All you need to know is that it is the first item on the command line. If a comparison is nothing except a variable name, we are checking to see whether or not that variable exists at all. That means it keeps going from top to bottom and back again until some condition is false . That is, it keeps going as long as the condition is true . Text variables are true if they exist, numeric variables are true if they are not zero.
Comparisons are true if they compare well.
You can also use two plusses to increase the variable's value by one. In general, you want your comparisons to be as robust as possible. If there are four values in the array, the loop will happen four times. Each time through the loop, it resets the value of the scalar variable to next value in the array.