Exploring History

Overview

Teaching: 15 min
Exercises: 0 min

Questions

• How can I identify old versions of files?

• How do I review my changes?

• How can I recover old versions of files?

Objectives

• Explain what the HEAD of a repository is and how to use it.

• Identify and use Git commit numbers.

• Compare various versions of tracked files.

• Restore old versions of files.

The clear advantage of tracking your documents with Git is that you can always go back to the previous version. Let’s see how this works.

The output of git log provides all the information you need to retrieve the older version of the document.

Let’s modify our git_steps.txt and add a note to it:

$ echo "Use git init only one time for every project" >>git_steps.txt

$ cat git_steps.txt

$ git add .
$ git commit -m "added note to git_steps.txt"

#view latest version
$ cat git_steps.txt

Now, this is our second version of git_steps.txt. Our first version was stored when we first created the file, and our second version when we added a new line to it.

Suppose that some time later, you want the original version of git_steps.txt. How would you do that?

To restore any of the original versions of the document, you use git checkout command:

#general syntax
$ git checkout <commitID> <file>

If we want the original git_steps.txt:

#see what commitID is associated with the original version
git log --oneline

#the commit that includes original file has message "Added steps for Git and GitHub" (or whatever you used when you committed the change)
$ git checkout e6a7d94 git_steps.txt

Now let’s make sure that we have original version of git_steps.txt

#view restored version
$ cat git_steps.txt

#restored version has not been committed, see HEAD commit
$ git log

#see what to do next
$ git status

#If you decide to go ahead with restoring file, commit the change since checkout command places file in the staging area (no need for 'git add')

$ git commit -m "changed to the original git_steps.txt version"

$ git status

#see HEAD now
$ git log

#if you run `git checkout, but would like to cancel restoring the old version:
#unstage restored file
$ git reset HEAD git_steps.txt 

#return to latest version before you attempted to restore the file
$ git checkout -- git_steps.txt

#see current file
$ cat git_steps.txt

Key Points

• git diff displays differences between commits.

• git checkout recovers old versions of files.