How do you interpret the output of the git diff command?

The git diff command is a powerful tool used to see changes between commits, commit and working tree, etc. It is widely used in version control systems to view what has been changed. When you run the command, it generates a lot of information and can be a bit daunting to understand if you are not familiar with it.

Here’s a detailed breakdown of how to interpret its output:

Firstly, let’s consider a simple example. Suppose we have a file named file.txt and we’ve modified it in some way. After that, we run the git diff command. The output will be something like this:

<code>diff --git a/file.txt b/file.txt
index 33db5c6..0d8c4ab 100644
--- a/file.txt
+++ b/file.txt
@@ -1,3 +1,4 @@
 This is an example file.
+This is a new line.
 This file is for demonstration.
-The last line.
+Changed the last line.

Here is a detailed breakdown of the output:

  1. diff –git a/file.txt b/file.txt: This line shows the input of the diff, i.e., it shows the files that are being compared. The “a” and “b” here represent the first and second version of the file, respectively.
  2. index 33db5c6..0d8c4ab 100644: The index line provides the checksums for the “a” and “b” versions of the file respectively, followed by the file mode. The mode 100644 represents a normal file type and an octal file permission in Unix-style.
  3. — a/file.txt +++ b/file.txt: These lines show the paths of the files “a” and “b”. They indicate what the file was (--- a/file.txt) and what it has changed to (+++ b/file.txt).
  4. @@ -1,3 +1,4 @@: This line is the chunk header and the most complex part of the output. It represents the range of lines in the file that the diff block is going to show you. The -1,3 means that the diff block starts at line 1 and shows 3 lines in the “a” version of the file. The +1,4 means that the diff block starts at line 1 and shows 4 lines in the “b” version of the file.
  5. The actual changes are displayed after the chunk header:
    • Lines that are added are marked with a + symbol at the start of the line.
    • Lines that are removed are marked with a - symbol at the start of the line.
    • Lines that haven’t been changed are displayed as they are.

In our example, we’ve added a line “This is a new line.” and changed “The last line.” to “Changed the last line.”.

Remember that git diff can do much more than this and has several options that can be used to tailor the output to your needs. To know more about these options, you can refer to the Git documentation or run the git diff --help command.