- Remote Repositories
- Managing a Website Using
- Essential Commands
[ Check out my latest post on the HP Security Blog: “HP Security and the Internet of Things” ]
git is a wicked-powerful distributed revision control system. It is confusing to many, so there are myriad tutorials and explanations online to help people understand it. This one will focus on the fundamental concepts and tasks rather than trying to compete with the documentation.
"I'm an egotistical bastard, and I name all my projects after myself. First Linux, now git." ~ Linus Torvalds
- Working Directory
- - the working directory is the directory where you have content that you want to manage with
- - a commit is a full snapshot of the contents of your working directory (everything being tracked by
git, anyway), and it's kept track of using a unique 40 character SHA1 hash. This way, the exact state of your project can be referred to, copied, or restored at any time.
- - the index can be considered a staging area. It's where snapshots of changes are placed (using
git add) before they get committed. The index is crucial to
gitbecause it sits between the working directory and your various commits.
- - a branch is similar in concept to other versioning systems, but in
gitit's simply a pointer to a particular commit. As you make additional commits within a branch, the branch pointer moves to point to your latest one. To revert to a previous version of your code, the branch pointer is simply moved to point to another commit within that branch.
Understanding how these components work together is the key to understanding
First and foremost, it's important to understand that
git has something called an index that sits between your working directory and your commits. It's basically a staging area, so when you
git add you copy a snapshot of your working directory to the index, and when you
git commit you copy that same thing from the index to create a new commit.
It is crucial to understand the intermediate (staging area) nature of the
git index in order to grasp the relationship and differences between adding and committing content.
git status is very helpful in understanding git because it shows you the differences between the working directory and the index and previous commits. The "status" refers to the status of the working directory, so if you make a change in your content -- say to index.php -- and you run
git status, it'll show you what's changed that isn't yet staged for a commit (in your index):
$ git status
On branch master # Changed but not updated: # (use "git add
..." to update what will be committed) # (use "git checkout -- ..." to discard changes in working directory) # # modified: index.php # no changes added to commit (use "git add" and/or "git commit -a")
git diff is similar to
git status, but it shows the differences between various commits, and between the working directory and the index.
git diff --cached, on the other hand, shows you the difference between what's in the index vs. in your last commit.
Branches are just pointers to various commits. Every project has a default pointer called Master. As you continue to create commits within the default branch, the Master pointer follows you along so that it always points to the latest commit in the branch.
Branches are created using the
git branch command, which creates a new branch label which points to the current commit. So, until a new commit is made, both the previous and new branch labels will point to the current commit. One way to think of them is "File->Save As Copy" for your codebase.
Using this model, someone can then checkout the Master branch, make changes, and commit them. The result would look like this.
Keep in mind that the "Next Commit" commit would only happen for someone who had the Master branch checked out. If the same person who performed the
git branch command immediately made changes, added them, and then committed them, they'd have made another commit along the Test branch instead, like so:
If both branches had been checked out, worked on, and had a commit made, the diagram would look like so:
To change to and start working on a given branch, you have to check it out. This is done with the
git checkout command:
$ git checkout
Keep in mind that when you do this you will pull the copy of your project that exists at that branch's latest commit point, and it will overwrite your current working directory with that version of your project.
Creating code branches on a single system is enjoyable enough, but the real purpose of
git is allowing people in disparate locations to contribute to a project. You add remote repositories in
git by using the
git remote command, like so:
$ git remote add remotesite ssh://yourdomain.tld/path/to/remote/gitdirectory.git
[ This is using SSH as the protocol, but
git supports many others as well. ]
Then you push your content from your local repository to the remote one, using the name you've set up:
$ git push remotesite +master:refs/heads/master
Then, to perform updates, you simply run
git push with your target defined:
$ git push remotesite
If you have more than one person working on the project -- each doing his own thing -- eventually you're going to have to synchronize.
git handles this quite elegantly by simply moving the necessary branch pointers upon completion.
In the diagram below we merge Test into Master, and we simply end up with a later version of Master, as you might expect -- only this version includes the changes that existed in Test as well.
NOTE: You want to make sure you're fully committed before you attempt a merge. Not doing so invites wrath.
So now that we've seen the various components of
git in play, let's see how it all works together by creating a setup that allows us to manage a website remotely.
On Your Local System
Either have, or create, a web directory on your local system. This is the main place you'll make changes from. Start by changing directory into it.
Create some content.
Add the content to
git add index.html
Commit the changes.
On Your Server
Initiate a new
git repository on the server.
mkdir /path/website.git && cd /path/website.git && git init --bare
Create your hook that will checkout the code to your actual web directory.
GIT_WORK_TREE=/path/htdocs git checkout -f
chmod +x /path/website.git/hooks/post-update
Back on the Local System
Add the remote directory to the local config.
git remote add website ssh://path/server/website.git
Push the contents of the local repository to the remote one.
git push website +master:refs/heads/master
Then, change things locally and to upload changes, simply do a:
git push website
The changes will upload to the remote
git repository and trigger the post-update hook, which copies the contents of the repository to the working directory -- your live website directory (htdocs).
[ To update from the server side you can execute
git commands as usual, but you must provide environment context with each command, like so: ]
GIT_DIR=/path/website.git GIT_WORK_DIR=/path/htdocs git $some_git_command
[ On the server you won't have to push after you add and commit, as using the environment variables above will mean the committed changes will already be present in the repository. ]
git has a powerful feature called tags, which allow you to define an intuitive name to a given commit -- like "Gold Version", or "Version 3", or "Before Production".
These are done using
git tag (easy enough). You can then check out that version just like a branch, only this will point to that specific point in time.
git tag Gold
...and later on...
git checkout Gold
There are a good number of
git commands, and the documentation is excellent, so I won't cover many here. Here are some I think are worth mentioning.
git bisect- allows you to isolate the exact code push that caused a problem by executing
git bisect badto mark the current (broken location), then restore to a known-good configuration, and then git will step you through each commit you've made in between the two. For each one you then either git bisect good or bad until you've found your error-causing commit.
git stash- sets changes you've made off to the side in a way that lets you bring them back later. Usage: you are about to checkout another branch which would crush your current changes, so you
git stashthem before doing your checkout.
git rm / mv- deletes / moves items in the working directory with visibility to
gitso that you can commit afterwards.
git reset- resets to a specified state (commit).
git status- show the status of the working tree -- including differences between the index and the current commit, differences between the working tree and the index, and items within the working tree that are not tracked by
git log- show a log of commits
git clone- clone a repository into a new directory.
git clean- remove untracked files from the working directory.
1 Be sure to restrict access to your
.git directory on your server if it resides within the presented content.
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