Persistent serial port names for USB-serial on OpenWrt hotplugd

10-persistent-serial.sh

[ "${ACTION}" = "bind" -o "${ACTION}" = "unbind" ] || exit 0
[ "${SUBSYSTEM}" = "usb-serial" ] || exit 0
[ -n "${DEVICENAME}" -a -n "${DEVPATH}" ] || exit 1

if [ "${ACTION}" = "bind" ]; then
        subsystem="$(basename $(readlink /sys${DEVPATH}/../subsystem))"

        [ "$subsystem" = "usb" ] || exit 0

	replace_whitespace="s/^[ \t]*|[ \t]*$//g; s/[ \t]+/_/g"
        manufacturer="$(sed -E "${replace_whitespace}" /sys${DEVPATH}/../../manufacturer)"
	# If manufacturer is blank, use vendor ID.
	: ${manufacturer:=$(cat /sys${DEVPATH}/../../idVendor)}
        product="$(sed -E "${replace_whitespace}" /sys${DEVPATH}/../../product)"
	# If product is blank, use product ID.
	: ${product:=$(cat /sys${DEVPATH}/../../idProduct)}
        serial="$(sed -E "${replace_whitespace}" /sys${DEVPATH}/../../serial)"
        interface="$(cat /sys${DEVPATH}/../bInterfaceNumber)"
        port="$(cat /sys${DEVPATH}/port_number)"

	replace_chars="s/[^0-9A-Za-z#+.:=@-]/_/g"
        id_link=$(echo "${subsystem}-${manufacturer}_${product}${serial:+_${serial}}-if${interface}${port:+-port${port}}" | \
	    sed "${replace_chars}")
        path_link=$(echo "${DEVPATH}${port:+-${port}}" | \
	    sed "s%/devices/%%; s%/${DEVICENAME}%%g; ${replace_chars}")

        mkdir -p /dev/serial/by-id /dev/serial/by-path
        ln -sf "/dev/${DEVICENAME}" "/dev/serial/by-id/${id_link}"
        ln -sf "/dev/${DEVICENAME}" "/dev/serial/by-path/${path_link}"
elif [ "${ACTION}" = "unbind" ]; then
        for link in $(find /dev/serial -type l); do
                [ -L ${link} -a "$(readlink ${link})" = "/dev/$DEVICENAME" ] && rm ${link}
        done
fi

I had a chance to test this against my CH340-based adapter which lacks manufacturer string - and I think the call to sed in line 11 breaks this detection, because result code of sed instead of cat is taken into account, causing fallback to idVendor to never happen, resulting in this:

lrwxrwxrwx    1 root     root            12 Mar 30 14:58 usb-_USB2.0-Ser_-if00-port0 -> /dev/ttyUSB3

Adding set -o pipefail at the start fixes this.
I noticed, that with bind/unbind events, hotplug reacts faster to changes in the system, btw.

Thanks for pointing this out. I've made a change that I believe fixes the problem.