Truncate a string inside an Excel cell if the char count is greater than than a value

Question

In Excel, for each row of the sheet I have various length strings (a1,a2,a3...). In cell B2 I have =Length(A1) to count the chars inside the string.

I need a formula/function that can truncate all strings in column A which have a character count > 20 Something like:

$string = THIS IS A LONG STRING I WANT TO TRUNCATE IF EXCEEDS 20 CHARS;
if ($string > 20)
{
   COUNT 20 CHARS FROM THE BEGINNING OF STRING AND CUT THE REST
} 
else 
{
 skip 
}