When synchronizing files between servers, developers often need more granular control than simple file copying. The ability to preview changes as diff patches before actual transfer helps in:

• Auditing security-sensitive modifications
• Verifying deployment changes in CI/CD pipelines
• Maintaining change documentation

While rsync doesn't natively output unified diffs, we can combine it with standard Unix tools:

rsync -avn --out-format="%f" source/ destination/ | \
while read file; do
    diff -u "source/$file" "destination/$file" >> changes.patch
done

For a more robust solution handling edge cases:

#!/bin/bash

# Configurable parameters
SOURCE_DIR="/path/to/source"
DEST_DIR="/path/to/dest"
PATCH_FILE="$(date +%Y%m%d)-changes.patch"

# Generate differential patch
rsync -avn --delete --out-format="%n" "$SOURCE_DIR/" "$DEST_DIR/" | \
grep -v '/$' | \
while read -r file; do
    if [ -f "$SOURCE_DIR/$file" ] && [ -f "$DEST_DIR/$file" ]; then
        diff -u "$DEST_DIR/$file" "$SOURCE_DIR/$file" >> "$PATCH_FILE"
    elif [ ! -f "$DEST_DIR/$file" ]; then
        echo "+++ NEW FILE: $file" >> "$PATCH_FILE"
        cat "$SOURCE_DIR/$file" | sed 's/^/+/' >> "$PATCH_FILE"
    fi
done

echo "Patch generated: $PATCH_FILE"

This technique proves particularly valuable when:

• Config file management: Review exactly which lines differ in nginx/Apache configurations
• Code deployments: Verify changes between development and staging environments
• Compliance tracking: Maintain audit trails of file modifications

For large directory trees, consider these optimizations:

# Use checksum comparison for accuracy
rsync -acn --out-format="%f" source/ dest/

# Parallel processing for speed
find source/ -type f -print0 | parallel -0 -j8 diff -u {} dest/{/}

When synchronizing files between servers, many developers want to preview changes before executing the actual transfer. While rsync's -n (dry run) flag shows which files will be modified, it doesn't reveal the content differences. Here's how to generate patch files that document these changes.

Rsync wasn't primarily designed as a diff tool, but we can combine it with other Unix utilities to achieve patch generation:

rsync -avn --out-format="%f" source/ destination/ | \
while read file; do 
    diff -u "source/$file" "destination/$file" >> changes.patch
done

For production use, consider this enhanced version that handles edge cases:

#!/bin/bash

SOURCE_DIR="/path/to/source/"
DEST_DIR="/path/to/dest/"
PATCH_FILE="rsync_changes_$(date +%Y%m%d).patch"

# First get the list of changed files
changed_files=$(rsync -aic --out-format="%f" "$SOURCE_DIR" "$DEST_DIR")

# Generate unified diff for each changed file
echo "" > "$PATCH_FILE"  # Clear existing patch file
while IFS= read -r file; do
    if [ -f "$SOURCE_DIR$file" ] && [ -f "$DEST_DIR$file" ]; then
        diff -u "$SOURCE_DIR$file" "$DEST_DIR$file" >> "$PATCH_FILE"
    fi
done <<< "$changed_files"

To apply the changes later:

patch -p0 < changes.patch

For binary files or more complex comparisons:

rsync -avc --dry-run --itemize-changes source/ dest/ | \
grep '^[><]' | awk '{print $2}' | \
xargs -I {} diff -u source/{} dest/{} > binary_changes.patch

• This method works best for text files
• Binary files require special handling
• Permission changes won't be captured in the patch
• New files will show as full diffs

For Git repositories, you can combine rsync with git diff:

rsync -avn --delete --exclude='.git' source/ dest/ | \
while read file; do
    (cd source && git diff --no-index -- "$file" "../dest/$file")
done > git_changes.patch