/etc/network/if-up.d/ script to set extra IP addresses

if-up.d_addresses

#!/bin/sh

ADDRESSES='
 eth0=2001:db8::2 eth0=2001:db8:2::1
 eth0=192.0.2.2 eth0=198.51.100.2 eth0=203.0.113.2 eth0=233.252.0.2
'

################################################################################

set -e

EXAMPLE_PREFIXES=''
EXAMPLE_PREFIXES="$EXAMPLE_PREFIXES"'^2001:0\?[dD][bB]8:'
EXAMPLE_PREFIXES="$EXAMPLE_PREFIXES"'\|192\.0\?0\?0\.0\?0\?2\.'
EXAMPLE_PREFIXES="$EXAMPLE_PREFIXES"'\|198\.0\?51\.100\.'
EXAMPLE_PREFIXES="$EXAMPLE_PREFIXES"'\|203\.0\?0\?0\.113\.'
EXAMPLE_PREFIXES="$EXAMPLE_PREFIXES"'\|233\.252\.0\?0\?0\.'

for spec in $ADDRESSES; do
 if (printf '%s\n' "$spec" | grep -q -e '='); then
  interface=${spec%%=*}
  address=${spec#*=}
  if [ x"$IFACE" = x"$interface" ]; then
   if ! (printf '%s\n' "$address" | grep -q -e "$EXAMPLE_PREFIXES"); then
    /sbin/ip addr add dev "$interface" "$address" || true
   fi
  fi
 fi
done

exit 0