diff — File & Output Comparison

Comparison patterns — unified diff, side-by-side, process substitution for comparing command outputs, and preview-before-apply workflows.

Basic diff

Default output — ed-style change commands

diff file1.txt file2.txt
# Output format: line-range + action (a=add, d=delete, c=change)
# 3c3 means "line 3 in file1 changes to line 3 in file2"

Exit codes — useful in scripts

diff -q file1.txt file2.txt
# Exit 0: files are identical
# Exit 1: files differ
# Exit 2: trouble (file not found, etc.)

Brief mode — just report whether files differ, no details

diff -q file1.txt file2.txt

Unified Format — -u

Unified diff — the standard for patches and code review

diff -u original.conf modified.conf

Unified diff with more context lines — default is 3

diff -u -U10 original.conf modified.conf

Unified diff with labels — cleaner headers

diff -u --label "before" --label "after" original.conf modified.conf

Side-by-Side — -y

Side-by-side comparison — visual alignment

diff -y file1.txt file2.txt

Side-by-side with custom width — fit terminal

diff -y -W "$(tput cols)" file1.txt file2.txt

Side-by-side showing only differences — suppress common lines

diff -y --suppress-common-lines file1.txt file2.txt

Context Format — -c

Context diff — older format, shows surrounding lines with ! for changes

diff -c file1.txt file2.txt

Context diff with more surrounding lines

diff -c -C5 file1.txt file2.txt

Recursive — -r

Compare two directory trees recursively

diff -r dir1/ dir2/

Recursive unified diff — suitable for patch files

diff -ru dir1/ dir2/ > changes.patch

Recursive diff excluding certain files

diff -r --exclude='*.log' --exclude='.git' dir1/ dir2/

Recursive diff with brief output — just list differing files

diff -rq dir1/ dir2/

Process Substitution — diff Without Temp Files

Compare command outputs directly — no temp files needed

diff <(sort file1.txt) <(sort file2.txt)

Compare remote and local config

diff <(ssh server cat /etc/nginx/nginx.conf) /etc/nginx/nginx.conf

Compare before and after a command — verify a change

diff <(systemctl list-units --type=service --state=running) <(sleep 5; systemctl list-units --type=service --state=running)

Compare package lists between two systems

diff <(ssh host1 'pacman -Q | sort') <(ssh host2 'pacman -Q | sort')

Compare two git branches — file list difference

diff <(git ls-tree -r --name-only main | sort) <(git ls-tree -r --name-only feature | sort)

colordiff

Colorized diff — easier to read in terminal

colordiff -u file1.txt file2.txt

Use colordiff as a drop-in replacement if available

diff -u file1.txt file2.txt | colordiff

Fallback pattern — use colordiff if installed, else plain diff

if command -v colordiff &>/dev/null; then
    colordiff -u file1.txt file2.txt
else
    diff -u file1.txt file2.txt
fi

diff3 — Three-Way Merge

Three-way diff — compare file with two variants

diff3 mine.txt original.txt yours.txt

Merge three files — output merged result

diff3 -m mine.txt original.txt yours.txt > merged.txt

Creating and Applying Patches

Create a patch file from unified diff

diff -u original.conf modified.conf > fix.patch

Apply a patch — transforms original into modified

patch < fix.patch

Apply a patch with backup — keeps .orig files

patch -b < fix.patch

Dry run — check if patch applies cleanly without modifying

patch --dry-run < fix.patch

Reverse a patch — undo the change

patch -R < fix.patch

Create a recursive patch for a directory tree

diff -ruN original_dir/ modified_dir/ > project.patch
# -N treats absent files as empty — includes new files in the patch

git diff vs diff

git diff uses unified format with enhanced headers

git diff                    # Unstaged changes
git diff --staged           # Staged changes
git diff HEAD~3..HEAD       # Last 3 commits
git diff main..feature      # Branch comparison

git diff with word-level highlighting — finer granularity than line diff

git diff --word-diff

git diff stat — summary of changes per file

git diff --stat main..feature

Use regular diff when git is not available or for non-repo files

# diff works on any two files — no repository needed
diff -u /etc/ssh/sshd_config /etc/ssh/sshd_config.bak

Practical Patterns

Verify a config change — before/after pattern

cp /etc/ssh/sshd_config /tmp/sshd_config.before
# ... make changes ...
diff -u /tmp/sshd_config.before /etc/ssh/sshd_config

Compare two URLs' content

diff <(curl -s https://example.com/config.json | jq .) <(curl -s https://staging.example.com/config.json | jq .)

Ignore whitespace differences — focus on content changes

diff -u -w file1.txt file2.txt     # Ignore all whitespace
diff -u -b file1.txt file2.txt     # Ignore changes in whitespace amount
diff -u -B file1.txt file2.txt     # Ignore blank lines

Ignore comments when comparing configs

diff <(grep -v '^#' original.conf | grep -v '^$') <(grep -v '^#' modified.conf | grep -v '^$')