Version Control with Git

Exploring History and Undoing Mistakes

Recall that git log shows us the history of our project by listing the commits we have made, their dates, and their commit messages:

$ git log

commit 005937fbe2a98fb83f0ade869025dc2636b4dad5
Author: Vlad Dracula <vlad@tran.sylvan.ia>
Date:   Thu Aug 22 10:14:07 2013 -0400

    Discuss concerns about Mars' climate for Mummy

commit 34961b159c27df3b475cfe4415d94a6d1fcd064d
Author: Vlad Dracula <vlad@tran.sylvan.ia>
Date:   Thu Aug 22 10:07:21 2013 -0400

    Add concerns about effects of Mars' moons on Wolfman

commit f22b25e3233b4645dabd0d81e651fe074bd8e73b
Author: Vlad Dracula <vlad@tran.sylvan.ia>
Date:   Thu Aug 22 09:51:46 2013 -0400

    Start notes on Mars as a base

Because git keeps track of not only the current state of our files but also their entire history, we can restore any file (or even the entire project) to any place in its history. This is very useful if we make a mistake and want to undo it!

For example, let’s suppose we accidentally overwrite our file:

$ nano mars.txt
$ cat mars.txt

We will need to manufacture our own oxygen

git status now tells us that the file has been changed, but those changes haven’t been staged to be committed:

$ git status

# On branch master
# Changes not staged for commit:
#   (use "git add <file>..." to update what will be committed)
#   (use "git checkout -- <file>..." to discard changes in working directory)
#
#   modified:   mars.txt
#
no changes added to commit (use "git add" and/or "git commit -a")

Let’s restore the version of mars.txt from the most recent commit. Each commit is uniquely identified by a long string of letters and digits; we can find a commit’s identifier from its entry in git log. Looking above, we see that our most recent commit has the identifier 005937fbe2a98fb83f0ade869025dc2636b4dad5. We can ask for the version of mars.txt from that commit using git checkout:

$ git checkout 005937fbe2a98fb83f0ade869025dc2636b4dad5 mars.txt
$ cat mars.txt

Cold and dry, but everything is my favorite color
The two moons may be a problem for Wolfman
But the Mummy will appreciate the lack of humidity

Ah, crisis averted – Git is a lifesaver for anyone who has every accidentally deleted a file.

Using git checkout, we can look at any commit in the history of the project. However, typing out random 40-character strings is annoying, so Git lets us use just the first few characters:

$ git checkout 34961b1 mars.txt
$ cat mars.txt

Cold and dry, but everything is my favorite color
The two moons may be a problem for Wolfman

Sometimes it’s easier to refer to commits not by their commit ID but by how many commits ago they were. The most recent commit is always called HEAD, the commit before that is called HEAD~1 (pronounced “HEAD minus one”), the commit before that is HEAD~2, etc. This makes it easy to recover from mistakes by going back to the most recent commit:

$ nano mars.txt
$ cat mars.txt

We must take care not to disturb the native lifeforms!

$ git checkout HEAD mars.txt
$ cat mars.txt

Cold and dry, but everything is my favorite color
The two moons may be a problem for Wolfman
But the Mummy will appreciate the lack of humidity

Finally, if we want to see what changed since a particular commit, we can use git diff again. For example, to see what has changed since the last commit:

$ git diff HEAD~1 mars.txt

diff --git a/mars.txt b/mars.txt
index 315bf3a..b36abfd 100644
--- a/mars.txt
+++ b/mars.txt
@@ -1,2 +1,3 @@
 Cold and dry, but everything is my favorite color
 The two moons may be a problem for Wolfman
+But the Mummy will appreciate the lack of humidity

We can also refer to a commit by its commit identifier:

$ git diff f22b25e mars.txt

diff --git a/mars.txt b/mars.txt
index df0654a..b36abfd 100644
--- a/mars.txt
+++ b/mars.txt
@@ -1 +1,3 @@
 Cold and dry, but everything is my favorite color
+The two moons may be a problem for Wolfman
+But the Mummy will appreciate the lack of humidity

Git Checkout

So, to put it all together:

The fact that files can be reverted one by one tends to change the way people organize their work. If everything is in one large document, it’s hard (but not impossible) to undo changes to the introduction without also undoing changes made later to the conclusion. If the introduction and conclusion are stored in separate files, on the other hand, moving backward and forward in time becomes much easier.

$ cd planets
$ nano venus.txt #input the following text: Venus is beautiful and full of love
$ git add venus.txt
$ nano venus.txt #add the following text: Venus is too hot to be suitable as a base
$ git commit -m "comments on Venus as an unsuitable base"
$ git checkout HEAD venus.txt
$ cat venus.txt #this will print the contents of venus.txt to the screen