If statement to compare strings

#!/bin/sh


#  IF  is not the tool for string compare
#  AWK for split a luxus, make once the loop
#  the PIPE delimiter hard to correctly controll, be save, change it

#  SED is also a good way to do that
#  i never write ksh-scripts, imo your if statement is buggy
#  in /bin/sh: [ -z "$aaa" -o "$bbb" = ERROR ] 

for what in `cut -d'|' -f2,4 data | tr '|' '!'`
do 
echo $what
case $what in !) echo both fields ARE EMPTY
;;              !?*) echo field2 IS EMPTY
;;              ?*!) echo field4 IS EMPTY
;;                *) echo both fields are NOT EMPTY
;; esac
done

field2=EMPTY
field4=EMPTY
case $what in !) echo both fields ARE EMPTY
;;     !*ERROR*) echo ERROR FOUND field2 is EMPTY
;;  ?*!*ERROR*) echo ERROR FOUND field2 is NOT empty
                      field2=`echo $what|sed 's/!.*//'
;;              !?*) echo field2 IS EMPTY
                      field4=`echo $what|sed 's/!//'
;;              ?*!) echo field4 IS EMPTY
                      field2=`echo $what|sed 's/!//'
;;                *) echo both fields are NOT EMPTY
                      field2=`echo $what|sed 's/!.*//'
                      field4=`echo $what|sed 's/.*!//'
;; esac
#now check the fields, and prefer case to if

# nota: cut and case are very fast
# sed will work on a short string only if it has to work on it :)