Randomly generate 8-bit string function

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# Randomly generate an 8-bit string function randstr
randstr() {
  index=0
  str=""
  for i in {a..z}; do arr[index]=$i; index=`expr ${index} + 1`; done
  for i in {A..Z}; do arr[index]=$i; index=`expr ${index} + 1`; done
  for i in {0..9}; do arr[index]=$i; index=`expr ${index} + 1`; done
  for i in {1..8}; do str="$str${arr[$RANDOM%$index]}"; done
  echo ${str}
}

echo `randstr`