(( bch_id += 1 )) 不支持 ++ 
(( next = pre + 1 )) 不能用 next=((pre+1))

let new_rank=$old_rank-1

let new_rank=10#$old_rank-1  十进制

if [[ x -eq "3456" ]] then
    echo "3456"
elif [[ x -eq "4567" ]] then
    echo "4567"
elif [[ x -eq "1234" ]] then
    echo "1234"
else
    echo "none"
fi