rsync

What it does

rsync synchronizes files and directories, either locally or over a network (typically tunneled through SSH), by comparing source and destination and transferring only the data that has actually changed. This delta-transfer approach makes it dramatically more efficient than a plain copy for repeated backups or deployments, and its --delete, --dry-run, and archive-mode options make it equally well suited to one-off mirroring and ongoing scheduled backup jobs.

Beginner examples

• rsync -av ./src/ ./dest/ — copy the contents of src into dest, preserving permissions and timestamps
• rsync -avz ./src/ user@host:/remote/dest/ — sync a local directory to a remote host over SSH with compression
• rsync -av file.txt ./backup/ — copy a single file, preserving its attributes
• rsync -n -av ./src/ ./dest/ — dry run: show what would change without copying anything

rsync -av ./notes/ ./notes-backup/

Advanced examples

• Mirror a directory exactly, removing anything at the destination not present in the source: rsync -av --delete ./src/ ./dest/
• Sync over SSH on a non-default port: rsync -avz -e "ssh -p 2222" ./src/ user@host:/remote/dest/
• Resume and watch progress on a large transfer: rsync -avP ./bigfile.iso user@host:/data/
• Exclude specific files or patterns from the sync: rsync -av --exclude='*.log' --exclude='node_modules/' ./app/ ./app-backup/
• Preview a mirror operation before committing to deletions: rsync -anv --delete ./src/ user@host:/remote/dest/

rsync -avz --delete -e ssh ./dist/ deploy@server.example.com:/var/www/app/

Common mistakes

• Getting the trailing slash wrong on the source path — rsync -a src/ dest/ copies the contents of src into dest, while rsync -a src dest/ (no trailing slash) copies src itself into dest, producing dest/src/ instead.
• Adding --delete without first running a dry run (-n) to confirm what would be removed — accidental deletions at the destination are hard to notice until it's too late.
• Assuming -a includes compression — it doesn't; add -z explicitly when transferring over a slow network link.
• Forgetting that rsync needs to be installed on both ends for remote transfers over SSH, not just the machine initiating the sync.

Tips

• Always pair --delete with -n (dry run) the first time you run a new mirroring command, to review exactly what would be removed.
• Use --exclude (or --exclude-from=file) to skip build artifacts, .git, or node_modules directories that shouldn't be part of a sync or backup.
• -P is shorthand for --partial --progress, which keeps partially transferred files around so an interrupted large transfer can resume instead of restarting from zero.

Best practices

• Use -a (archive mode) as your default baseline for anything beyond trivial copies — it bundles recursion, symlink preservation, permissions, timestamps, and ownership into one flag.
• Run a --dry-run before any --delete operation, especially in scripted or scheduled backup jobs, to avoid silently wiping data due to a path typo.
• For scheduled backups, combine rsync with --link-dest to create space-efficient, hard-linked incremental snapshots instead of full copies each run.

Real-world use cases

• Deploying a built static site or application bundle to a web server, transferring only changed files on each deploy.
• Nightly incremental backups of a home directory or database dump directory to a remote backup host over SSH.
• Mirroring large media or dataset directories between machines where re-copying everything each time would be prohibitively slow.

Common interview questions

• Why is rsync generally preferred over scp/cp for repeated syncs? It uses a delta-transfer algorithm that only sends changed parts of files, making repeat syncs of large or mostly-unchanged directories far faster.
• What does a trailing slash on the source path change? src/ syncs the contents of src into the destination; src (no slash) syncs the src directory itself, nesting it one level deeper at the destination.
• How do you make rsync mirror a source exactly, including removing extra files at the destination? Add --delete, ideally verified first with a --dry-run (-n) pass to review what would be deleted.