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    Command:

    gittutorial-2

    
    
    

    SYNOPSIS

           git *
    
    
    

    DESCRIPTION

           You should work through 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 files changed, 1 insertions(+), 0 deletions(-)
                create mode 100644 file.txt
               $ echo ?hello world!? >file.txt
               $ git commit -a -m "add emphasis"
               [master c4d59f3] add emphasis
                1 files changed, 1 insertions(+), 1 deletions(-)
    
           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:
           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:
    
               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 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
           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
               +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 "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
               #
               # Changed but not updated:
               #   (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
           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 Everyday git[1]. You should be able to
           find any unknown jargon in gitglossary(7).
    
           The Git User's Manual[2] provides a more comprehensive introduction to
           git.
    
           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 howtos[3].
    
           For git developers, gitcore-tutorial(7) goes into detail on the
           lower-level git mechanisms involved in, for example, creating a new
           commit.
    
            3. howtos
               file:///usr/share/doc/git-1.7.1/howto-index.html
    
    
    

    Git 1.7.1 03/04/2013 GITTUTORIAL-2(7)

    
    
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