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Diffs

What does “diff” mean?​

Diffing means comparing two versions of something to see:

  • What changed
  • What was added
  • What was removed

This can be done with:

  • Regular UNIX tools (diff)
  • Git commands (git diff)

Using the UNIX diff command​

The standard UNIX diff command compares:

  • Two files
  • Or two directories (recursively)

Simple file comparison​

Suppose:

file1

This is the contents of a simple file.

file2

This is the contents of a slightly different file.

Run:

diff file1 file2

Example output:

2,3c2,3
< contents
< of a simple
---
> contents of a slightly
> different

This output is correct, but not very easy to read.

Most people use the unified diff format with -u:

diff -u file1 file2

Example output:

--- file1
+++ file2
@@ -1,4 +1,4 @@
 This is the
-contents
-of a simple
+contents of a slightly
+different
 file.

How to read unified diff output​

  • --- → original file
  • +++ → new file
  • @@ → line number context
  • - → line removed
  • + → line added
  • No symbol → unchanged context

This format is also used by patch tools.

Diffing directories​

To compare two directory trees:

diff -Nur directory1 directory2

Common options explained​

  • -r → recursive (check subdirectories)
  • -N → show added or deleted files
  • -u → unified output

This is useful when comparing two project folders.

Diffing with Git​

Git provides more powerful and convenient diff tools.

Basic Git diff​

git diff

Shows differences between:

  • Your working directory
  • The last commit (HEAD)

Compare working directory with a commit​

git diff earlier_commit

Compares your current files with a specific commit or branch.

Compare staged changes​

git diff --cached

or (more intuitive):

git diff --staged

Shows what is staged and will go into the next commit.

You can also compare staged changes to a commit:

git diff --cached earlier_commit

Compare two commits​

git diff commit1 commit2

Shows exactly what changed between two commits.

Useful Git diff options​

  • Ignore whitespace:
git diff --ignore-all-space
  • Show summary statistics:
git diff --stat
  • Show numeric statistics:
git diff --numstat

Limit diff to part of a project​

You can focus on a specific directory.

Example (Linux kernel repo):

git diff v4.2.1 v4.2.2 Documentation/vm

Only shows changes inside Documentation/vm.

Show stats for a subdirectory​

git diff --stat v4.2.1 v4.2.2 arch/x86_64

Shows a brief summary instead of full diffs.

Summary
  • diff compares files or directories
  • diff -u gives readable, standard output
  • git diff is more powerful and Git-aware
  • You can compare working files, staged files, commits, or directories
  • Essential for reviewing changes before committing