JCL Interview Questions and Answers (1 - 10)
1. What is JCL?
JCL stands for Job Control Language. It is the command language used in IBM mainframe operating systems, such as Multiple Virtual Storage (MVS). JCL is used to instruct the system on how to run batch jobs, specifying details like which programs to execute and the resources required.
2. What is the use of JCL?
JCL is used to identify the program to be executed, the inputs required, and the location of input/output data. It informs the operating system about these requirements through job control statements, enabling the system to process the job accordingly.
3. What are the types of job control statements?
The main types of job control statements in JCL are:
- JOB Statement: Specifies the start of a job and provides job-level parameters.
- EXEC Statement: Identifies the program or procedure to execute within the job.
- DD (Data Definition) Statement: Defines the datasets (files) to be used, including their attributes and disposition.
4. What does a JCL statement consist of?
A JCL statement typically consists of the following fields:
- Name Field (Optional): Provides a unique identifier for the statement.
- Operation Field: Specifies the action to be performed (e.g., JOB, EXEC, DD).
- Operand Field: Contains parameters or arguments related to the operation.
- Comments: Optional field for adding explanatory notes.
5. What are the differences between JES2 and JES3?
JES2 and JES3 are Job Entry Subsystems in IBM mainframes that manage job processing. Key differences include:
| Feature | JES2 | JES3 |
|---|---|---|
| Job Scheduling | Processes jobs individually. | Uses centralized control, analyzing all jobs before execution. |
| Job Execution Order | Jobs are queued and selected based on priority. | Jobs can be pre-sequenced for optimized execution. |
| Checkpoint Restart | Each job is handled independently, so restart must be managed manually. | Can dynamically control job restarts within the same execution cycle. |
| Dataset Allocation | Allocates datasets at execution time | Can pre-allocate datasets before execution begins. |
| System Configuration | More decentralized control, simpler to configure. | More centralized control, complex but efficient for large environments. |
JES2 is preferred for smaller, independent job processing, while JES3 is better for interdependent jobs requiring centralized control.
6. State the difference between positional parameter and keyword parameter in JCL.
| Parameter Type | Positional Parameter | Keyword Parameter |
|---|---|---|
| Definition | Parameters that must be specified in a fixed order. | Parameters identified by a keyword, allowing flexible ordering. |
| Specification | Must be placed in the correct position in the statement. | Can appear in any order since they are identified by names. |
| Example | | |
7. What is the difference between the JOBLIB and the STEPLIB statements?
Both JOBLIB and STEPLIB are used to specify libraries that the system should search to find programs to execute. However, they differ in scope:
- JOBLIB: Placed at the beginning of a job, it applies to all steps within that job, specifying the libraries to search for the programs to be executed.
- STEPLIB: Placed after an EXEC statement within a job step, it applies only to that specific step, indicating the libraries to search for the program to be executed in that step.
Additionally, JCLLIB is used to specify the procedure libraries to be searched for any procedures or INCLUDE members.
8. What happens if both JOBLIB and STEPLIB are specified?
When both JOBLIB and STEPLIB are specified, the STEPLIB takes precedence for the specific step in which it is defined. This means that for that particular step, the system will search the libraries listed in the STEPLIB before those in the JOBLIB. The JOBLIB applies to all steps that do not have a STEPLIB specified.
9. What is the order of searching of the libraries in a JCL?
When executing a program, the system searches for the program libraries in the following order:
- STEPLIB: If specified, the system searches the libraries defined in the STEPLIB for the current step.
- JOBLIB: If STEPLIB is not specified, the system searches the libraries defined in the JOBLIB for the entire job.
- System Libraries: If neither STEPLIB nor JOBLIB are specified, the system searches the default system libraries, such as
SYS1.LINKLIB.
10. How to restart a job from STEP3 if it abends in STEP3?
To restart a job from step3 after an abend (abnormal end),
add the RESTART parameter to the JOB statement as follows: RESTART=STEP3. This directs the system to begin execution from STEP3.