while (<condition>) <action>
while (<condition>) [{ <action> }]

The WHILE loop is a sort of hybrid between the FOR loop and the IF control statement. It allows for repetitive action, as with FOR, but the loop iterates (performs the action) only if a specific condition is met, as with IF.

The “condition” portion may contain any comparison or assignment allowed in an IF statement.

To display a warning message 3 times:

    @ xx = 3
    while ( xx > 0 ) {
       echo WARNING!  This ship will self destruct in $xx seconds!
       @ xx--
    }

A infinite loop that behaves like the Unix 'yes' command:

    while ( 1 ) echo yes

UNTIL is the exact opposite of WHILE. It is essentially the same applying the negation operator (!) to the entire WHILE condition.

WHILE has all of the capabilities of FOR, only in a different syntax. The distinction between the two is not great enough to warrant a recommendation of one over the other. If anything, FOR tends to be more concise than WHILE; however, this is not always the case.