linux shell script stops after first line - linux

Question

I try to execute etherwake based on a MQTT topic.
The output of mosquitto_sub stops if I pipe it in a while statement.
works:
# mosquitto_sub -L mqtt://... | grep -o -E '([[:xdigit:]]{2}:){5}[[:xdigit:]]{2}'
00:00:00:00:de:ad
00:00:00:00:be:ef
00:00:00:00:ca:fe
(goes on and on)
does not work:
mosquitto_sub -L mqtt://... \
| grep -o -E '([[:xdigit:]]{2}:){5}[[:xdigit:]]{2}' \
| hexdump
Output stops after a single line:
0000000 1234 5678 9abc def0 abcd cafe 3762 3a65
The big picture is this one:
mosquitto_sub -L mqtt://... \
| grep -o -E '([[:xdigit:]]{2}:){5}[[:xdigit:]]{2}' \
| while read macaddr; do
echo "send WOL to " $macaddr;
/usr/bin/etherwake -D -b "$macaddr" 2>&1;
done
Usually I am fine with the Linux shell but this time it simply gets stuck after the first line.
My guess is there is some problem with stdin or stdout (is not read or full etc.) in some kind. But I am out ideas.
By the way its an OpenWRT shell so an ash and no bash.

Answer 1

The problem is indeed the "buffering" of grep when used with pipes.
Usually the '--line-buffered' switch should be used to force grep to process the data line by line instead of buffer the data.
Because grep on OpenWRT (busybox) does not have this switch 'awk' is used:
mosquitto_sub -L mqtt://... \
| awk '/([[:xdigit:]]{2}:){5}[[:xdigit:]]{2}/{ print $0 }' \
| hexdump
If there is no busybox version of grep used the solution would be like:
mosquitto_sub -L mqtt://... \
| grep -o --line-buffered -E '([[:xdigit:]]{2}:){5}[[:xdigit:]]{2}' \
| hexdump
Thank you all a lot for your help.