How to evaluate or process if statements in data? - awk

Question

Background
I wrote a bash script that pulls simple user functions from a PostgreSQL database, using awk converts pgplsql commands to SQL (like PERFORM function() to SELECT function(), removes comments --.*, etc.), stores the SQL commands to a file (file.sql) and reads and executes them in the database:
$ psql ... -f file.sql db
The functions are simple, mostly just calling other user defined functions. But how to "evaluate" or process an IF statement?:
IF $1 = 'customer1' THEN -- THESE $1 MEANS ARGUMENT TO PGPL/SQL FUNCTION
PERFORM subfunction1($1); -- THAT THIS IF STATEMENT IS IN:
ELSE -- SELECT function('customer1');
PERFORM subfunction2($1); -- $1 = 'customer1'
END IF;
Tl;dr:
IFs and such are not SQL so they should be pre-evaluated using awk. It's safe to assume that above is already processed into one record with comments removed:
IF $1 = 'customer1' THEN PERFORM subfunction1($1); ELSE PERFORM subfunction2($1); END IF;
After "evaluating" above should be replaced with:
SELECT subfunction1('customer1');
if the awk to evaluate it was called:
$ awk -v arg1="customer1' -f program.awk file.sql
or if arg1 is anything else, for example for customer2:
SELECT subfunction2('customer2');
Edit
expr popped into my mind first thing when I woke up:
$ awk -v arg="'customer1'" '
{
gsub(/\$1/,arg) # replace func arg with string
n=split($0,a,"(IF|THEN|ELSE|ELSE?IF|END IF;)",seps) # seps to get ready for SQL CASE
if(seps[1]=="IF") {
# here should be while for ELSEIF
c="expr " a[2]; c|getline r; close(c) # use expr to solve
switch (r) { # expr has 4 return values
case "1": # match
print a[3]
break
case "0": # no match
print a[4]
break
default: # (*) see below
print r
exit # TODO
} } }' file.sql
(*) expr outputs 0,1,2 or 3:
$ expr 1 = 1
1
$ expr 1 = 2
0
However, if you omit spaces:
$ expr 1=1
1=1

Answer 1

Without writing a full language parser, if you're looking for something cheap and cheerful then this might be a decent starting point:
$ cat tst.awk
{ gsub(/\$1/,"\047"arg1"\047") }
match($0,/^IF\s+(\S+)\s+(\S+)\s+(\S+)\s+THEN\s+(\S+)\s+(\S+)\s+ELSE\s+(\S+)\s+(\S+)\s+END\s+IF/,a) {
lhs = a[1]
op = a[2]
rhs = a[3]
trueAct = (a[4] == "PERFORM" ? "SELECT" : a[4]) FS a[5]
falseAct = (a[6] == "PERFORM" ? "SELECT" : a[6]) FS a[7]
if (op == "=") {
print (lhs == rhs ? trueAct : falseAct)
}
}
$ awk -v arg1='customer1' -f tst.awk file
SELECT subfunction1('customer1');
$ awk -v arg1='bob' -f tst.awk file
SELECT subfunction2('bob');
The above uses GNU awk for the 3rd arg to match(). Hopefully it's easy enough to understand that you can massage as needed to handle other constructs or other variations of this construct.