Why are we here?

• An introduction to version control.
• Some basic recipes for using git.
• Questions and answers.

Ask lots of questions.

Commit

• (noun) The state of your project at a certain point in time.
• (verb) To record the current state of your files in the repository.

In git, commits are uniquely identified by a 40 character hash, like this:

d8c554432e316851f7e66e0e7d8bfc8f08083618

But you can also name them using abbreviated hashes:

d8c55

And by a variety of symbolic names:

master
HEAD
version_1.0

If you are curious:

git rev-parse --help

Branches and tags

Symbolic names that refer to commits in your repository.

• Branches move forward with each new commit.

• Tags point at a fixed commit.

Creating a repository

From scratch:

$ git init myproject

With existing files:

$ cd myproject
$ git init

Add all your files to the repository:

$ git add .
$ git commit -m "added all my files"

Which looks something like this:

$ cd hello-2.9
$ git init
Initialized empty Git repository in /home/lkellogg/projects/harvard-git-workshop/sandbox/hello-2.9/.git/
$ git add .
$ git commit -m 'initial import'
[master (root-commit) d8c5544] initial import
 291 files changed, 90160 insertions(+)
 create mode 100644 ABOUT-NLS
 create mode 100644 AUTHORS
.
.
.
 create mode 100755 tests/last-1
 create mode 100755 tests/traditional-1

Adding files/modifications

Stage changes to be committed:

$ git add file1
$ git add file2

Commit the changes:

$ git commit

This will bring up an editor in which you can enter your commit message. You can also provide a message on the command line:

$ git commit -m "This is my commit message."

Tips

Stage all local modifications:

$ git add -u

(This does not add new files.)

What's changed?

To see files in your working copy that have been modified:

$ git status

To see how your working copy differs from the repository:

$ git diff

Output of git status with no pending changes:

$ git status
# On branch master
nothing to commit, working directory clean

Output of git status with modified files in the working copy:

$ git status
# On branch master
# 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:   src/hello.c
#
no changes added to commit (use "git add" and/or "git commit -a")

Output of git diff with modified files in the working copy:

$ git diff
diff --git a/src/hello.c b/src/hello.c
index 8c36915..b546447 100644
--- a/src/hello.c
+++ b/src/hello.c
@@ -16,6 +16,9 @@
    You should have received a copy of the GNU General Public License
    along with this program.  If not, see <http://www.gnu.org/licenses/>.  */

+#define PACKAGE_NAME "harvard_hello"
+#define PACKAGE_URL "http://harvard.edu/harvard_hello"
+
 #include <config.h>
 #include "system.h"
 #include "progname.h"

Viewing history

To view the commit messages in your repository:

$ git log

To see the commits along with the changes they introduced:

$ git log -p

The output of git log looks something like this:

commit 99bbf5e53f3d4c5cc7d3f9f0da0e03426e200695
Author: Sami Kerola <kerolasa@iki.fi>
Date:   Wed Nov 20 00:59:37 2013 +0000

    add git-version-gen from gnulib/build-aux

    * configure.ac: change AC_INIT version to call a script
    * git-version-gen: add version determinator script

commit 7037566cb87ea8ad59a9ab7079225510cef84ddd
Author: Reuben Thomas <rrt@sc3d.org>
Date:   Thu Oct 10 14:19:04 2013 +0100

    Update Thai strings.

The output of git log -p looks something like this:

commit 3a74f02ba241a8e2517362df6fd68dac462e60d5
Author: Lars Kellogg-Stedman <lars@redhat.com>
Date:   Mon Feb 24 23:05:52 2014 -0500

    a sample change

diff --git a/src/hello.c b/src/hello.c
index 8c36915..b546447 100644
--- a/src/hello.c
+++ b/src/hello.c
@@ -16,6 +16,9 @@
    You should have received a copy of the GNU General Public License
    along with this program.  If not, see <http://www.gnu.org/licenses/>.  */

+#define PACKAGE_NAME "harvard_hello"
+#define PACKAGE_URL "http://oddbit.com/harvard_hello"
+
 #include <config.h>
 #include "system.h"
 #include "progname.h"

Branches: Merging

To merge the changes in another branch into your current branch:

$ git merge newfeature

$ git merge newfeature
Updating 25d039f..1baae6b
Fast-forward
 slides.md | 14 ++++++++++++++
  1 file changed, 14 insertions(+)

Throwing away history

git reset discards history from your repository. To get rid of all commits after a specific commit:

$ git reset <commit>

For example:

$ git reset HEAD~

This command does not affect your working copy.

To also affect your working copy, add --hard:

$ git reset --hard HEAD~

Reasons to reset:

• You've made a series of bad local commits and you want to do them over in a more organized fashion.

• You want to discard a bunch of local (uncommitted) modifications and return return to HEAD:

  git reset --hard HEAD

$ git remote add origin git@code.seas.harvard.edu:myteam/myproject.git
$ git push -u origin master
Counting objects: 4193, done.
Delta compression using up to 8 threads.
Compressing objects: 100% (1072/1072), done.
Writing objects: 100% (4193/4193), 7.65 MiB | 0 bytes/s, done.
Total 4193 (delta 3095), reused 4193 (delta 3095)
To ../hello-remote.git/
 * [new branch]      master -> master
Branch master set up to track remote branch master from origin.

$ git clone git@code.seas.harvard.edu:myteam/myproject.git
Cloning into 'myproject'...
done.

$ git remote -v
origin  git@code.seas.harvard.edu:myteam/myproject.git (fetch)
origin  git@code.seas.harvard.edu:myteam/myproject.git (push)

$ git push
Counting objects: 5, done.
Delta compression using up to 8 threads.
Compressing objects: 100% (3/3), done.
Writing objects: 100% (3/3), 348 bytes | 0 bytes/s, done.
Total 3 (delta 1), reused 0 (delta 0)
To git@code.seas.harvard.edu:myteam/myproject.git
   f95bae7..f9120d5  master -> master

$ git pull
remote: Counting objects: 5, done.
remote: Compressing objects: 100% (3/3), done.
remote: Total 3 (delta 1), reused 0 (delta 0)
Unpacking objects: 100% (3/3), done.
From git@code.seas.harvard.edu:myteam/myproject.git
   f95bae7..f9120d5  master     -> origin/master
Updating f95bae7..f9120d5
Fast-forward
 TODO | 2 ++
 1 file changed, 2 insertions(+)

$ git push
To git@code.seas.harvard.edu:myteam/myproject.git
 ! [rejected]        master -> master (non-fast-forward)
error: failed to push some refs to '../hello-remote.git/'
hint: Updates were rejected because the tip of your current branch is behind
hint: its remote counterpart. Merge the remote changes (e.g. 'git pull')
hint: before pushing again.
hint: See the 'Note about fast-forwards' in 'git push --help' for details.

$ git pull
Auto-merging TODO
Merge made by the 'recursive' strategy.
 TODO | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

But what if Alice and Bob made overlapping changes?

$ git pull
remote: Counting objects: 10, done.
remote: Compressing objects: 100% (6/6), done.
remote: Total 6 (delta 3), reused 0 (delta 0)
Unpacking objects: 100% (6/6), done.
From git@code.seas.harvard.edu:myteam/myproject.git
   1827d91..dab8832  master     -> origin/master
Auto-merging TODO
CONFLICT (content): Merge conflict in TODO
Automatic merge failed; fix conflicts and then commit the result.

WRITE MORE CODE

GNU hello --- TODO

Todo:

<<<<<<< HEAD
    * Write more awesome code.
=======
    * buy new laptop
>>>>>>> dab88328f5f521bbd40135e36110d33924d0f529
    * submit a new hello.pot.

end of file TODO

To fix the conflict:

• Manually correct the file.

• Add the changes to the repository:

  git add TODO

• Commit the changes

  git commit

Initial configuration: Identity

Tell git who you are:

$ git config --global user.name "Lars Kellogg-Stedman"
$ git config --global user.email "lars@oddbit.com"

These values will be included in your commits:

$ git log
commit 60f3de47e36eebbcf4cde4655096e73d0dc24f3e
Author: Lars Kellogg-Stedman <lars@oddbit.com>
Date:   Fri Feb 14 17:52:01 2014 -0500

    This is my commit message.