Dangerous Commands

Dry run / preview

Run Rsync Without Deleting Your Backup

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

Command

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

Before you run this

System impact: Preview only. Still verify source and destination paths before running the real command.

When not to use it: Do not assume dry-run output is safe if the source and destination paths are reversed.

Expected output

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

System impact

Dry run / preview. The dry run prints intended changes without modifying files.

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.

Recovery / rollback

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

Watch this command run

Command transcript

This sanitized transcript shows the commands and output shape without exposing host details.

demo@lab:~$

$ find source backup -maxdepth 2 -type f | sort

backup/app.conf
backup/stale.txt
source/app.conf
source/index.html

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

sending incremental file list
deleting stale.txt
app.conf
index.html

sent 98 bytes  received 31 bytes  258.00 bytes/sec
total size is 24  speedup is 0.19 (DRY RUN)

$ rsync -avh --delete ./source/ ./backup/

sending incremental file list
deleting stale.txt
app.conf
index.html

sent 206 bytes  received 67 bytes  546.00 bytes/sec
total size is 24  speedup is 0.09

View commands shown

These are the commands shown in the sanitized transcript.

Commands shown

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

next steps

Related commands

Dangerous Commands Dry run

Preview What Rsync Would Delete

`rsync --delete` is useful. It is also how people erase the wrong side.

rsync -avhn --delete ./source/ ./backup/ | grep '^deleting'

Hosting Operations Dry run

Preview Backup Drift with rsync

Rsync can tell you what would change before it changes anything.

rsync -ain --delete source/ backup/

Dangerous Commands Deletes data

Print a Dry-Run Removal Script

The reviewable cleanup command is the one you print before you run.

find /lab/disk-inode-cleanup/var/tmp/uploads -xdev -type f -mtime +7 -printf 'rm -i -- %p\n'

Hosting Operations Can be slow

Compare Source and Backup File Lists

A backup can be missing files and still look plausible at a glance.

comm -3 <(find source -type f | sed 's#^source/##' | sort) <(find backup -type f | sed 's#^backup/##' | sort)

Dangerous Commands Read-only

Inspect Permissions Before Changing Them

The permission fix was easy. Knowing what not to chmod was the hard part.

namei -l /var/www/example/index.html

Study mapping

Use this as independent command practice: read the notes, predict the output, then compare it with the example before using a real shell.