/IF [expression]
 /THEN command
 /ELSE command

Evaluates expression and stores true if expression evaluates to non-zero integer or non-unset string, false if it evaluates to unset value, integer zero or unset string.
Then and else commands can be called any number of times - they will use the same stored value.
For details on expression syntax see help for eval command.
Example:
 /if fifo_name
 /then load fifo
 /else echo "fifo_name not set, fifo module not loaded"