diff

line-by-line file comparison tool

TLDR

Compare files

$ diff [file1.txt] [file2.txt]

Unified format

$ diff -u [file1.txt] [file2.txt]

Compare directories

$ diff -r [dir1] [dir2]

Side-by-side comparison

$ diff -y [file1.txt] [file2.txt]

Ignore whitespace

$ diff -w [file1.txt] [file2.txt]

diff compares files line by line and outputs the differences. It's essential for version control, code review, and understanding changes between file versions.The tool forms the basis of patch files and is used by version control systems.

PARAMETERS

-u, --unified

Unified diff format (recommended)

-c, --context

Context diff format

-y, --side-by-side

Side-by-side comparison

-r, --recursive

Recursively compare directories

-q, --brief

Report only if files differ

-s, --report-identical-files

Report when files are identical

-w, --ignore-all-space

Ignore whitespace changes

-b, --ignore-space-change

Ignore whitespace amount changes

-B, --ignore-blank-lines

Ignore blank line changes

-i, --ignore-case

Case-insensitive comparison

-N, --new-file

Treat missing files as empty

OUTPUT FORMATS

Normal format:

$ 3c3
< old line
---
> new line

Unified format (-u):

$ --- file1.txt
+++ file2.txt
@@ -1,3 +1,3 @@
context
-old line
+new line
context

Context format (-c):

$ *** file1.txt
--- file2.txt
***************
*** 1,3 ****
  context
! old line
  context
--- 1,3 ----
  context
! new line
  context

WORKFLOW

$ # Basic comparison
diff file1.txt file2.txt

# Unified diff (best for patches)
diff -u original.txt modified.txt

# Save as patch
diff -u original.txt modified.txt > changes.patch

# Compare directories
diff -r old_version/ new_version/

# Ignore whitespace
diff -w file1.txt file2.txt

# Brief output
diff -q file1.txt file2.txt

# Side by side
diff -y file1.txt file2.txt | less

CREATING PATCHES

$ # Create patch
diff -u old.c new.c > fix.patch

# Apply patch
patch old.c < fix.patch

# Reverse patch
patch -R new.c < fix.patch

DIRECTORY COMPARISON

$ # Compare directories
diff -r dir1/ dir2/

# Exclude files
diff -r --exclude="*.log" dir1/ dir2/

# Brief directory diff
diff -rq dir1/ dir2/

EXIT STATUS

- 0 - Files are identical- 1 - Files differ- 2 - Error occurred

INSTALL

sudo apt install diffutils

sudo dnf install diffutils

sudo pacman -S diffutils

sudo apk add diffutils

sudo zypper install diffutils

brew install diffutils

nix profile install nixpkgs#diffutils

CAVEATS

Large files can be slow. Binary files show as "differ" without details. Whitespace handling varies. Line endings (CRLF vs LF) can cause issues. Not suitable for binary comparison (use cmp).

HISTORY

diff was written by James W. Hunt and Doug McIlroy at Bell Labs around 1974, with the algorithm published in their joint paper in 1976.