If you need to go back in time to a very specific point in a repo's history, Git enables time travel very nicely with the checkout command. This is really useful for troubleshooting a specific working version of the app, or going back in time when a certain bug was present.
The checkout command changes all the contents of your local project directory to a specified state, like the most recent commit on a different branch, or a specific commit on any branch.
It's most frequently used to switch between different branches. In this case, we will change the local directory contents to a specific commit.
The syntax is simple:
git checkout <commitSha>
In case you didn't know what the commitSha is, it's an alphanumeric 'id' of the commit. It's 40 characters long. For example: 73f33074ada943eb5faf3cb3b5bc11059bda30a8. One way to see it is with the git log command.
On GitHub you can see the first 7 characters, so 73f3307. This is called the partial hash. The partial hash for a git command can be as short as 4 characters if it's distinct from other repo commits' partial hashes.
The partial hash is enough for the checkout command. So you could do git checkout 73f3307 to check out that specific commit.
Checking out a specific commit is is not like checking out a branch. If you want to make some changes in reference to that commit, it's best to create a new branch in reference to that commit, or else you may lose your progress. Here is an excellent explainer from GitTower.
The HEAD pointer in Git determines your current working revision (and thereby the files that are placed in your project's working directory). Normally, when checking out a branch, Git automatically moves the HEAD pointer along when you create new commits: you're automatically and always on the newest commit of the chosen branch.
When you instead choose to check out a specific commit hash, Git will NOT do this for you. This means that when you make changes and commit them, these changes do NOT belong to any branch. The consequence is that these changes can easily get lost once you check out a different revision or branch: not being recorded in the context of a branch, you lack the possibility to access that state easily.
To make a long story short: be very careful when checking out a specific commit instead of a branch (and make sure this is really what you want and need).
To create a branch from a specific commit, use the git branch command.
git branch <branchName> [<commit-id>]
This creates a new branch, branchName whose head points to specified commit-id. For example, the following creates a develop branch from the specified commit hash.
git branch newBranch 73f33074ada943eb5faf3cb3b5bc11059bda30a8
This creates a new branch newBranch whose head points to commit 73f33074ada943eb5faf3cb3b5bc11059bda30a8.