typeset -i mNbr
typeset -i mInput
typeset -i mCnt
echo 'Enter Number: '
read mInput
mFound='N'
while read mNbr
do
  if [ $mNbr -eq $mInput ]; then
    mCnt=$mNbr
    mFound='Y'
  fi
  if [ "$mFound" = "Y" ]; then
    if [ $mNbr -ne $mCnt ]; then
      echo 'Input = '$mInput' Output = '$mCnt
      exit
    fi
    mCnt=$mCnt+1
  else
    if [ $mNbr -gt $mInput ]; then
      echo 'Input = '$mInput' not found'
      exit
    fi
  fi
done < input_file

$ for v in 177 220 1001 2000 2001 2003 2005; do a[$v]=1; done
$ read -p "Input = " i; while [[ ${a[$i]} = 1 ]]; do ((i+=1)); done; echo Output = $i
Input = 2000
Output = 2002