Ah, the pitfalls of quoting:

• You need to double any single quote within your SQL statement when using it within a string.
• You need an enclosing double quote pair around the whole CMD, that seems to be necessary for the CMD interpreter.

The following works for me when run against the SA16 demo database (note, I'm using the SQLANY16 environment variable and the 64-bit binary here, and are using the "2>" stderr redirection):

call xp_cmdshell('""%SQLANY16%\\Bin64\\dbisql" -q -c "UID=DBA;PWD=sql;Server=demo16;Database=demo" UNLOAD select * from Employees TO ''c:\\employee_data.csv'' 2> C:\\MyTest.log"', 'no_output');

The call creates a CSV file and an empty MyTest.log file.

In case your command contains an error (say, you scripted the table name in the SQL statement wrong), the via "2>" generated log file will contain an error message:

call xp_cmdshell('""%SQLANY16%\\Bin64\\dbisql" -q -c "UID=DBA;PWD=sql;Server=demo16;Database=demo" UNLOAD select * from EmployeesX TO ''c:\\employee_data.csv'' 2> C:\\MyTest.log"', 'no_output');

I still would think running the UNLOAD statement without xp_cmdshell would be easier. When using xp_cmdshell, it might also be easier to put the command in a batch file and make xp_cmdshell run that.

Here's a short code snippet of how to check for errors (and you would usually delete the log file afterwards):

begin
   declare strLog long varchar;
   declare bSuccess bit = 0;
   call xp_cmdshell('""%SQLANY16%\\Bin64\\dbisql" -q -c "UID=DBA;PWD=sql;Server=demo16;Database=demo" UNLOAD select * from Employees TO ''c:\\employee_data.csv'' 2> C:\\MyTest.log"', 'no_output');
   set strLog = cast(xp_read_file('C:\\MyTest.log') as long varchar);
   if strLog = '' then
      set bSuccess = 1;
   end if;
   select bSuccess, strLog;
end;

This returns "1, ''" in the above case but might return the following if you spelt the table wrong:

"0, 'Anweisung konnte nicht ausgef\x81hrt werden.\x0a\x0aTabelle 'EmployeesX' nicht gefunden\x0aSQLCODE=-141, ODBC 3-Status="42S02"\x0aZeile 1, Spalte 1\x0d\x0a'"