diff (Differential)

The diff command is used to compare two files line by line and display the differences between them. It tells you which lines in one file must be changed to make it identical to the second file. This is the foundation of version control systems like Git.

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1. Basic Usage

diff [options] file1 file2

• < denotes lines from the first file.
• > denotes lines from the second file.

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2. Key Options

Option	Name	Description
-c	context	Provides a few lines of context around each difference.
-u	unified	Unified format. The standard format used for creating patches and in Git.
-y	side-by-side	Displays the differences in two columns.
-i	ignore-case	Ignores case differences in file contents.
-w	ignore-all-space	Ignores all white space when comparing lines.
-r	recursive	Directory comparison. Compares folders and their subfolders.

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3. Practical Examples

① Visualizing differences side-by-side

diff -y original.c updated.c

② Generating a patch file (Unified format)

diff -u config.old config.new > config.patch

③ Comparing two directories

diff -qr dir1/ dir2/
# -q shows only whether files differ, not the actual content changes.

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4. [Tip] Symbolic Indicators

• a: Addition
• c: Change
• d: Deletion Modern developers often use colordiff or git diff for colorized, easier-to-read comparisons.