check command line input is a number in c

1bool isNumber(char number[])
2{
3    int i = 0;
4
5    //checking for negative numbers
6    if (number[0] == '-')
7        i = 1;
8    for (; number[i] != 0; i++)
9    {
10        //if (number[i] > '9' || number[i] < '0')
11        if (!isdigit(number[i]))
12            return false;
13    }
14    return true;
15}