validint

#!/bin/bash
# validint--Validates integer input, allowing negative integers too

validint()
{
    # Validate first field and test that value against min value $2 and/or
    #  max value $3 if they are supplied. If the value isn't within range
    #  or it's not composed of just digits, fail.
    number="$1";    min="$2";   max="$3"

    if [ -z $number ]; then
        echo "You didn't enter anything. Please enter a number." >&2
        return 1
    fi

    # Is the first character a '-' sign?
    if [ "${number%${number#?}}" = "-" ]; then 
        testvalue="${number#?}"  # Grab all but the first character to test.

    # Create a version of the number that has no digits for testing.
    nodigits="$(echo $testvalue | sed 's/[[:digit:]]//g')"

    # Check for nondigit characters.
    if [ ! -z nodigits ]; then
        echo "Invalid number format! Only digits, no commas, spaces, etc." >&2
        return 1
    fi

    if [ ! -z $min ]; then
        # Is the input less than the minimum value?
        if [ "$number" -lt "$min" ]; then
            echo "Your value is too small: smallest acceptable value is $min." >&2
            return 1
        fi
    fi
    if [ ! -z $max ]; then
        # Is the input greater than the maximum value?
        if [ "$number" -gt "$max" ]; then
            echo "Your value is too big: largest acceptable value is $max." >&2
            return 1
        fi
    fi
    return 0cle
}

# Input validation
if validint "$1" "$2" "$3" ; then
    echo "Input is a valid integer within your constrains."
fi