gittutorial-2 - A tutorial introduction to Git: part two


   git *


   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.


   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
       $ git cat-file commit 54196cc2
       tree 92b8b694ffb1675e5975148e1121810081dbdffe
       author J. Bruce Fields <> 1143414668 -0500
       committer J. Bruce Fields <> 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

   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

       $ find .git/objects/

   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

       $ 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
       $ git cat-file -t c4d59f39
       $ git cat-file commit c4d59f39
       tree d0492b368b66bdabf2ac1fd8c92b39d3db916e59
       parent 54196cc2703dc165cbd373a65a4dcf22d50ae7f7
       author J. Bruce Fields <> 1143418702 -0500
       committer J. Bruce Fields <> 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 <> 1143414668 -0500
       committer J. Bruce Fields <> 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

   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

   *   "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 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
       $ 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

   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
       $ 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

   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

       $ 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
   gitcore-tutorial(7) and the relevant man pages for details.


   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 giteveryday(7). You should be able to
   find any unknown jargon in gitglossary(7).

   The Git User's Manual[1] provides a more comprehensive introduction to

   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[2].

   For Git developers, gitcore-tutorial(7) goes into detail on the
   lower-level Git mechanisms involved in, for example, creating a new


   gittutorial(7), gitcvs-migration(7), gitcore-tutorial(7),
   gitglossary(7), git-help(1), giteveryday(7), The Git User's Manual[1]


   Part of the git(1) suite.


    1. Git User's Manual

    2. howtos


Personal Opportunity - Free software gives you access to billions of dollars of software at no cost. Use this software for your business, personal use or to develop a profitable skill. Access to source code provides access to a level of capabilities/information that companies protect though copyrights. Open source is a core component of the Internet and it is available to you. Leverage the billions of dollars in resources and capabilities to build a career, establish a business or change the world. The potential is endless for those who understand the opportunity.

Business Opportunity - Goldman Sachs, IBM and countless large corporations are leveraging open source to reduce costs, develop products and increase their bottom lines. Learn what these companies know about open source and how open source can give you the advantage.

Free Software

Free Software provides computer programs and capabilities at no cost but more importantly, it provides the freedom to run, edit, contribute to, and share the software. The importance of free software is a matter of access, not price. Software at no cost is a benefit but ownership rights to the software and source code is far more significant.

Free Office Software - The Libre Office suite provides top desktop productivity tools for free. This includes, a word processor, spreadsheet, presentation engine, drawing and flowcharting, database and math applications. Libre Office is available for Linux or Windows.

Free Books

The Free Books Library is a collection of thousands of the most popular public domain books in an online readable format. The collection includes great classical literature and more recent works where the U.S. copyright has expired. These books are yours to read and use without restrictions.

Source Code - Want to change a program or know how it works? Open Source provides the source code for its programs so that anyone can use, modify or learn how to write those programs themselves. Visit the GNU source code repositories to download the source.


Study at Harvard, Stanford or MIT - Open edX provides free online courses from Harvard, MIT, Columbia, UC Berkeley and other top Universities. Hundreds of courses for almost all major subjects and course levels. Open edx also offers some paid courses and selected certifications.

Linux Manual Pages - A man or manual page is a form of software documentation found on Linux/Unix operating systems. Topics covered include computer programs (including library and system calls), formal standards and conventions, and even abstract concepts.