| 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 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). 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 SHA1 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 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. |
| |
| 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 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 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 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) |
| # |
| # 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 link:everyday.html[Everyday Git]. 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], |
| link:everyday.html[Everyday Git], |
| link:user-manual.html[The Git User's Manual] |
| |
| GIT |
| --- |
| Part of the linkgit:git[1] suite. |