TLDR: The git commit --amend command combines the content of the tip of the current branch (HEAD) with the content of the index and creates a new commit with the same parent as HEAD. It also opens an editor for you to update the commit message.

Setup

Let’s create a new commit.

This leads to the following commits graph.

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how to change the commit message of the last commit?

The common way to use git commit --amend is to edit the last commit message. The command will open an editor where you can change the commit message.

When you save the file with the new message, a new commit is created with the same content as the previous one but with the updated message.

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As always, git does not actually change a commit; it creates a new one (e9d13f) that is a copy of the previous one (396811) but with a different message.

The reference HEAD now points to the new commit, and the previous commit 396811 is no longer referenced by any branch and will eventually be cleaned up by git’s garbage collector.

how to change the content of the last commit?

Let’s update the file and add it to the index.

This time, git commit --amend will not only open an editor to change the commit message but will also update the content of the previous commit.

When you save the editor, the last commit is updated with the new content and results in this new commit graph:

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Same as before: we now have a new commit 06d0e3 referenced by HEAD, and the other two will eventually be cleaned up by git’s garbage collector.

How to see the content of a commit?

If you want to check the content of all those commits, the git show command is your friend.

The command git show [commit]:[file's name] can be used to see the content of a file in a given commit.

Let’s start with our HEAD commit 06d0e3 and check the content of the file file.txt:

That’s indeed our most recent change. Let’s check the previous commit e9d13f:

That’s the previous content of the file. And finally, let’s check the first commit 396811:

These commands only show the content of a specific file.

If you want to see the content of the entire commit, you can use the git show [commit] command:

This command shows the content of the commit, the commit message, and the diff between the commit and its parent.

The command can also be used to check the commit message as it was in the previous commit:

In a typical scenario, you can’t always see the commit graph on disk, but you can use the git reflog command to see the history of the commits you have been working on.