Re: Variable checking routine...

Catherine Hampton wrote,

| I want to write a recipe which checks to see if a variable is set, and if
| it isn't, sets it to a default value.  Will this recipe work?
| 
| :0:
| * ! VARIABLE ?? .*
| VARIABLE=defaultvalue
| 

Rik Kabel and W. Wesley Groleau have pretty much answered this; I just want
to gather what they've said into one place.

First, it is critical whether Catherine means "set" literally (and a null
value is acceptable if the variable is set but null) or she really means
"non-null" (if the variable is set but null, that's unacceptable and the
default value should be used).

As Wesley said, it can be done without a recipe:

VARIABLE=${VARIABLE-defaultvalue} # if set but null is acceptable
VARIABLE=${VARIABLE:-defaultvalue} # if set but null is unacceptable

If you really want to put it into a recipe,

  # if set but null is acceptable
  :0
  * $ ${VARIABLE+!}
  { VARIABLE=defaultvalue }

  # if set but null is unacceptable
  :0
  * $ ${VARIABLE:+!}
  { VARIABLE=defaultvalue }

  # another way if set but null is unacceptable
  :0
  * VARIABLE ?? ^^^^
  { VARIABLE=defaultvalue }