Манипуляция (перестановка) строк

    { a[$1] = a[$1] " " $2 }
END { for(v in a) { print v, a[v] } }

cat test.txt | sort | sed 'N;s/\n/ /'| awk '{print $1" "$2" "$4}'

$ sed -r ':a;N;s/((\n|^)([^ ]+ ))([^\n]*)(.*)\3([^\n]*)$/\1\4 \6\5/;s/\n*$//;ba' << END
> vlan 123
> master 1.1.1.1
> slave 2.2.2.1
> vlan 345
> master 1.1.1.2
> slave 2.2.2.2
> END
vlan 123 345
master 1.1.1.1 1.1.1.2
slave 2.2.2.1 2.2.2.2

perl -anE '$H{$F[0]}.=" $F[1]" if @F}{say@_ while @_=each %H}'

while read key val; do
   case $key in
      vlan)
         vlan="$vlan $val"
      ;;
      master)
         master="$master $val"
      ;;
      slave)
         slave="$slave $val"
      ;;
   esac

  # вместо case можно eval
  # eval "$key=\$$key $val"
done

echo "vlan:   $vlan"
echo "master: $master"
echo "slave:  $slave"