Git Basics

Git is a version control system for software development.

There are two ways to manage versions using Git. Either through a series of commits or through the use of branches.

In this tutorial you will learn how to use the version control of Git using commit and go back to a previous commit.


Prerequisite:

Having a programming background is required to do this tutorial.


Install a Git Client

  1. Go to Git.

  2. Download the Git client installer and install.

  3. Test if the Git client is installed successfully. Open a terminal window.

    > git

    Output:

    usage: git [--version] [--help] [-C <path>] [-c name=value]
           [--exec-path[=<path>]] [--html-path] [--man-path] [--info-path]
           [-p|--paginate|--no-pager] [--no-replace-objects] [--bare]
           [--git-dir=<path>] [--work-tree=<path>] [--namespace=<name>]
           <command> [<args>]

The most commonly used git commands are:
   add        Add file contents to the index
   bisect     Find by binary search the change that introduced a bug
   :
   :

    You should see the help screen of the Git client.


Create several text files and use Git to manage the versions of the files

In this tutorial, you will be creating several .txt files that will contain a list of words representing each letter of the alphabet. The use of this approach will help you understand how Git works. Once you are familiar with Git, you may use Git with your source code (e.g., .java file).

  1. Create the directory gittemp in the root directory. Go to the created directory.

    > mkdir gittemp
> cd gittemp


  2. Create the subdirectory alphabet in the gittemp directory. Go to the created directory.

    > mkdir alphabet
> cd alphabet

    The alphabet directory represents a working directory containing your source code. As mentioned earlier, instead of source code, text files will be used in this tutorial.


  3. Make the alphabet directory a local Git repository.

    > git init

    Output:

    Initialized empty Git repository in /gittemp/alphabet/.git/

    A hidden directory named .git is created after you issued the git init command. You should not modify the contents of the .git hidden directory. This contains information that will track the changes you made inside the alphabet directory.


  4. Check the status of your local Git repository.

    > git status

    Output:

    On branch master

Initial commit

nothing to commit (create/copy files and use "git add" to track)

    As stated in the status, there is nothing to commit. This will change once you start creating files.


  5. Create a file animal.txt in the alphabet directory with the following contents:

    ant
bat
cat
dog
eagle
fox
goat
horse


  6. Check the status of your local Git repository.

    > git status

    Output:

    On branch master

Initial commit

Untracked files:
  (use "git add <file>..." to include in what will be committed)

        animal.txt

nothing added to commit but untracked files present (use "git add" to track)

    Git detected the animal.txt file that you created. However, it is currently untracked by Git. Untracked files are those in the working directory but are NOT in index.

    Git does not track all the files you created since it is possible that some of the files are non-essential. You need to explicitly tell Git to track a file.


  7. Let Git track animal.txt.

    > git add animal.txt

    If there are multiple files that you want Git to track, you may use the same git add command and just separate the files with a space.

    The git add animal.txt command places the animal.txt in the index. Those files in the index are staged. Staging means that a file is being prepared to be committed.


  8. Check the status of your local Git repository.

    > git status

    Output:

    On branch master

Initial commit

Changes to be committed:
  (use "git rm --cached <file>..." to unstage)

        new file:   animal.txt

    At this point, animal.txt is already in index and being tracked by Git. However, changes made in animal.txt is not yet committed.


  9. Commit the changes made in animal.txt.

    > git commit -m "added words from ant to horse"

    Output:

    [master (root-commit) 828389d] added words from ant to horse
 1 file changed, 8 insertions(+)
 create mode 100644 animal.txt

    Note that if there are other files that are staged aside from animal.txt, the changes made in those files will also be committed.


  10. Check the status of your local Git repository.

    > git status

    Output:

    On branch master
nothing to commit, working directory clean

    At this point, changes made in animal.txt are committed. In the git commit command, you included a message added words from ant to horse to give a short description on what changes were committed. This is very useful when you perform several commits later and you want to undo previous commits.


  11. Visually look at the the details regarding the commit you performed.

    > gitk

    Output:

    *master added words from ant to horse

    gitk is a tool which allows you to visualize the commits you performed. You may use gitk as a guide when you need to undo previous commits.


  12. Update animal.txt to include words iguana to pig:

    ant
bat
cat
dog
eagle
fox
goat
horse
iguana
jaguar
kangaroo
lion
monkey
newt
octopus
pig


  13. Track and commit the changes made in animal.txt.

    > git add animal.txt
> git commit -m "added words from iguana to pig"

    Output:

    [master 5394740] added words from iguana to pig
 1 file changed, 9 insertions(+), 1 deletion(-)


  14. Update animal.txt to include words quail to zebra. In addition, delete dog and change monkey to mouse:

    ant
bat
cat
eagle
fox
goat
horse
iguana
jaguar
kangaroo
lion
mouse
newt
octopus
pig
quail
rabbit
snail
tiger
uakari
vulture
wolf
x-ray tetra
yak
zebra


  15. Track and commit the changes made in animal.txt.

    > git add animal.txt
> git commit -m "added words from quail to zebra, deleted dog, and changed monkey to mouse"

    Output:

    [master f1fbe3c] added words from quail to zebra, deleted dog, and changed monkey to mouse
 1 file changed, 12 insertions(+), 3 deletions(-)


Checkout Previous Commits

Checking out previous commits allows you to inspect the state of the files after a particular commit. This is done by changing back the contents of the working directory. However, the latest committed changes remain intact.

  1. Get a summary of the commits performed in the local Git repository.

    > git log

    Output:

    commit f1fbe3cfccfa0343f0b4ffe0d9c35967174b5f77
Author: Alexis V. Pantola <pantolav@gmail.com>
Date:   Sun Jan 10 23:32:48 2016 +0800

    added words from quail to zebra, deleted dog, and changed monkey to
commit 539474028f1a73dcb01111ce79eefa4a50a31baf
Author: Alexis V. Pantola <pantolav@gmail.com>
Date:   Sun Jan 10 23:31:45 2016 +0800

    added words from iguana to pig

commit 828389d84eedf01b1ca4fa842c879139bc8e62b5
Author: Alexis V. Pantola <pantolav@gmail.com>
Date:   Sun Jan 10 23:29:40 2016 +0800

    added words from ant to horse

    NOTE: You may press Q to exit the log console.

    Notice that the three commits performed earlier are listed in the log. In addition, there are unique hash values identifying each commit. As an example, in the log above, the commit added words from ant to horse has a hash value of 828389d84eedf01b1ca4fa842c879139bc8e62b5. Note that the hash value in your repository is not necessarily the same as the one shown in this tutorial.

    The hash value is important to undo previous commits and go to a particular state of a file.


  2. Inspect the state of the file when the words ant to horse has just been added.

    > git checkout <hash of commit for "added words from ant to horse">

    Example:

    > git checkout 828389d84eedf01b1ca4fa842c879139bc8e62b5

    Output:

    Note: checking out '828389d84eedf01b1ca4fa842c879139bc8e62b5'.

You are in 'detached HEAD' state. You can look around, make experimental
changes and commit them, and you can discard any commits you make in this
state without impacting any branches by performing another checkout.

If you want to create a new branch to retain commits you create, you may
do so (now or later) by using -b with the checkout command again. Example:

  git checkout -b new_branch_name

HEAD is now at 828389d... added words from ant to horse


  3. Open (or reload) animal.txtand verify that its content is now the following:

    ant
bat
cat
dog
eagle
fox
goat
horse

    As expected, you went back to the state when the words ant to horse has just been added to animal.txt.


  4. Check the contents of the log.

    > git log

    Output:

    commit 828389d84eedf01b1ca4fa842c879139bc8e62b5
Author: Alexis V. Pantola <pantolav@gmail.com>
Date:   Sun Jan 10 23:29:40 2016 +0800

    added words from ant to horse

    Notice that the log has only one commit.


  5. Go back to the last commit you made (i.e., added words from quail to zebra, deleted dog, and changed monkey to mouse).

    > git checkout master

    Output:

    Previous HEAD position was 828389d... added words from ant to horse
Switched to branch 'master'


  6. Open (or reload) animal.txtand verify that its content is now the following:

    ant
bat
cat
eagle
fox
goat
horse
iguana
jaguar
kangaroo
lion
mouse
newt
octopus
pig
quail
rabbit
snail
tiger
uakari
vulture
wolf
x-ray tetra
yak
zebra

    As expected, you are able to return to the latest state of the file.


Go back to a particular Commit

The git checkout command that was demonstrated above allows you to inspect the state of a file of a particular commit. However, the latest committed changes remain intact. In the event that you really want to go back to a particular commit (e.g., you realized that you erroneously changed the contents of a file and want to go back to its correct state), you may use the git reset command.

  1. Get a summary of the commits performed in the local Git repository.

    > git log

    Output:

    commit f1fbe3cfccfa0343f0b4ffe0d9c35967174b5f77
Author: Alexis V. Pantola <pantolav@gmail.com>
Date:   Sun Jan 10 23:32:48 2016 +0800

    added words from quail to zebra, deleted dog, and changed monkey to
commit 539474028f1a73dcb01111ce79eefa4a50a31baf
Author: Alexis V. Pantola <pantolav@gmail.com>
Date:   Sun Jan 10 23:31:45 2016 +0800

    added words from iguana to pig

commit 828389d84eedf01b1ca4fa842c879139bc8e62b5
Author: Alexis V. Pantola <pantolav@gmail.com>
Date:   Sun Jan 10 23:29:40 2016 +0800

    added words from ant to horse

    This is the same log that was shown in the previous steps.


  2. Go back to the state of the file when the words ant to horse has just been added.

    > git reset --hard <hash of commit for "added words from ant to horse">

    Example:

    > git reset --hard 828389d84eedf01b1ca4fa842c879139bc8e62b5

    Output:

    HEAD is now at 828389d added words from ant to horse


  3. Open (or reload) animal.txtand verify that its content is now the following:

    ant
bat
cat
dog
eagle
fox
goat
horse

    Similar to git checkout, you are able to go back to the state when the words ant to horse has just been added to animal.txt using git reset. You will see later the difference of these two commands.


  4. Check the contents of the log.

    > git log

    Output:

    commit 828389d84eedf01b1ca4fa842c879139bc8e62b5
Author: Alexis V. Pantola <pantolav@gmail.com>
Date:   Sun Jan 10 23:29:40 2016 +0800

    added words from ant to horse


  5. TRY to go back to the last commit you made (i.e., added words from quail to zebra, deleted dog, and changed monkey to mouse).

    > git checkout master

    Output:

    Already on 'master'

    Notice that the output is Already on 'master'. This means that you have reset the last commit from added words from quail to zebra, deleted dog, and changed monkey to mouse to added words from ant to horse. The added words from quail to zebra, deleted dog, and changed monkey to mousecommit as well as the added words from iguana to pig commit are already removed.


  6. Open (or reload) animal.txtand verify that its content is STILL ant to horse:

    ant
bat
cat
dog
eagle
fox
goat
horse


End of Tutorial

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