This tutorial will address the source code management (SCM) tool named Git. By following these steps you should learn about the basic usage of Git, which is required for the whole practical course. Furthermore, Git is a great SCM tool, and it's good to know how to use it. During this tutorial, we will follow Alan Turing's thoughts towards developing the Turing Machine.

More in-depth documentation can be found on the official home page, which mentions books, videos, and links to other tutorials and references.

Creating Commits

1. Read the Git for Computer Scientists introduction (skip this if you are already familiar with Git). 
2. For Linux, Git is available in its own package. Windows users can install msysGit. For MacOS, Git is available as part of Xcode; if you cannot install that, use Git for OSX.

3. Configure your name and email address (will be included in all commits you create):

   $ git config --global --add user.name "Your Name"
   $ git config --global --add user.email "<login>@informatik.uni-kiel.de"

4. Create a local repository for the "Turing Project":

   $ mkdir turing
   $ cd turing
   $ git init
   Initialized empty Git repository in ~/turing/.git/

   The .git subdirectory contains all history and metadata of the repository. You should not modify it.

5. Add and commit some content: copy notes.txt to your turing directory.

   $ git add notes.txt
   $ git commit -m "wrote some first notes"
   [master (root-commit) 2e73b34] wrote some first notes
    1 files changed, 5 insertions(+), 0 deletions(-)
    create mode 100644 notes.txt

   The file is now stored in the local history of your repository.

6. Edit notes.txt:
   
   1. Replace "fixed" with "infinite" in line 1.
   2. Replace "... (TODO)" with "a finite state machine" in line 4.

7. View the status of your current working copy:

   $ git status
   # On branch master
   # Changed but not updated:
   #   (use "git add <file>..." to update what will be committed)
   #   (use "git checkout -- <file>..." to discard changes in working directory)
   #
   #       modified:   notes.txt
   #
   no changes added to commit (use "git add" and/or "git commit -a")

8. Mark the modified file to include it in the next commit, then view the status again and compare with the previous output:

   $ git add notes.txt
   $ git status
   # On branch master
   # Changes to be committed:
   #   (use "git reset HEAD <file>..." to unstage)
   #
   #       modified:   notes.txt
   #

9. Commit the modified content to your local repository and view the status:

   $ git commit -m "modified tape length, found a controller for tape head"
   [master 52e2d49] modified tape length, found a controller for tape head
    1 files changed, 2 insertions(+), 2 deletions(-)
   $ git status
   # On branch master
   nothing to commit (working directory clean)

After the preceding steps you have two commits in your local repository, each with one file in the index. You have different commands for viewing these commits:

$ git log
commit 52e2d4946791c2725015853e5e261ce143c6fe8a
Author: Miro Spoenemann <msp@informatik.uni-kiel.de>
Date:   Mon Oct 15 15:00:14 2012 +0200

    modified tape length, found a controller for tape head

commit 2e73b34ac44480773fc0e52875b7353a087d8c6d
Author: Miro Spoenemann <msp@informatik.uni-kiel.de>
Date:   Mon Oct 15 12:14:06 2012 +0200

    wrote some first notes
 
$ git show 52e2d49
commit 52e2d4946791c2725015853e5e261ce143c6fe8a
Author: Miro Spoenemann <msp@informatik.uni-kiel.de>
Date:   Mon Oct 15 15:00:14 2012 +0200

    modified tape length, found a controller for tape head

diff --git a/notes.txt b/notes.txt
index 4ded2b3..bd422b3 100644
--- a/notes.txt
+++ b/notes.txt
@@ -1,5 +1,5 @@
- * A tape with fixed length
+ * A tape with infinite length
  * Tape head can read or write data
  * Tape head can move left or right
- * The head is controlled by ... (TODO)
+ * The head is controlled by a finite state machine

Note that each commit is identified by a looong hash value, but it is possible to use only a prefix when referencing them (if the prefix is not ambiguous): the example above uses 52e2d49 to identify the second commit. The commit hashes in your repository will be different from those seen in this tutorial, because the name of the author and the exact time of committing is also considered in the hash calculation. Also try the command gitk to get an overview of your commits (a better alternative available for MacOS is GitX).

Branching and Merging

In the previous section you have created two commits on the default branch, which is named master. Now you will create a new branch and commit there, thus adding complexity to the commit graph.

1. Create a branch with name sketches:

   $ git branch sketches

2. View the list of branches:

   $ git branch
   * master
     sketches

   The star reveals that you are still on the old master branch.

3. Switch to the new branch:

   $ git checkout sketches
   Switched to branch 'sketches'
   $ git branch
     master
   * sketches

   It is also possible to create a branch and switch immediately to it using the option -b of git checkout.

4. Add the new file examples.txt:

   $ git add examples.txt
   $ git commit -m "wrote first examples"
   [sketches cd63135] wrote first examples
    1 files changed, 20 insertions(+), 0 deletions(-)
    create mode 100644 examples.txt