When using rsync -avh, the command combines several flags that affect how file transfers occur:

-a (archive mode) 
-v (verbose)
-h (human-readable)

The crucial point is that -a implies -t (preserve modification times), which affects rsync's decision-making process about whether to overwrite files.

rsync uses a sophisticated algorithm to determine whether to transfer a file. Here's what happens when a file exists at the destination:

• Compares file sizes first (quick check)
• If sizes match, compares modification timestamps
• If timestamps differ, performs a checksum comparison

Example scenario where existing files might be overwritten:

# Source file (newer)
-rw-r--r-- 1 user user 1024 Jun 10 15:30 source/file.txt

# Destination file (older)
-rw-r--r-- 1 user user 1024 Jun 9 14:20 dest/file.txt

# Running:
rsync -avh source/ dest/

In this case, file.txt would be overwritten because the source file is newer.

Several options modify how rsync handles existing files:

--ignore-existing       # Never overwrite existing files
--update (-u)           # Skip newer files on receiver
--checksum (-c)         # Force checksum comparison
--size-only             # Compare only sizes
--backup                # Make backups of overwritten files

Always test with --dry-run first to see what would happen:

rsync -avh --dry-run source/ dest/

This outputs what files would be transferred without actually making changes.

When dealing with configuration files where you might have local modifications you want to preserve:

rsync -avh --ignore-existing configs/ /etc/

This ensures only new config files are copied, never overwriting existing ones.

The default timestamp comparison is fast but can sometimes lead to unnecessary transfers. For large files, consider:

rsync -avh --checksum large_files/ backup/

This performs a slower but more accurate checksum comparison to verify if files truly differ.

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When using rsync -avh (archive mode, verbose, human-readable), the command will:

• Compare source and destination files using checksums
• Only transfer files that differ between source and destination
• Overwrite existing files when they're different from source
• Preserve permissions, ownership, and timestamps

Consider this common directory structure:

source_dir/
├── config.json
└── app.js

dest_dir/
├── config.json
└── old_script.js

Running:

rsync -avh source_dir/ dest_dir/

Here's what happens:

• config.json will be overwritten if different
• app.js will be copied (new file)
• old_script.js remains untouched

For more granular control:

# Skip newer files on receiver
rsync -avh --ignore-existing source/ dest/

# Backup overwritten files (appends ~)
rsync -avh --backup source/ dest/

# Dry run to see what would happen
rsync -avh --dry-run source/ dest/

Always verify with:

rsync -avh --stats source/ dest/

This shows detailed transfer statistics including number of files overwritten.

If files aren't overwriting as expected:

1. Check file permissions
2. Verify no trailing slashes are missing
3. Test with --verbose to see decision-making

Remember that rsync uses size and modification time checks by default. For absolute certainty, use --checksum for full file comparison.