In the previous tutorial, the checkout subcommand was used to switch betweeen branches, but there’s a lot more that it can do. “Checking out” is how you switch between different points of the repository’s history. You can checkout branches, tags, and commits.

Tags

Tags are just named pointers to commits (with optional extra data). They’re usually used for marking versions of software, which makes it really easy to jump between versions in a git repository.

For example, to check out the latest version of Linux (at the time of writing), you can use the following command in the Linux mainline git repository directory:

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$ git checkout v5.10-rc6

This works because there is a tag named v5.10-rc6, which points to the commit for version 5.10, release candidate 6 of Linux.

A similar command can be used to checkout version 5.8:

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$ git checkout v5.8

Branches

To switch to a branch, use git checkout followed by the branch name.

This command will switch to the master branch:

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$ git checkout master

Or for the devel branch:

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$ git checkout devel

Create and checkout a branch

The -b flag will tell Git to create the branch, then check it out.

Here’s an example:

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$ git checkout master
$ git checkout -b feature-foo

This will first switch to the master branch, then create a new branch called feature-foo which will be based off of master.

git checkout -b will create a branch which is based off of the currently checked-out branch.

Commits

You can also check out specific commits using their SHA-1 hash. To find commits to check out, use the git log command.

Here’s a commit we’re going to check out:

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commit e0ab2c0029936fbf4db72bfdc5e71aa30beffdfd (HEAD -> master)
Author: Kian Kasad <kian@kasad.com>
Date:   Mon Nov 30 22:52:51 2020 -0800

    add option for building docs package

To check it out, you can use the following command:

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$ git checkout e0ab2c0029936fbf4db72bfdc5e71aa30beffdfd

Since that’s pretty tricky to type, especially without copy/paste abilities, Git allows using as few characters of the hash as possible while staying unique. Usually 6 or 8 characters is ideal. Here are two examples using the same commit as before:

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$ git checkout e0ab2c
$ git checkout e0ab2c00

Getting back

After checking out a tag or commit, Git will usually print a message that looks like the following:

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Note: switching to 'e0ab2c0'.

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 switching back to a branch.

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

  git switch -c <new-branch-name>

Or undo this operation with:

  git switch -

Turn off this advice by setting config variable advice.detachedHead to false

HEAD is now at e0ab2c0 add option for building docs package

Basically, it’s saying that you’re in a state called “detached HEAD” state (remember HEAD is another name for the latest commit).

To exit “detached HEAD” state and return to the normal HEAD, use the following command:

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$ git switch -

If you make changes in “detached HEAD” mode and you want to save them, use the following command to create a new branch with the changes:

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$ git switch -c <new branch name>