Version Control with Git

Viewing a Repositories History

If we want to know what we’ve done recently, we can ask Git to show us the project’s history using git log:

$ git log
commit cdb7fa654c3f5aee731a655e57f2ba74d9c74582 (HEAD -> forecast)
Author: Joanne Simpson <j.simpson@mo-weather.uk>
Date:   Mon Nov 4 18:35:21 2024 +0000

    Add in the temperature to the forecast and create the weather atlas file

git log lists all commits made to a repository in reverse chronological order. The listing for each commit includes the commit’s full identifier (which starts with the same characters as the short identifier printed by the git commit command earlier), the commit’s author, when it was created, and the log message Git was given when the commit was created. The output above only shows the latest commit in the log for brevity, you should see all your commits!

git log is equivalent to:

$ fcm log
Paging the Log

When the output of git log is too long to fit in your screen, git uses a program to split it into pages of the size of your screen. When this “pager” is called, you will notice that the last line in your screen is a :, instead of your usual prompt.

  • To get out of the pager, press Q.
  • To move to the next page, press Spacebar.
  • To search for some_word in all pages, press / and type some_word. Navigate through matches pressing N.
Limit Log Size

To avoid having git log cover your entire terminal screen, you can limit the number of commits that Git lists by using -N, where N is the number of commits that you want to view. For example, if you only want information from the last commit you can use:

$ git log -1
commit cdb7fa654c3f5aee731a655e57f2ba74d9c74582 (HEAD -> forecast)
Author: Joanne Simpson <j.simpson@mo-weather.uk>
Date:   Mon Nov 4 18:35:21 2024 +0000

    Add in the temperature to the forecast and create the weather atlas file

You can also reduce the quantity of information using the --oneline option:

$ git log --oneline
cdb7fa6 (HEAD -> forecast) Add in the temperature to the forecast and create the weather atlas file
62a9457 Modify the forecast to add a chance of Sun
d3e4637 Add tomorrows forecast to forecast.md
590c40c Create a md file with the forecast

You can also combine the --oneline option with others. One useful combination adds --graph to display the commit history as a text-based graph and to indicate which commits are associated with the current HEAD, the current branch main, or [other Git references][git-references]:

$ git log --oneline --graph
* cdb7fa6 (HEAD -> forecast) Add in the temperature to the forecast and create the weather atlas file
* 62a9457 Modify the forecast to add a chance of Sun
* d3e4637 Add tomorrows forecast to forecast.md
* 590c40c Create a md file with the forecast

A common alias for git log

It is often useful to use the --decorate, --oneline, and --graph flags all at once. To avoid us having to write out the three flags each time we can set an alias:

$ git config --global alias.dog "log --decorate --oneline --graph"

This alias makes these two commands equivalent:

$ git dog
$ git log --decorate --oneline --graph

--decorate ensures commits with reference names1 are displayed when using older versions of Git.

git show

The git show command lets you view information for specific commits. By default git show will show information for the latest commit on the current branch.

$ git show
commit cdb7fa654c3f5aee731a655e57f2ba74d9c74582 (HEAD -> forecast)
Author: Joanne Simpson <j.simpson@mo-weather.uk>
Date:   Mon Nov 4 18:35:21 2024 +0000

    Add in the temperature to the forecast and create the weather atlas file

diff --git a/atlas.md b/atlas.md
new file mode 100644
index 0000000..18fac28
--- /dev/null
+++ b/atlas.md
@@ -0,0 +1,5 @@
+# Weather Atlas
+
+- rain
+- sunshine
+- fog
:

Identifying Commits

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 reference name HEAD.

We’ve been adding small changes at a time to forecast.md, so it’s easy to track our progress by looking, so let’s do that using our HEADs. Before we start, let’s make a change to forecast.md, adding yet another line with an ill-considered change.

$ nano forecast.md
$ cat forecast.md
# Forecast

## Today

Cloudy with a chance of sun.
Mild temperatures around 16 °C.

## Tomorrow

Morning rainbows followed by light showers.
An ill-considered change.

Now, let’s see what we get.

$ git diff HEAD forecast.md
diff --git a/forecast.md b/forecast.md
index b36abfd..0848c8d 100644
--- a/forecast.md
+++ b/forecast.md
@@ -8,3 +8,4 @@
 Mild temperatures around 16 °C.
 
 ## Tomorrow
 
 Morning rainbows followed by light showers.
+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.

$ git diff HEAD~1 forecast.md

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:

$ git diff HEAD~2 forecast.md
diff --git a/forecast.md b/forecast.md
index df0654a..b36abfd 100644
--- a/forecast.md
+++ b/forecast.md
@@ -2,8 +2,10 @@
 
 ## Today

-Cloudy with a chance of pizza.
+Cloudy with a chance of sun.
+Mild temperatures around 16 °C.
 
 ## Tomorrow
 
 Morning rainbows followed by light showers.
+An ill-considered change.

We can also use identifiers with git show.

$ git show HEAD~2 forecast.md
Author: Joanne Simpson <j.simpson@mo-weather.uk>
Date:   Mon Nov 4 18:16:29 2024 +0000

    Add tomorrows forecast to forecast.md

diff --git a/forecast.md b/forecast.md
index d8bc6ce..5b5d97e 100644
--- a/forecast.md
+++ b/forecast.md
@@ -3,3 +3,7 @@
 ## Today
 
 Cloudy with a chance of pizza.
+
+## Tomorrow
+
+Morning rainbows followed by light showers.

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 both git log and git show display. 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 on the forecast branch was given the ID f22b25e3233b4645dabd0d81e651fe074bd8e73b, so let’s try this:

$ git diff f22b25e3233b4645dabd0d81e651fe074bd8e73b forecast.md
diff --git a/forecast.md b/forecast.md
index df0654a..93a3e13 100644
--- a/forecast.md
+++ b/forecast.md
@@ -2,4 +2,10 @@
 
 ## Today

-Cloudy with a chance of pizza.
+Cloudy with a chance of sun.
+Mild temperatures around 16 °C.
+
+## Tomorrow
+
+Morning rainbows followed by light showers.
+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):

$ git diff f22b25e forecast.md
diff --git a/forecast.md b/forecast.md
index df0654a..93a3e13 100644
--- a/forecast.md
+++ b/forecast.md
@@ -2,4 +2,10 @@
 
 ## Today

-Cloudy with a chance of pizza.
+Cloudy with a chance of sun.
+Mild temperatures around 16 °C.
+
+## Tomorrow
+
+Morning rainbows followed by light showers.
+An ill-considered change.

So far we have only been comparing a previous commit to the working copy. To get a difference between two specific commits use both their IDs:

$ git diff d3e4637 62a9457 forecast.md
diff --git a/forecast.md b/forecast.md
index 4c96be7..541eee7 100644
--- a/forecast.md
+++ b/forecast.md
@@ -2,7 +2,7 @@
 
 ## Today
 
-Cloudy with a chance of pizza.
+Cloudy with a chance of Sun.
 
 ## Tomorrow

Understanding Workflow and History

What is the output of the last command in

$ cd weather
$ git switch -c add_CMIP_data
$ echo "Global Climate Data" > CMIP7.md
$ git add CMIP7.md
$ echo "Data from the 7th model intercomparison project" >> CMIP7.md
$ git commit -m "Adds in CMIP7 data file"
$ git restore CMIP7.md
$ cat CMIP7.md  # this will print the content of CMIP7.md on screen
  1.   Data from the 7th model intercomparison project
  2.   Global Climate Data
  3.   Global Climate Data
  Data from the 7th model intercomparison project
  4.   Error because you have changed CMIP7.md without committing the changes

Solution (Solution). The answer is 2.

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 "Adds in CMIP7 data file" is executed, the version of CMIP7.md committed to the repository is the one from the staging area and only has one line, Global Climate Data.

At this time, the working copy still has the second line (and git status will show that the file is modified). However, git restore CMIP7.md removes all unstaged modifications to the CMIP7.md file, so the second line is removed. So, cat CMIP7.md will output

Global Climate Data

Checking Understanding of git diff

Consider this command: git diff HEAD~9 forecast.md. 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] forecast.md, where [ID] is replaced with the unique identifier for your most recent commit. What do you think will happen, and what does happen?

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 weather project has more than 50 files. You would like to find a commit that modifies some specific text in forecast.md. 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 forecast.md. We can apply a similar idea here.

$ git log forecast.md

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:

$ git log --patch forecast.md

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?

$ git log --patch HEAD~9 *.md

Keypoints

  • git log displays the repositories history.
  • git diff displays differences between commits.
  • HEAD references the last commit.
  • HEAD~1 references the commit before last.

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Footnotes

  1. References in Git are user friendly links to specific commits. For instance HEAD is a reference to the latest commit on a branch. Programs with regular releases might add reference tags such as v1.0 to a specific commit to mark a new release. These references can be used instead of a commit identifier such as e48heu0.↩︎


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