■ space · the safety net

■ the answer first

Dry-run by default, an explicit arm step, and a backup net — the generator refuses to emit a bare --delete.

The net is two flags — --backup --backup-dir=DEST/_rsync_backup_$(date +%F) — and rsync moves every file it would have deleted into that dated folder instead of erasing it.

Paths, remote host, excludes and bandwidth limits in; a paste-ready command out. Nothing you type leaves the page.

WHY --DELETE DESERVES A WARNING

Because rsync's contract is make the destination match the source, and --delete extends it to removals. The man page: "delete extraneous files from the destination dir."

Extraneous = anything the source doesn't have — including things missing because you pointed at the wrong source, mounted the wrong USB stick, or typo'd a path.

One page-one generator even labels its output "AI-generated and not yet checked for validity" — above a delete command. The four flags that do the real damage:

flag	what it does	what it does when you're wrong
`--delete`	destination files missing from source get removed	wrong source path → destination files "extraneous" → gone
`src/` vs `src`	trailing slash copies the contents; no slash copies the directory itself	one character, entirely different destination layout
`-a`	archive: recursive + permissions, times, owners, symlinks	skipped → a "backup" that silently isn't one
`--partial`	keep half-transferred files on interruption	flaky link → restart the whole transfer from zero

The trailing-slash row is the classic. rsync's docs say a trailing slash on the source "changes this behavior to avoid creating an extra directory at the destination."

Plainly: src/ pours the contents into the destination; src creates dest/src and pours them in there. Two characters.

The machine below shows which sentence applies as you type — the trailing slash has eaten more home directories than rm -rf ever did.

THE SAFETY NET

Paths in, paste-ready command out. The command ships with -n: read what would happen, then flip the arm switch.

With --delete involved, the generator refuses to emit a bare delete — the net attaches itself.

paths — local or user@host:/path on either side source destination ssh port — wired through -e "ssh -p NNNN" only when it isn't 22

direction — the trailing slash

the non-negotiable

the arm lane

the loaded gun

filters — --exclude presets, the usual suspects

more patterns — comma or space separated

manners

--bwlimit in KB/s — so the 3am backup stops saturating the household

output — your terminal's choice

Arming a delete safely: don't remove the safety — add a net. With --backup --backup-dir, rsync moves removed or overwritten files into the dated folder first.

The mirror still ends up exact. Reversibility is the difference between a mirror and a memory.

# the record's worked example — armed, net attached:
rsync -a --delete \
  --backup --backup-dir=/mnt/backup/_rsync_backup_$(date +%F) \
  /mnt/usb/ /mnt/backup/

verify: man rsync → --backup reads "make a backup of every file installed" · the --backup-dir rows land in DEST/_rsync_backup_<date>/, not the void

WHAT THE GENERATOR WIRES

The set that covers nearly every real transfer, grouped the way you actually decide about them:

Paths — local or remote on either side (user@host:/path), with the SSH port wired through -e "ssh -p 2222" when it isn't 22.
Direction — the trailing-slash toggle, with the contents-vs-directory sentence printed live.
Filters — --exclude patterns with presets for the usual suspects: .git, node_modules, caches, logs.
Manners — -z compression, --bwlimit so the 3am backup stops saturating the household, -P for progress and resumability, -c when mtime lies.
The arm lane — dry-run default, explicit arm step, net auto-attached to any --delete.

Like the GGUF VRAM calculator: a single HTML file doing arithmetic in your browser — nothing you type leaves the page, because a tool about safe copies shouldn't phone home.

MIT, one file, no build step, at github.com/mrsaynothing/rsync-builder. If the arm flow feels like nagging, that's the feature working.

WHERE THIS SITS IN THE TOOLBOX

The generator composes the command; the siblings hold the theory. Rsync vs scp — why delta transfer beats a dumb copy, and when scp is fine.

Cron vs systemd timers is the scheduling half; the journal side of knowing whether it ran is in the journalctl cheat sheet.

A dry-run you skip is a delete you didn't mean.

■ honest stamp Tested on: rsync(1) man-page semantics, protocol 32 · in-browser, nothing uploaded. Recorded 2026-10-16, shipped as Tool Friday during the bench outage — so no transfer was benched for this page, and none needed: every flag row above is the man page's own contract, the trailing-slash sentence is rsync's documented behaviour, and the machine runs the same two net flags the record's worked example ships. The tool lives at github.com/mrsaynothing/rsync-builder — if you can name a flag combination the net doesn't cover, that's an issue worth filing.

end of the safety net · nothing fetched · everything local