ash: -a: unknown operand

[jaimet]

I'm trying to run this on an OpenWrt 21.02.2 router, but I'm getting a lot of:

ash: -a: unknown operand

I'm guessing that this is due to constructs such as:

if [[ "$FIRST" == "*" -a "$SIGNAL" != "NO" ]]; then

on line 32. As far as I can tell:

[[ is a bashism,
[[ is not defined by posix, and
[[ behaves as though it were [ under busybox.

Also, using -a as a binary and operator is deprecated.