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This two schedued task will download new stable version and install this package. After 10 min script upgrade routerboard and reboot router.
/system scheduler
add interval=1d name=UPDATE-download-and-install on-event="system package upda\
te set channel=stable\r\
\nsystem package update install" policy=\
ftp,reboot,read,write,policy,test,password,sniff,sensitive,romon \
start-time=03:00:00
add interval=1d name=UPGRADE-routerboard on-event=":global Var1\r\
\n:global Var2\r\
\n:set Var1 \"\$[/system routerboard get upgrade-firmware]\"\r\
\n:set Var2 \"\$[/system routerboard get current-firmware]\"\r\
\n:if (\$Var1>\$Var2) do={/system routerboard upgrade;\r\
\n/system reboot;\r\
\n}" policy=\
ftp,reboot,read,write,policy,test,password,sniff,sensitive,romon \
start-time=03:10:00