`${1,,}` is bash 4.0+. macOS ships bash 3.2 as /bin/bash, where it raises
"bad substitution"; the subshell then exited 1 — read as "not local" — so
`url-guard.sh host` returned rc 0 for localhost, 127.x, 10.x, 192.168.x,
172.16-31.x, 169.254.169.254 and metadata.google.internal. The guard failed
OPEN on every Mac, and url-guard.test.sh recorded it as 13 "got[0] want[2]".
`shopt -s nocasematch` (bash 3.1+) keeps both the ASCII-only folding that
LC_ALL=C gives and the no-fork property the lowercase expansion had.
Verified on /bin/bash 3.2: the ten local/private/metadata targets now return
rc 2 (case-folded variants included), legitimate hosts still rc 0.
bmottin