argument=${1:-"default if unset or empty"}

awk -v argument=${1:-"default if unset or empty"} 'BEGIN{print argument}'

#!/bin/ksh
date > now
awk -v v1="v1 -v" '
function setvars() {
	v1 = v1 ? v1 : "v1 unset, empty string, or 0"
	sub("^$", "v2 is unset or empty string", v2)
	printf("fn=%d, FILENAME=\"%s\"\nv1=\"%s\"\nv2=\"%s\"\n\n",
		fn, FILENAME, v1, v2)
}
BEGIN {	setvars()
}
FNR == 1 {
	fn++
	setvars()
	print fn, $0
	print ""
}
END {	print "in END clause"
	setvars()
}' v2='v2 before 1st file operand' now v1= v2= now v1=0 v2=0 now v1=abc \
   v2=def now v1='v1 after last file operand' v2=0

fn=0, FILENAME=""
v1="v1 -v"
v2="v2 is unset or empty string"

fn=1, FILENAME="now"
v1="v1 -v"
v2="v2 before 1st file operand"

1 Thu Apr 30 20:54:59 PDT 2015

fn=2, FILENAME="now"
v1="v1 unset, empty string, or 0"
v2="v2 is unset or empty string"

2 Thu Apr 30 20:54:59 PDT 2015

fn=3, FILENAME="now"
v1="v1 unset, empty string, or 0"
v2="0"

3 Thu Apr 30 20:54:59 PDT 2015

fn=4, FILENAME="now"
v1="abc"
v2="def"

4 Thu Apr 30 20:54:59 PDT 2015

in END clause
fn=4, FILENAME="now"
v1="v1 after last file operand"
v2="0"