shell中如何生成随机数？

########################
# Generate a random string
# Arguments:
#   -t|--type - String type (ascii, alphanumeric, numeric), defaults to ascii
#   -c|--count - Number of characters, defaults to 32
# Arguments:
#   None
# Returns:
#   None
# Returns:
#   String
#########################
generate_random_string() {
    local type="ascii"
    local count="32"
    local filter
    local result
    # Validate arguments
    while [[ "$#" -gt 0 ]]; do
        case "$1" in
        -t | --type)
            shift
            type="$1"
            ;;
        -c | --count)
            shift
            count="$1"
            ;;
        *)
            echo "Invalid command line flag $1" >&2
            return 1
            ;;
        esac
        shift
    done
    # Validate type
    case "$type" in
    ascii)
        filter="[:print:]"
        ;;
    alphanumeric)
        filter="a-zA-Z0-9"
        ;;
    numeric)
        filter="0-9"
        ;;
    *)
        echo "Invalid type ${type}" >&2
        return 1
        ;;
    esac
    # Obtain count + 10 lines from /dev/urandom to ensure that the resulting string has the expected size
    # Note there is a very small chance of strings starting with EOL character
    # Therefore, the higher amount of lines read, this will happen less frequently
    result="$(head -n "$((count + 10))" /dev/urandom | tr -dc "$filter" | head -c "$count")"
    echo "$result"
}