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diff --git a/Documentation/gittutorial-2.txt b/Documentation/gittutorial-2.txt new file mode 100644 index 0000000000..30d2119565 --- /dev/null +++ b/Documentation/gittutorial-2.txt @@ -0,0 +1,436 @@ +gittutorial-2(7) +================ + +NAME +---- +gittutorial-2 - A tutorial introduction to Git: part two + +SYNOPSIS +-------- +[verse] +git * + +DESCRIPTION +----------- + +You should work through linkgit:gittutorial[7] 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 +Initialized empty Git repository in .git/ +$ echo 'hello world' > file.txt +$ git add . +$ git commit -a -m "initial commit" +[master (root-commit) 54196cc] initial commit + 1 file changed, 1 insertion(+) + create mode 100644 file.txt +$ echo 'hello world!' >file.txt +$ git commit -a -m "add emphasis" +[master c4d59f3] add emphasis + 1 file changed, 1 insertion(+), 1 deletion(-) +------------------------------------------------ + +What are the 7 digits of hex that Git responded to the 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 +a 40-digit hex name. That name is the SHA-1 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 SHA-1 +name), and that the contents of a Git object will never change (since +that would change the object's name as well). The 7 char hex strings +here are simply the abbreviation of such 40 character long strings. +Abbreviations can be used everywhere where the 40 character strings +can be used, so long as they are unambiguous. + +It is expected that the content of the commit object you created while +following the example above generates a different SHA-1 hash than +the one shown above because the commit object records the time when +it was created and the name of the person performing the commit. + +We can ask Git about this particular object with the `cat-file` +command. Don't copy the 40 hex digits from this example but use those +from your own version. Note that you can shorten it to only a few +characters to save yourself typing all 40 hex digits: + +------------------------------------------------ +$ git cat-file -t 54196cc2 +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 +------------------------------------------------ + +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 hierarchy. You can examine the contents of +any tree using ls-tree (remember that a long enough initial portion +of the SHA-1 will also work): + +------------------------------------------------ +$ git ls-tree 92b8b694 +100644 blob 3b18e512dba79e4c8300dd08aeb37f8e728b8dad file.txt +------------------------------------------------ + +Thus we see that this tree has one file in it. The SHA-1 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 SHA-1 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. + +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 SHA-1 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 blob 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 linkgit: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 SHA-1 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 add 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! +hello world, again +------------------------------------------------ + +So what our 'git add' 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 8b9743b2 +goodbye, world +------------------------------------------------ + +The "status" command is a useful way to get a quick summary of the +situation: + +------------------------------------------------ +$ git status +On branch master +Changes to be committed: + (use "git reset HEAD <file>..." to unstage) + + new file: closing.txt + +Changes not staged for commit: + (use "git add <file>..." to update what will be committed) + (use "git checkout -- <file>..." to discard changes in working directory) + + modified: file.txt + +------------------------------------------------ + +Since the current state of closing.txt is cached in the index file, +it is listed as "Changes to be committed". 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 linkgit:gitcore-tutorial[7] 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 linkgit:giteveryday[7]. You +should be able to find any unknown jargon in linkgit:gitglossary[7]. + +The link:user-manual.html[Git User's Manual] provides a more +comprehensive introduction to Git. + +linkgit:gitcvs-migration[7] 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, linkgit:gitcore-tutorial[7] goes +into detail on the lower-level Git mechanisms involved in, for +example, creating a new commit. + +SEE ALSO +-------- +linkgit:gittutorial[7], +linkgit:gitcvs-migration[7], +linkgit:gitcore-tutorial[7], +linkgit:gitglossary[7], +linkgit:git-help[1], +linkgit:giteveryday[7], +link:user-manual.html[The Git User's Manual] + +GIT +--- +Part of the linkgit:git[1] suite. |