shell脚本实现把不连续的序号扩成连续

#!/bin/bash


#sh replace2.sh demo.txt demo2.txt


declare -i old
declare -i i


old=0
cat $1 | while read line; do
arr=(${line// / }) 
for (( i = $old+1; i < ${arr[0]}; i++ )); do
echo ${i} >> $2
done
old=${arr[0]}
echo ${line} >> $2


done