How to check if a SQL*Plus variable has been defined or not

Refer below thread...

https://forums.oracle.com/forums/thread.jspa?threadID=976322

http://vbegun.blogspot.in/2008/04/on-sqlplus-defines.html (referred in above thread)

use the following:

variable variablex varchar2(30)

begin
  select to_char(sysdate) into :variablex from dual;
end;
/

print variablex

See if this is what you are after.

It uses sqplus variables not DEFINE.  I don't know that you can alter a sqpllus defined variable in a PL/SQL block.

var variablex char(1);

begin
	if :variablex is null then
		:variablex := 'A';
	end if;
	dbms_output.put_line('Got: ' || :variablex);
end;
/


exec :variablex := 'X';

begin
	if :variablex is null then
		:variablex := 'A';
	end if;
	dbms_output.put_line('Got: ' || :variablex);
end;
/

Select allOpen in new window

Ok, here is the full picture of what I need.

S1[.sql]
define user_a = 'A'
define user_b = 'B'
define user_c = 'C'

S2[.sql]
@script1 &user_a
@script1 &user_b
@script1 &user_c

When running above scripts batch mode
SQL >@S1
SQL >@S2

[ there should be no error as all variable are defined in S1 ]

Now, no change to S1
But S2 has extra line:

@script1 &user_a
@script1 &user_b
@script1 &user_c
@script1 &user_d


When running above scripts batch mode
SQL >@S1
SQL >@S2

[S2 will prompt the value for user_d as it is not defined in S1]

I wish to add Addition code in S2 to avoid such prompt.

You may ask why I do not define user_d in S1.sql,  this is the requirement that user wish to have the flixbility to define or not define user_d in S1.sql,  

In case the user_d does define in S1 the "@script1 &user_d" will be executed by use the value defined in S1, if not defined in S1, user_d will be set to a defualt value in S2 (from the additional code).

You solution should be like:

S2[.sql]
-----
Extra code to set user_d to a default value if it is not defined in the SQL session
-----
@script1 &user_a
@script1 &user_b
@script1 &user_c
@script1 &user_d

As none of the feedback give me a better solution that I have composed. I need to close this thread.