diff: Guide to Comparing Files/Directories

Overview

diff compares changes between two files or directories, showing which lines have been added, deleted, or modified. Output formats include unified, context, and side-by-side. It can also adjust comparison sensitivity for whitespace or case.

Key Features

Line-by-line comparison and human-readable formats provided
Recursive comparison between directories possible (-r)
Various ignore rules for whitespace, case, empty lines, etc.
Supports unified format (-u) suitable for patch (.patch) generation
Offers optimized options for large file comparison performance

Example Scenarios

Code review: Extracting only changed files as a patch
Configuration file change tracking: Comparing settings before and after deployment
Backup/snapshot comparison: Checking differences between specific points in time
Detecting unintended changes in document/data pre-processing stages
Security checks: Verifying suspicious changes in binaries/scripts deployed from external sources

diff Output Format

The output format of diff indicates instructions for changing the first file to the second file.

`a` (add): Lines present in the second file but not in the first file have been **added**. (e.g., `1a2`)
`c` (change): The corresponding lines in both files have been **changed**. (e.g., `1c2`)
`d` (delete): Lines present in the first file but not in the second file have been **deleted**. (e.g., `1d2`)

Key Options

Frequently used options grouped by purpose.

1) Basic Behavior & Output Format

2) Specify Comparison Scope/Targets

3) Output Control / Ignore Rules

4) Help/Version

Generated command:

Try combining the commands.

Description:

`diff` Executes the command.

Combine the above options to virtually execute commands with AI.

Usage Examples

Common comparison scenarios organized as examples.

Generate Patch in Unified Format

diff -u old.conf new.conf > change.patch

Recursive Directory Comparison

diff -ruN ./v1 ./v2

Compare Ignoring Whitespace Differences

diff -uw src_old.c src_new.c

Side-by-Side View + Hide Common Lines

diff -y --suppress-common-lines a.txt b.txt

Exit Codes

Refer to these when handling conditional logic in scripts.

diff Exit Status Values
Code	Meaning
0	Two targets are identical
1	Differences found
2	Error occurred (e.g., usage error, file access denied)

Installation

Most distributions include diffutils by default, but if not, install the diffutils package.

Debian/Ubuntu

sudo apt update && sudo apt install -y diffutils

RHEL/CentOS/Fedora

sudo dnf install -y diffutils

Arch Linux

sudo pacman -S --needed diffutils

openSUSE

sudo zypper install -y diffutils

Tips & Cautions

Points often overlooked in practice.

The patch format commonly uses -u as the standard. Git also internally uses a similar unified format.
Overuse of whitespace handling options (-w, -b, -B) can obscure actual meaningful changes.
Differences in Windows/Linux newlines can be easily compared by normalizing with --strip-trailing-cr.
When comparing directories, use -r along with -x/-X (exclude rules) to filter out unnecessary files.
If you only need to quickly check for existence, use the -q option. It will not output detailed content.