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1.6. Datatypes and Variables

NASL allows for
the assignment of values to variables that can be manipulated by a
NASL script. Unlike a strongly typed language such as C, NASL does
not require you to predefine a variable's type. In
NASL, the variable type is determined automatically when a variable
is assigned a specific value. NASL recognizes two valid datatypes:
scalars and arrays. A
scalar can be a number or a string, while an array is a collection of
scalars.

1.6.1. Numbers

NASL
allows variables to hold integer
valuesfor example, the number 11. It is also possible to
assign numeric values to variables using a hexadecimal
representation. You write hexadecimal numbers in NASL using a
leading "0x" prefix. For example,
the hexadecimal number 0x1b holds the value 27 when represented as an
integer in base-10 notation. Type the following script into a file:

h=0x1b;
display ("The value of h is ",h,"\n");

Now run it using the NASL interpreter to see the output:

[notroot]$ nasl hex.nasl
The value of h is 27

It is also possible to input numerical values in octal notation form,
which uses base- 8 notation by placing a leading
"0" prefix. For example, the
x and y are equivalent in the
following example:

x=014; #octal
y=12; #decimal

1.6.2. Strings

A
string
is a collection of characters.
abcdefg, Hello
World, and Boeing
747 are all examples of strings. Consider the
following NASL script:

mystring="Hello. I am a string!\n";
display(mystring);

The \n at the end of
mystring is an escape character and is equivalent
to a newline character. Table 1-1 lists common
escape characters applicable to NASL.

\'

\"

\\

\r

\n

\t

\x(integer)

\v

Note that a string inside double quotes (")
is left as is. Therefore, if you define a string using double quotes,
escape sequences will not be translated. Also note that the
display( ) function calls the string(
) function before displaying data on the
console, and it is the string( ) function that
converts the escape sequences. That is why our escape sequences are
translated in the preceding examples even though we define them using
double quotes.

1.6.3. Arrays and Hashes

An
array
is a collection of numbers or strings that can be indexed using a
numeric subscript. Consider the following NASL script:

myarray=make_list(1,"two");
display("The value of the first item is ",myarray[0]," \n");
display("The value of the second item is ",myarray[1]," \n");

The script displays the following when executed:

The value of the first item is 1
The value of the second item is two

Notice that the array subscripts begin at 0, and that is why the
first element is obtained using the [0] subscript.

Like
arrays,
hashes are also collections of numbers or
strings. However, elements in hashes have a key value associated
with them that can be used to obtain the element. You can use the
make_array( ) function call to define a hash.
Because every element must have an associated key value, the function
call requires an even number of arguments. The following is a
definition of a hash that contains port numbers for the Telnet
protocol (port 23) and HTTP (port 80):

myports=make_array('telnet',23,'http',80);

Now, myports['telnet'] gives you the value of
23, while myports['http']
evaluates to 80.

1.6.4. Local and Global Variables

Variables
exist only within the blocks in which
they are defined. A block is a collection of
statements enclosed by special statements such as loops and function
calls. For example, if you define a variable within a particular
function call, it will not exist when the function call returns. At
times, it is necessary to define variables that should exist
globally; in such cases you should use global_var
to define them:

global_var myglobalvariable;

Variables are local by default. You can also use
local_var to state this explicitly.