Undoing things

Overview

Teaching: 10 min
Exercises: 15 min

Questions

• How can I undo things?

Objectives

• Learn to undo changes safely

• See when changes are permanently deleted and when they can be retrieved

Undoing things

The whole point of version control is that you can be see and retrieve previous snapshots of your work.

With Git, if you have committed (made a snapshot) of a file, you can get it back, even if it gets deleted or modified in the future.

Some commands will modify the hitory of a Git repository. This is generally a very bad idea, and you should only do it if you’re really confident you know what you’re doing.

If changes are uncommitted, they are not safe, and if they are deleted, they are gone.

Reverting commits

• Imagine we made a few commits.
• We realize that the latest commit was a mistake and we wish to undo it:

$ git log --oneline

f960dd3 (HEAD -> master) not sure this is a good idea
40fbb90 draft a readme
dd4472c we should not forget to enjoy
2bb9bb4 add half an onion
2d79e7e adding ingredients and instructions

A safe way to undo the commit is to revert the commit with git revert:

$ git revert f960dd3

This creates a new commit that does the opposite of the reverted commit. The old commit remains in the history:

$ git log --oneline

d62ad3e (HEAD -> master) Revert "not sure this is a good idea"
f960dd3 not sure this is a good idea
40fbb90 draft a readme
dd4472c we should not forget to enjoy
2bb9bb4 add half an onion
2d79e7e adding ingredients and instructions

Challenge 1

Make a new commit in your guacamole repo (it can be whatever you like)

Inspect the history, and revert the commit.

Undo unstaged/uncommitted changes

DANGER!!

This is a command that permanently deletes changes that haven’t been staged/committed!

Modify before staging

• Make a silly change to repo, do not stage it or commit it.
• Inspect the change with git status and git diff.
• Now undo the change with git checkout <file>.
• Verify that the change is gone with git status and git diff.

Modify after staging

• Make a reasonable change to a project, stage it.
• Make a silly change after you have staged the reasonable change.
• Inspect the situation with git status, git diff, git diff --staged, and git diff HEAD.
• Now undo the silly change with git checkout <file>.
• Inspect the new situation with git status, git diff, git diff --staged, and git diff HEAD.

Challenge 2

How much do you trust Git…?

Delete one (or more) of your committed files using any method you like.

Can you get it/them back?

Solution to Challenge 2

As long as a file was committed, you can get it back with git checkout <file>, but Git isn’t magic - you won’t have any changes that weren’t committed.

Key Points

• Git history can be reverted without modifying it

• Once changes are committed they are safe

• Changes that are not committed can be deleted