Note, by the way, that lots of commands take a tree as an argument. +But as we can see above, a tree can be referred to in many different +ways--by the SHA1 name for that tree, by the name of a commit that +refers to the tree, by the name of a branch whose head refers to that +tree, etc.--and most such commands can accept any of these names. + +In command synopses, the word "tree-ish" is sometimes used to +designate such an argument. + +The index file +-------------- + +The primary tool we've been using to create commits is "git commit +-a", which creates a commit including every change you've made to +your working tree. But what if you want to commit changes only to +certain files? Or only certain changes to certain files? + +If we look at the way commits are created under the cover, we'll see +that there are more flexible ways creating commits. + +Continuing with our test-project, let's modify file.txt again: + +------------------------------------------------ +$ echo "hello world, again" >>file.txt +------------------------------------------------ + +but this time instead of immediately making the commit, let's take an +intermediate step, and ask for diffs along the way to keep track of +what's happening: + +------------------------------------------------ +$ git diff +--- a/file.txt ++++ b/file.txt +@@ -1 +1,2 @@ + hello world! + +hello world, again +$ git update-index file.txt +$ git diff +------------------------------------------------ + +The last diff is empty, but no new commits have been made, and the +head still doesn't contain the new line: + +------------------------------------------------ +$ git-diff HEAD +diff --git a/file.txt b/file.txt +index a042389..513feba 100644 +--- a/file.txt ++++ b/file.txt +@@ -1 +1,2 @@ + hello world! + +hello world, again +------------------------------------------------ + +So "git diff" is comparing against something other than the head. +The thing that it's comparing against is actually the index file, +which is stored in .git/index in a binary format, but whose contents +we can examine with ls-files: + +------------------------------------------------ +$ git ls-files --stage +100644 513feba2e53ebbd2532419ded848ba19de88ba00 0 file.txt +$ git cat-file -t 513feba2 +blob +$ git cat-file blob 513feba2 +hello world, again +------------------------------------------------ + +So what our "git update-index" did was store a new blob and then put +a reference to it in the index file. If we modify the file again, +we'll see that the new modifications are reflected in the "git-diff" +output: + +------------------------------------------------ +$ echo 'again?' >>file.txt +$ git diff +index 513feba..ba3da7b 100644 +--- a/file.txt ++++ b/file.txt +@@ -1,2 +1,3 @@ + hello world! + hello world, again ++again? +------------------------------------------------ + +With the right arguments, git diff can also show us the difference +between the working directory and the last commit, or between the +index and the last commit: + +------------------------------------------------ +$ git diff HEAD +diff --git a/file.txt b/file.txt +index a042389..ba3da7b 100644 +--- a/file.txt ++++ b/file.txt +@@ -1 +1,3 @@ + hello world! ++hello world, again ++again? +$ git diff --cached +diff --git a/file.txt b/file.txt +index a042389..513feba 100644 +--- a/file.txt ++++ b/file.txt +@@ -1 +1,2 @@ + hello world! ++hello world, again +------------------------------------------------ + +At any time, we can create a new commit using "git commit" (without +the -a option), and verify that the state committed only includes the +changes stored in the index file, not the additional change that is +still only in our working tree: + +------------------------------------------------ +$ git commit -m "repeat" +$ git diff HEAD +diff --git a/file.txt b/file.txt +index 513feba..ba3da7b 100644 +--- a/file.txt ++++ b/file.txt +@@ -1,2 +1,3 @@ + hello world! + hello world, again ++again? +------------------------------------------------ + +So by default "git commit" uses the index to create the commit, not +the working tree; the -a option to commit tells it to first update +the index with all changes in the working tree. + +Finally, it's worth looking at the effect of "git add" on the index +file: + +------------------------------------------------ +$ echo "goodbye, world" >closing.txt +$ git add closing.txt +------------------------------------------------ + +The effect of the "git add" was to add one entry to the index file: + +------------------------------------------------ +$ git ls-files --stage +100644 8b9743b20d4b15be3955fc8d5cd2b09cd2336138 0 closing.txt +100644 513feba2e53ebbd2532419ded848ba19de88ba00 0 file.txt +------------------------------------------------ + +And, as you can see with cat-file, this new entry refers to the +current contents of the file: + +------------------------------------------------ +$ git cat-file blob a6b11f7a +goodbye, word +------------------------------------------------ + +The "status" command is a useful way to get a quick summary of the +situation: + +------------------------------------------------ +$ git status +# +# Updated but not checked in: +# (will commit) +# +# new file: closing.txt +# +# +# Changed but not updated: +# (use git-update-index to mark for commit) +# +# modified: file.txt +# +------------------------------------------------ + +Since the current state of closing.txt is cached in the index file, +it is listed as "updated but not checked in". Since file.txt has +changes in the working directory that aren't reflected in the index, +it is marked "changed but not updated". At this point, running "git +commit" would create a commit that added closing.txt (with its new +contents), but that didn't modify file.txt. + +Also, note that a bare "git diff" shows the changes to file.txt, but +not the addition of closing.txt, because the version of closing.txt +in the index file is identical to the one in the working directory. + +In addition to being the staging area for new commits, the index file +is also populated from the object database when checking out a +branch, and is used to hold the trees involved in a merge operation. +See the link:core-tutorial.txt[core tutorial] and the relevant man +pages for details. + +What next? +---------- + +At this point you should know everything necessary to read the man +pages for any of the git commands; one good place to start would be +with the commands mentioned in link:everday.html[Everyday git]. You +should be able to find any unknown jargon in the +link:glossary.html[Glosssay]. + +The link:cvs-migration.html[CVS migration] document explains how to +import a CVS repository into git, and shows how to use git in a +CVS-like way. + +For some interesting examples of git use, see the +link:howto-index.html[howtos]. + +For git developers, the link:core-tutorial.html[Core tutorial] goes +into detail on the lower-level git mechanisms involved in, for +example, creating a new commit. diff --git a/Documentation/tutorial.txt b/Documentation/tutorial.txt index 4c298c6e71..79781adf4f 100644 --- a/Documentation/tutorial.txt +++ b/Documentation/tutorial.txt @@ -442,7 +442,25 @@ $ git diff v2.5:Makefile HEAD:Makefile.in Next Steps ---------- -Some good commands to explore next: +This tutorial should be enough to perform basic distributed revision +control for your projects. However, to fully understand the depth +and power of git you need to understand two simple ideas on which it +is based: + + * The object database is the rather elegant system used to + store the history of your project--files, directories, and + commits. + + * The index file is a cache of the state of a directory tree, + used to create commits, check out working directories, and + hold the various trees involved in a merge. + +link:tutorial-2.html[Part two of this tutorial] explains the object +database, the index file, and a few other odds and ends that you'll +need to make the most of git. + +If you don't want to consider with that right away, a few other +digressions that may be interesting at this point are: * gitlink:git-format-patch[1], gitlink:git-am[1]: These convert series of git commits into emailed patches, and vice versa, @@ -456,8 +474,6 @@ Some good commands to explore next: smart enough to perform a close-to-optimal search even in the case of complex non-linear history with lots of merged branches. -Other good starting points include link:everyday.html[Everday GIT -with 20 Commands Or So] and link:cvs-migration.html[git for CVS -users]. Also, link:core-tutorial.html[A short git tutorial] gives an -introduction to lower-level git commands for advanced users and -developers. + * link:everyday.html[Everday GIT with 20 Commands Or So] + + * link:cvs-migration.html[git for CVS users]. |