Exploring History
Last updated on 2023-09-07 | Edit this page
Estimated time 25 minutes
Overview
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.
As we saw in the previous episode, we can refer to commits by their
identifiers. You can refer to the most recent commit of the
working directory by using the identifier HEAD
.
We’ve been adding one line at a time to mars.txt
, so
it’s easy to track our progress by looking, so let’s do that using our
HEAD
s. Before we start, let’s make a change to
mars.txt
, adding yet another line.
BASH
$ nano mars.txt
$ cat mars.txt
OUTPUT
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
An ill-considered change
Now, let’s see what we get.
BASH
$ git diff HEAD mars.txt
OUTPUT
diff --git a/mars.txt b/mars.txt
index b36abfd..0848c8d 100644
--- a/mars.txt
+++ b/mars.txt
@@ -1,3 +1,4 @@
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
+An ill-considered change.
which is the same as what you would get if you leave out
HEAD
(try it). The real goodness in all this is when you
can refer to previous commits. We do that by adding ~1
(where “~” is “tilde”, pronounced [til-duh])
to refer to the commit one before HEAD
.
BASH
$ git diff HEAD~1 mars.txt
If we want to see the differences between older commits we can use
git diff
again, but with the notation HEAD~1
,
HEAD~2
, and so on, to refer to them:
BASH
$ git diff HEAD~3 mars.txt
OUTPUT
diff --git a/mars.txt b/mars.txt
index df0654a..b36abfd 100644
--- a/mars.txt
+++ b/mars.txt
@@ -1 +1,4 @@
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
+An ill-considered change
We could also use git show
which shows us what changes
we made at an older commit as well as the commit message, rather than
the differences between a commit and our working directory that
we see by using git diff
.
BASH
$ git show HEAD~3 mars.txt
OUTPUT
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
diff --git a/mars.txt b/mars.txt
new file mode 100644
index 0000000..df0654a
--- /dev/null
+++ b/mars.txt
@@ -0,0 +1 @@
+Cold and dry, but everything is my favorite color
In this way, we can build up a chain of commits. The most recent end
of the chain is referred to as HEAD
; we can refer to
previous commits using the ~
notation, so
HEAD~1
means “the previous commit”, while
HEAD~123
goes back 123 commits from where we are now.
We can also refer to commits using those long strings of digits and
letters that git log
displays. These are unique IDs for the
changes, and “unique” really does mean unique: every change to any set
of files on any computer has a unique 40-character identifier. Our first
commit was given the ID
f22b25e3233b4645dabd0d81e651fe074bd8e73b
, so let’s try
this:
BASH
$ git diff f22b25e3233b4645dabd0d81e651fe074bd8e73b mars.txt
OUTPUT
diff --git a/mars.txt b/mars.txt
index df0654a..93a3e13 100644
--- a/mars.txt
+++ b/mars.txt
@@ -1 +1,4 @@
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
+An ill-considered change
That’s the right answer, but typing out random 40-character strings is annoying, so Git lets us use just the first few characters (typically seven for normal size projects):
BASH
$ git diff f22b25e mars.txt
OUTPUT
diff --git a/mars.txt b/mars.txt
index df0654a..93a3e13 100644
--- a/mars.txt
+++ b/mars.txt
@@ -1 +1,4 @@
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
+An ill-considered change
All right! So we can save changes to files and see what we’ve
changed. Now, how can we restore older versions of things? Let’s suppose
we change our mind about the last update to mars.txt
(the
“ill-considered change”).
git status
now tells us that the file has been changed,
but those changes haven’t been staged:
BASH
$ git status
OUTPUT
On branch main
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")
We can put things back the way they were by using
git checkout
:
BASH
$ git checkout HEAD mars.txt
$ cat mars.txt
OUTPUT
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
As you might guess from its name, git checkout
checks
out (i.e., restores) an old version of a file. In this case, we’re
telling Git that we want to recover the version of the file recorded in
HEAD
, which is the last saved commit. If we want to go back
even further, we can use a commit identifier instead:
BASH
$ git checkout f22b25e mars.txt
BASH
$ cat mars.txt
OUTPUT
Cold and dry, but everything is my favorite color
BASH
$ git status
OUTPUT
On branch main
Changes to be committed:
(use "git reset HEAD <file>..." to unstage)
modified: mars.txt
Notice that the changes are currently in the staging area. Again, we
can put things back the way they were by using
git checkout
:
BASH
$ git checkout HEAD mars.txt
Don’t Lose Your HEAD
Above we used
BASH
$ git checkout f22b25e mars.txt
to revert mars.txt
to its state after the commit
f22b25e
. But be careful! The command checkout
has other important functionalities and Git will misunderstand your
intentions if you are not accurate with the typing. For example, if you
forget mars.txt
in the previous command.
BASH
$ git checkout f22b25e
ERROR
Note: checking out 'f22b25e'.
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 performing another checkout.
If you want to create a new branch to retain commits you create, you may
do so (now or later) by using -b with the checkout command again. Example:
git checkout -b <new-branch-name>
HEAD is now at f22b25e Start notes on Mars as a base
The “detached HEAD” is like “look, but don’t touch” here, so you
shouldn’t make any changes in this state. After investigating your
repo’s past state, reattach your HEAD
with
git checkout main
.
It’s important to remember that we must use the commit number that
identifies the state of the repository before the change we’re
trying to undo. A common mistake is to use the number of the commit in
which we made the change we’re trying to discard. In the example below,
we want to retrieve the state from before the most recent commit
(HEAD~1
), which is commit f22b25e
:
So, to put it all together, here’s how Git works in cartoon form:
Simplifying the Common Case
If you read the output of git status
carefully, you’ll
see that it includes this hint:
OUTPUT
(use "git checkout -- <file>..." to discard changes in working directory)
As it says, git checkout
without a version identifier
restores files to the state saved in HEAD
. The double dash
--
is needed to separate the names of the files being
recovered from the command itself: without it, Git would try to use the
name of the file as the commit identifier.
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.
Recovering Older Versions of a File
Jennifer has made changes to the Python script that she has been working on for weeks, and the modifications she made this morning “broke” the script and it no longer runs. She has spent ~ 1hr trying to fix it, with no luck…
Luckily, she has been keeping track of her project’s versions using
Git! Which commands below will let her recover the last committed
version of her Python script called data_cruncher.py
?
$ git checkout HEAD
$ git checkout HEAD data_cruncher.py
$ git checkout HEAD~1 data_cruncher.py
$ git checkout <unique ID of last commit> data_cruncher.py
Both 2 and 4
The answer is (5)-Both 2 and 4.
The checkout
command restores files from the repository,
overwriting the files in your working directory. Answers 2 and 4 both
restore the latest version in the repository of the
file data_cruncher.py
. Answer 2 uses HEAD
to
indicate the latest, whereas answer 4 uses the unique ID of the
last commit, which is what HEAD
means.
Answer 3 gets the version of data_cruncher.py
from the
commit before HEAD
, which is NOT what we
wanted.
Answer 1 can be dangerous! Without a filename,
git checkout
will restore all files in the
current directory (and all directories below it) to their state at the
commit specified. This command will restore
data_cruncher.py
to the latest commit version, but it will
also restore any other files that are changed to that version,
erasing any changes you may have made to those files! As discussed
above, you are left in a detached HEAD
state, and
you don’t want to be there.
Reverting a Commit
Jennifer is collaborating with colleagues on her Python script. She
realizes her last commit to the project’s repository contained an error,
and wants to undo it. Jennifer wants to undo correctly so everyone in
the project’s repository gets the correct change. The command
git revert [erroneous commit ID]
will create a new commit
that reverses the erroneous commit.
The command git revert
is different from
git checkout [commit ID]
because git checkout
returns the files not yet committed within the local repository to a
previous state, whereas git revert
reverses changes
committed to the local and project repositories.
Below are the right steps and explanations for Jennifer to use
git revert
, what is the missing command?
________ # Look at the git history of the project to find the commit ID
Copy the ID (the first few characters of the ID, e.g. 0b1d055).
git revert [commit ID]
Type in the new commit message.
Save and close
The command git log
lists project history with commit
IDs.
The command git show HEAD
shows changes made at the
latest commit, and lists the commit ID; however, Jennifer should
double-check it is the correct commit, and no one else has committed
changes to the repository.
Understanding Workflow and History
What is the output of the last command in
BASH
$ cd planets
$ echo "Venus is beautiful and full of love" > venus.txt
$ git add venus.txt
$ echo "Venus is too hot to be suitable as a base" >> venus.txt
$ git commit -m "Comment 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
OUTPUT
Venus is too hot to be suitable as a base
OUTPUT
Venus is beautiful and full of love
OUTPUT
Venus is beautiful and full of love Venus is too hot to be suitable as a base
OUTPUT
Error because you have changed venus.txt without committing the changes
The answer is 2.
The command git add venus.txt
places the current version
of venus.txt
into the staging area. The changes to the file
from the second echo
command are only applied to the
working copy, not the version in the staging area.
So, when
git commit -m "Comment on Venus as an unsuitable base"
is
executed, the version of venus.txt
committed to the
repository is the one from the staging area and has only one line.
At this time, the working copy still has the second line (and
git status
will show that the file is modified). However,
git checkout HEAD venus.txt
replaces the working copy with
the most recently committed version of venus.txt
.
So, cat venus.txt
will output
OUTPUT
Venus is beautiful and full of love.
Checking Understanding of
git diff
Consider this command: git diff HEAD~9 mars.txt
. What do
you predict this command will do if you execute it? What happens when
you do execute it? Why?
Try another command, git diff [ID] mars.txt
, where [ID]
is replaced with the unique identifier for your most recent commit. What
do you think will happen, and what does happen?
Getting Rid of Staged Changes
git checkout
can be used to restore a previous commit
when unstaged changes have been made, but will it also work for changes
that have been staged but not committed? Make a change to
mars.txt
, add that change using git add
, then
use git checkout
to see if you can remove your change.
After adding a change, git checkout
can not be used
directly. Let’s look at the output of git status
:
OUTPUT
On branch main
Changes to be committed:
(use "git reset HEAD <file>..." to unstage)
modified: mars.txt
Note that if you don’t have the same output you may either have forgotten to change the file, or you have added it and committed it.
Using the command git checkout -- mars.txt
now does not
give an error, but it does not restore the file either. Git helpfully
tells us that we need to use git reset
first to unstage the
file:
BASH
$ git reset HEAD mars.txt
OUTPUT
Unstaged changes after reset:
M mars.txt
Now, git status
gives us:
BASH
$ git status
OUTPUT
On branch main
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")
This means we can now use git checkout
to restore the
file to the previous commit:
BASH
$ git checkout -- mars.txt
$ git status
OUTPUT
On branch main
nothing to commit, working tree clean
Explore and Summarize Histories
Exploring history is an important part of Git, and often it is a challenge to find the right commit ID, especially if the commit is from several months ago.
Imagine the planets
project has more than 50 files. You
would like to find a commit that modifies some specific text in
mars.txt
. When you type git log
, a very long
list appeared. How can you narrow down the search?
Recall that the git diff
command allows us to explore
one specific file, e.g., git diff mars.txt
. We can apply a
similar idea here.
BASH
$ git log mars.txt
Unfortunately some of these commit messages are very ambiguous, e.g.,
update files
. How can you search through these files?
Both git diff
and git log
are very useful
and they summarize a different part of the history for you. Is it
possible to combine both? Let’s try the following:
BASH
$ git log --patch mars.txt
You should get a long list of output, and you should be able to see both commit messages and the difference between each commit.
Question: What does the following command do?
BASH
$ git log --patch HEAD~9 *.txt