Seeing What Changed

Goals

By the end of this lesson you should be able to…

• Use the git show and git diff commands to inspect changes recorded in your repository

Show

The git show command shows all the changes that were made in a particular commit.

# Format for git show
git show [<commit_id>]

If no commit ID is specified, the commit indicated by HEAD is used.

Show the changes in the most recent commit:

$ git show
commit 7d7596afd8f34ed4e393472ab66842ef9f0d6fbf (HEAD -> master)
Author: Rodney Martin <rod@email.com>
Date:   Thu May 21 10:56:08 2020 -0400

    Added another change tracking description

diff --git a/git.txt b/git.txt
index 99de68c..35fd8aa 100644
--- a/git.txt
+++ b/git.txt
@@ -2,6 +2,7 @@ Git
 
 Git is a distributed version control system
 Git records changes in files, not the current state of files
+Changes in the working tree that have not been staged will not be committed
 
 
 The 'git init' command is used to initialize a Git repository

The output of git show includes the commit information that you have already encountered with git log, but it also includes a ‘diff’. A diff is a record of only the changes between one set of files and another.

Reading Diff Output

The output of a diff may seem like random symbols at first but it all has meaning. You do not need to know all the details except for the following¹:

Lines starting and ending with @@ indicate a chunk of lines in the file that have changed in some way. Below each @@ heading will be any number of lines, some of which begin with a - or +. Such lines are deletions or insertions, respectively, when comparing the two revisions of the file in question. Lines that do not begin with a - or + are just context around the insertions and deletions being shown.

Thus, in the output above Git is indicating that in commit 7d75 git.txt had a single insertion: the line Changes in the working tree that have not been staged will not be committed.

Let's look at a more complicated example. Suppose there were two revisions of a file, A and B

Hello, world!

This is some text.
There are several lines of text.
This is the last line of text.

MY DOCUMENT

Hello, world!

This is some more text.
There are several lines of text.

Here is what the diff for these two revisions would look like:

diff --git a/a.txt b/a.txt
index 94e1883..2ceb5b9 100644
--- a/a.txt
+++ b/a.txt
@@ -1,5 +1,6 @@
+MY DOCUMENT
+
 Hello, world!
 
-This is some text.
+This is some more text.
 There are several lines of text.
-This is the last line of text.

You can see here the three different kinds of changes you can make to a file: Insertions, deletions, and modifications. Insertions and deletions are simply chunks that all start with either + or - respectively. Modifications are indicated with one set of deletions followed immediately by one set of insertions, as you can see in the change from This is some text. to This is some more text..

Diff

The git diff command can be used to examine the difference between revisions of a file.

Working Tree vs Index

The default behaviour of git diff is to compare the unstaged changes with the staged changes of a file.

# Compare all changes
git diff

# Compare all changes in a file named foo.txt
git diff foo.txt

# Compare all changes in a directory named foo
git diff foo/*

Working Tree vs Most Recent Commit

You can specify HEAD as the specifc commit to compare with instead of the index. (If term HEAD is not familiar see this page.)

# Compare all changes
git diff HEAD

# Compare all changes in a file named foo.txt
git diff HEAD foo.txt

# Compare all changes in a directory named foo
git diff HEAD foo/*

Working Tree vs Older Commit

You can specify a commit ID as the specifc commit to compare with instead of the index. You may use a unique prefix of any commit ID. (Your commit IDs will be different than the one used below.)

# Compare all changes
git diff 2e44

# Compare all changes in a file named foo.txt
git diff 2e44 foo.txt

# Compare all changes in a directory named foo
git diff 2e44 foo/*

Staged Changes vs Most Recent Commit

The --staged option allows you to compare staged changes instead of working tree changes. Staged changes are compared with the most recent commit.

# Compare all changes
git diff --staged

# Compare all changes in a file named foo.txt
git diff --staged foo.txt

# Compare all changes in a directory named foo
git diff --staged foo/*

Staged Changes vs Older Commit

Just like the normal command you can include a commit ID to pick a specific commit other than the most recent one to compare with.

# Compare all changes
git diff --staged 2e44

# Compare all changes in a file named foo.txt
git diff --staged 2e44 foo.txt

# Compare all changes in a directory named foo
git diff --staged 2e44 foo/*

One Commit vs Another Commit

Finally, you can specify two commit IDs to show the differences between those two commits.

# Compare all changes
git diff 2e44 f33d

# Compare all changes in a file named foo.txt
git diff 2e44 f33d foo.txt

# Compare all changes in a directory named foo
git diff 2e44 f33d foo/*

Use the git diff command in its various formats to examine the differences between various parts of your repository, index, and working directory.