if
Condition clause.
<expr1> <operator> <expr2> <cmd1> [else <cmd1>]
expr1
A string where you usually use your own text, macros, and values of variables (like $volume).
operator
==, =, !=, <>, >, <, >=, <=, isin, !isin. Where 'isin' means "expr1 is a substring of expr2" and '!isin' is a negation of this. The others are standard mathematical comparison.
expr2
See expr1
cmd1
Will get executed if the binary operation succeeds.
cmd2
Will get executed if the binary operation fails.