checking if a variable value is null on bash script - sql

Question

i have this script that retrieves a parameter (a number) from a SQL query and assigns it to a variable.
there are two options - either the SQL query finds a value and then the script preforms
echo "the billcycle number is $v_bc", or it doesnt find a value and it suppose to
echo "no billcycle parameter found".
im having a problem with the if condition.
this is what i came up with:
#!/bin/bash
v_bc=`sqlplus -s /#bscsprod <<EOF
set pagesize 0
select billcycle from bc_run
where billcycle not in (50,16)
and control_group_ind is null
and billseqno=6043;
EOF`
if [ -z "$v_bc" ]; then echo no billcycle parameter found
else echo "the billcycle parameter is $v_bc"
fi
when billseqno=6043, then it means that v_bc=25, and when i run the script, the result is:
"the billcycle parameter is 25". which is what i ment it to do.
when i set billseqno=6042, according to the above SQL query, v_bc will get no value, therefore what i want it to do is echo "no billcycle parameter found".
instead i get
"the billcycle parameter is
no rows were selected".
any suggestions ?
thanks very much
Assaf.

Answer 1

Your code is correctly checking for an empty value -- v_bc is just not empty, even with -s.
You may either:
parse the output of sqplus when no rows are returned, so add this:
if [[ "$v_bc" == "no rows were selected" ]]; then v_bc=""; fi
This uses the bash [[ ]] command with "==" for pattern matching so we don't need to worry about leading/trailing whitespace. This is not as robust as dogbane's SET FEEDBACK OFF since it's entirely possible for "no rows were selected" to be valid data.
write a better query which always returns data, like this:
http://asktom.oracle.com/pls/asktom/f?p=100:11:0::::P11_QUESTION_ID:594023455752#followup-221571500346463844
The trick being to reformulate your query and use select/union with a fallback query that conditionally provides output when your query is empty:
with data as
(select billcycle from bc_run where [...])
select * from data
union all
select 'NA', null from dual where not exists (select null from data);
(see also What is the dual table in Oracle? )

Answer 2

Try turning feedback off in sqlplus so that you don't get any output if no rows are selected:
v_bc=`sqlplus -s /#bscsprod <<EOF
SET FEEDBACK OFF
set pagesize 0
select billcycle from bc_run
where billcycle not in (50,16)
and control_group_ind is null
and billseqno=6043;
EOF`

Answer 3

Try something like
if [ "${v_bc:-SHOULDNTHAPPEN}" = "SHOULDNTHAPPEN" ]; then
echo no billcycle parameter found
else......

Answer 4

The idiomatic way to assign a value to a variable if that variable is empty is with an = in a parameter expansion. In other words, this:
: ${v_bc:=no billcycle paramter found}
is equivalent to:
test -z "$v_bc" && v_bc='no billcycle paramter found'
In your case, it would also be easy to do:
echo ${v_bc:-no billcycle parameter found}
but the question asked in the title is not really the problem in your case. (Since the problem is not that v_bc is the empty string, but rather that it is a value you do not expect.)