Restoring old versions of a file

The point of a version control system such as Git is that you can move between different versions of a file (or, really, of a working directory). This means that you can make changes to files while safe in the knowledge that the old versions of the file are still safe. This can save you from having to save multiple versions of a file, e.g. README.md, README_01.md, README_01_cats_meow.md etc. etc.

To see all of the available versions of your working directory, type the command

git log

You should see output that is similar (but definitely not identical) to this

commit d70cb25dae45df90a63b34314a6132d8d9a19728 (HEAD -> main)
Author: Jean Golding <ab01234@bristol.ac.uk>
Date:   Fri Jan 15 10:08:10 2021 +0000

    Added some text to something.md to see if this was found by Git

commit 26101aef6ce05d115d060bcd2b9dec97d5c2dc95
Author: Jean Golding <ab01234@bristol.ac.uk>
Date:   Fri Jan 15 10:07:54 2021 +0000

    Added in another file

commit 3bdce3794b66af27bed3d28ee8d94aff3cc74fad
Author: Jean Golding <ab01234@bristol.ac.uk>
Date:   Thu Jan 14 17:46:23 2021 +0000

    Fixing a typo in README.md. Cats do not go woof.

commit 1ca35e3ad694fee566ab83341938699c726a4d0d
Author: Jean Golding <ab01234@bristol.ac.uk>
Date:   Thu Jan 14 17:23:27 2021 +0000

    Added the file README.md so that we have an initial file to
    play with in Git

Note that if you have a lot of commits this will open a text viewer that will let you scroll through the log. You can exit the text viewer by pressing the “q” key.

Each commit is listed, one after another, starting from the latest commit and moving back to the first commit. Each commit represents a different version of the working directory, and is given its own unique ID, e.g. commit 3bdce3794b66af27bed3d28ee8d94aff3cc74fad is the version where I changed the file so that cats go meow.

The version ID is a very large number which is written in hexadecimal. This is a random large number. It is so large that it is completely unique, meaning that, in the whole World across all time, only the commit where I made cats go meow will have version number 3bdce3794b66af27bed3d28ee8d94aff3cc74fad.

Each commit is also tagged with the name and email address of the person who committed, together with the commit message. Hopefully you can now see why this information needed to be given to git.

By default, git log will show information about all commits. To limit the output to only the last N commits, use the -n option. For example, to print out the log of the last 3 commits, type

git log -n 3

The log also records the date and time of each commit, and indeed, each commit represents the working directory at a different point in time. You can “move” your working directory through time by “checking out” different versions. To do this, you will need to use the unique ID of the commit you want to change to, and you will need to use the command git checkout ID. where ID is the number of the version you want to change to.

Now, change back to the first version of your working directory. In my case, this is version 1ca35e3ad694fee566ab83341938699c726a4d0d (the ID of the last commit printed out by git log). For me, I do this by typing

git checkout 1ca35e3ad694fee566ab83341938699c726a4d0d

You will have to use the ID number of your first version.

You should see git output something similar to this;

Note: switching to '1ca35e3ad694fee566ab83341938699c726a4d0d'.

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 1ca35e3 Added the file README.md so that we have an initial file to play with in Git

This confirms that git has now changed the working directory to match version 1ca35e3ad694fee566ab83341938699c726a4d0d. It also outputs some useful information that we will come back to later…

Type ls. You should see now that there is only the file README.md in your working directory. Take a look at README.md using your text editor. You should see that this is the original version of the file, in which the cat goes woof.

To return back to the newest version of the working directory, type

git checkout main

This is a safe command that will always bring your working directory back so that it matches the latest version snapshot that is saved in the repository. You should get output similar to;

Previous HEAD position was 1ca35e3 Added the file README.md so that we have an initial file to play with in Git
Switched to branch 'main'

Now, typing ls should reveal that you have both README.md and something.md. If you take a look at README.md, you should see that this is the fixed version of the file, in which the cat goes meow.

Note that you should not edit files in an old version of the working directory. At the moment, you should treat this as a read-only view of past versions of files (you will learn how to make this an editable view in the next workshop, Git for Collaboration).

You can bring old versions of files into the current working directory by using git checkout VERSION FILENAME, where VERSION is the version you wish to restore, and FILENAME is the name of the file you wish to restore. For example, if I want to restore the version of README.md where cats go woof, I can type;

git checkout 1ca35e3ad694fee566ab83341938699c726a4d0d README.md

To save this change, I will need to commit, e.g.

git commit -a

But, if I wanted to revert README.md back to the latest version as saved in the repository, I should type;

git checkout main README.md

This command tells git to discard any changes made to README.md and to revert that file back to what it is like in the last committed version of the working directory in the repository. Note here that main is a special (global) git version, which refers to the latest commit of the working directory in the repository.

If you now run git status you should see that the working directory is clean, and a quick check of README.md in a text editor should show you that the cat is indeed going meow again.