QL> set term on echo on
SQL> @a.sql
SQL> spool desctab.sql
SQL> select 'desc '||table_name from user_tables order by table_name;

'DESC'||TABLE_NAME
-----------------------------------
desc COUNTRIES
desc DEPARTMENTS
desc EMP
desc EMPLOYEE
desc EMPLOYEES
desc JOBS
desc JOB_HISTORY
desc LOCATIONS
desc REGIONS
desc TESTER

10 rows selected.

SQL> spool off
SQL> set term on echo on
SQL> @desctab.sql
SQL> SQL> select 'desc '||table_name from user_tables order by table_name;
SP2-0734: unknown command beginning "SQL> selec..." - rest of line ignored.
SQL> 
SQL> 'DESC'||TABLE_NAME
SP2-0734: unknown command beginning "'DESC'||TA..." - rest of line ignored.
SQL> -----------------------------------
SQL> desc COUNTRIES
 Name					   Null?    Type
 ----------------------------------------- -------- ----------------------------
 COUNTRY_ID				   NOT NULL CHAR(2)
 COUNTRY_NAME					    VARCHAR2(40)
 REGION_ID					    NUMBER

SQL> desc DEPARTMENTS
 Name					   Null?    Type
 ----------------------------------------- -------- ----------------------------
 DEPARTMENT_ID				   NOT NULL NUMBER(4)
 DEPARTMENT_NAME			   NOT NULL VARCHAR2(30)
 MANAGER_ID					    NUMBER(6)
 LOCATION_ID					    NUMBER(4)

SQL> desc EMP
 Name					   Null?    Type
 ----------------------------------------- -------- ----------------------------
 EMPNO					   NOT NULL NUMBER(4)
 ENAME						    VARCHAR2(10)
 JOB						    VARCHAR2(9)
 MGR						    NUMBER(4)
 HIREDATE					    DATE
 SAL						    NUMBER(7,2)
 COMM						    NUMBER(7,2)
 DEPTNO 					    NUMBER(2)

SQL> desc EMPLOYEE
 Name					   Null?    Type
 ----------------------------------------- -------- ----------------------------
 EMP_ID 					    VARCHAR2(10)
 COMPANY_NAME					    VARCHAR2(50)

SQL> desc EMPLOYEES
 Name					   Null?    Type
 ----------------------------------------- -------- ----------------------------
 EMPLOYEE_ID				   NOT NULL NUMBER(6)
 FIRST_NAME					    VARCHAR2(20)
 LAST_NAME				   NOT NULL VARCHAR2(25)
 EMAIL					   NOT NULL VARCHAR2(25)
 PHONE_NUMBER					    VARCHAR2(20)
 HIRE_DATE				   NOT NULL DATE
 JOB_ID 				   NOT NULL VARCHAR2(10)
 SALARY 					    NUMBER(8,2)
 COMMISSION_PCT 				    NUMBER(2,2)
 MANAGER_ID					    NUMBER(6)
 DEPARTMENT_ID					    NUMBER(4)

SQL> desc JOBS
 Name					   Null?    Type
 ----------------------------------------- -------- ----------------------------
 JOB_ID 				   NOT NULL VARCHAR2(10)
 JOB_TITLE				   NOT NULL VARCHAR2(35)
 MIN_SALARY					    NUMBER(6)
 MAX_SALARY					    NUMBER(6)

SQL> desc JOB_HISTORY
 Name					   Null?    Type
 ----------------------------------------- -------- ----------------------------
 EMPLOYEE_ID				   NOT NULL NUMBER(6)
 START_DATE				   NOT NULL DATE
 END_DATE				   NOT NULL DATE
 JOB_ID 				   NOT NULL VARCHAR2(10)
 DEPARTMENT_ID					    NUMBER(4)

SQL> desc LOCATIONS
 Name					   Null?    Type
 ----------------------------------------- -------- ----------------------------
 LOCATION_ID				   NOT NULL NUMBER(4)
 STREET_ADDRESS 				    VARCHAR2(40)
 POSTAL_CODE					    VARCHAR2(12)
 CITY					   NOT NULL VARCHAR2(30)
 STATE_PROVINCE 				    VARCHAR2(25)
 COUNTRY_ID					    CHAR(2)

SQL> desc REGIONS
 Name					   Null?    Type
 ----------------------------------------- -------- ----------------------------
 REGION_ID				   NOT NULL NUMBER
 REGION_NAME					    VARCHAR2(25)

SQL> desc TESTER
 Name					   Null?    Type
 ----------------------------------------- -------- ----------------------------
 ID						    NUMBER

SQL> 
SQL> 10 rows selected.
SQL> 
SQL> SQL> spool off
SP2-0734: unknown command beginning "SQL> spool..." - rest of line ignored.
SQL>