Dangerous Commands

Run Rsync Without Deleting Your Backup

You need to preview an rsync operation before moving or deleting files.

Command

rsync -avhn --delete ./source/ ./backup/

What changed

The dry run prints intended changes without modifying files.

Danger

caution

When to use it

Use this before any sync involving deletes, production data, or backups.

When not to use it

Do not assume dry-run output is safe if the source and destination paths are reversed.

Undo or recovery

No state is changed during the dry run. If you remove `-n`, make a backup first.

Expected output

A list of files that would be copied, updated, or deleted.

demo script

Disposable terminal steps

find source backup -maxdepth 2 -type f | sort
rsync -avhn --delete ./source/ ./backup/
rsync -avh --delete ./source/ ./backup/

Terminal transcript Demo JSON

simulated output

What it looks like

disposable vessel

::fixture-ready::
$ find source backup -maxdepth 2 -type f | sort
backup/app.conf
backup/stale.txt
source/app.conf
source/index.html
::exit-code::0
$ rsync -avhn --delete ./source/ ./backup/
sending incremental file list
deleting stale.txt
app.conf
index.html

sent 101 bytes  received 31 bytes  264.00 bytes/sec
total size is 24  speedup is 0.18 (DRY RUN)
::exit-code::0
$ rsync -avh --delete ./source/ ./backup/
sending incremental file list
deleting stale.txt
app.conf
index.html

sent 209 bytes  received 67 bytes  552.00 bytes/sec
total size is 24  speedup is 0.09
::exit-code::0

YouTube Short

Preview rsync before it bites.

The `-n` flag is the pause button. It shows what rsync would do before you let it touch the files.

experiments

A/B tests to run

Metric: youtube_retention_15s

A: One rsync flag can save you. Another can erase the wrong side.

B: Run this before `rsync --delete`.