Running commands from string
This is actually pretty simple:

if (s == "digitalWrite(8,LOW);") {
    digitalWrite(8,LOW);
}


Obviously, it will not work is s contains any other string... If you
want something more general, that is able to interpret a wide range of
possible commands, you will have to define a language, and write an
interpreter for that language. From the example you give, it seems you
would like your language to look like C++. This is most likely a bad
design choice. If you want an interpreter that understands the whole C++
language: forget it. You will never fit something this big into an
Arduino Uno.

Here is, for inspiration, a very simple interpreter I
wrote that understand the following commands:

mode &lt;pin&gt; &lt;mode&gt;: pinMode()
read &lt;pin&gt;: digitalRead()
aread &lt;pin&gt;: analogRead()
write &lt;pin&gt; &lt;value&gt;: digitalWrite()
awrite &lt;pin&gt; &lt;value&gt;: analogWrite()
echo &lt;value&gt;: set echo off (0) or on (1)


You can use it a basis for writing your custom interpreter. Otherwise
you can do a Web search for “Arduino interpreter”: you should be able to
find interpreters implementing a wide variety of languages, including
compact binary languages (Firmata), Forth (another one),
Lisp, Basic, and even a C-like language.