mkdir /etc/config/scripts
vi /etc/config/scripts/wifi-signal
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#!/bin/sh
/usr/bin/iwinfo wlan0 info | /bin/grep -i signal | /usr/bin/awk '{ print $2 }
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chmod 777 /etc/config/scripts/wifi-signal
vi /etc/config/snmpd
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config 'exec'
        option 'name' 'signaldb'
        option 'prog' '/etc/config/scripts/wifi-signal'
        option 'args' ''
        option miboid 1.2.3.4
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/etc/init.d/snmpd restart