File Synchronization Rules

rsync Fundamentals

Trailing slash matters: rsync src/ copies contents, rsync src copies the folder itself — this is the #1 cause of wrong directory structures
Always use -avz baseline: archive mode preserves permissions/timestamps, verbose shows progress, compress speeds transfers
Add --delete only when you want destination to mirror source exactly — without it, deleted source files remain on destination
Use --dry-run before any destructive sync — shows what would change without modifying anything

Create an exclude file instead of multiple --exclude flags: rsync -avz --exclude-from=.syncignore src/ dest/
Standard excludes for code projects: .git/, node_modules/, __pycache__/, .venv/, *.pyc, .DS_Store, Thumbs.db
Exclude patterns are relative to source root — /logs/ excludes only top-level logs, logs/ excludes logs/ anywhere

rclone sync deletes destination files not in source; rclone copy only adds — use copy when unsure
Configure remotes interactively: rclone config — never hardcode cloud credentials in scripts
Test with --dry-run first, then --progress for visual feedback during actual sync
For S3-compatible storage, set --s3-chunk-size 64M for large files to avoid timeouts

After critical syncs, verify with checksums: rsync -avzc uses checksums instead of size/time (slower but certain)
For rclone, use rclone check source: dest: to compare without transferring
Log sync operations to file for audit: rsync -avz src/ dest/ | tee sync.log

rsync is one-way only — for true bidirectional sync, use unison: unison dir1 dir2
Unison detects conflicts when both sides change — resolve manually or set prefer rules
Cloud services like Dropbox/Syncthing handle bidirectional automatically — don't reinvent with rsync

For SSH remotes, use key-based auth: rsync -avz -e "ssh -i ~/.ssh/key" src/ user@host:dest/
Specify non-standard SSH port: -e "ssh -p 2222"
Use --partial --progress for large files over unreliable connections — allows resume on failure

Syncing to mounted drives that unmount silently creates a local folder with the mount name — verify mount before sync
Running sync without --delete repeatedly causes destination to accumulate deleted files forever
Time-based sync fails across machines with clock skew — use --checksum for accuracy or sync NTP first