What is JCL (Job Control Language)?

Definition: JCL (Job Control Language)

Job Control Language (JCL) is a scripting language used on IBM mainframe operating systems to instruct the system on how to execute batch jobs or start subsystems. JCL is crucial for defining the specifics of a job, including input and output data, resources needed, and the sequence of operations to be performed.

Overview of JCL

Job Control Language (JCL) is an essential component in the operation of IBM mainframes, particularly in environments running z/OS. It provides the framework for submitting jobs, controlling job execution, and managing output. Given its importance, a thorough understanding of JCL is critical for IT professionals working in mainframe environments.

Structure of JCL

JCL scripts are composed of three main types of statements: Job, Exec, and DD (Data Definition). Each statement serves a specific purpose:

• JOB Statement: This statement marks the beginning of a job and specifies job-related parameters such as job name, accounting information, and scheduling priorities. It sets the stage for the job’s execution.
• EXEC Statement: This indicates the execution of a specific program or procedure within the job. It defines what action should be taken.
• DD Statement: This defines the data sets (files) to be used in the job, specifying where data is located, how it should be accessed, and what resources are required.

Key Concepts in JCL

1. Jobs and Job Steps: A job in JCL is composed of one or more job steps, each of which executes a program or a series of commands. Job steps are executed sequentially unless specified otherwise.
2. Data Sets: Data sets are the files used by job steps for input and output. They can reside on various storage media and must be defined with DD statements in JCL.
3. Procedures: These are pre-written sets of JCL statements that can be invoked by a job step. Procedures simplify job management by allowing reuse of common job steps.
4. Parameters: JCL allows the use of parameters to control job execution. These can include runtime options, resource requirements, and conditional execution paths.

Benefits of JCL

JCL provides numerous benefits in a mainframe environment:

• Efficiency: JCL scripts automate repetitive tasks, reducing the need for manual intervention and increasing operational efficiency.
• Consistency: By standardizing job definitions, JCL ensures consistent execution of batch processes.
• Control: JCL offers granular control over job execution, allowing precise specification of resources, priorities, and dependencies.
• Scalability: JCL can manage complex job streams involving multiple steps and data sets, making it suitable for large-scale data processing tasks.

Uses of JCL

JCL is widely used in various scenarios on IBM mainframes:

• Batch Processing: It is primarily used for batch processing tasks, where large volumes of data are processed in scheduled jobs.
• Data Management: JCL scripts manage data input and output, ensuring that the right data sets are accessed and manipulated.
• System Operations: JCL is employed for system maintenance tasks, such as backups, restores, and system updates.
• Application Deployment: It is used to deploy and execute application programs in a controlled environment.

Features of JCL

Several features make JCL a powerful tool for mainframe operations:

• Conditional Processing: JCL supports conditional execution of job steps based on the success or failure of previous steps, enabling complex workflows.
• Symbolic Parameters: These allow for the use of variables within JCL scripts, making them more flexible and reusable.
• Dataset Management: JCL provides detailed control over data set allocation, disposition, and cataloging, essential for managing large volumes of data.
• Error Handling: Comprehensive error handling capabilities help in diagnosing and recovering from job failures.

Writing a Basic JCL Script

Writing a JCL script involves understanding its syntax and the proper use of statements. Here’s an example of a simple JCL script:

//JOBNAME  JOB  (ACCT#),'PROGRAMMER NAME',CLASS=A,MSGCLASS=A<br>//STEP1    EXEC PGM=MYPROGRAM<br>//INPUT    DD  DSN=MY.INPUT.DATASET,DISP=SHR<br>//OUTPUT   DD  DSN=MY.OUTPUT.DATASET,DISP=(NEW,CATLG,DELETE),<br>//             UNIT=SYSDA,SPACE=(CYL,(5,5)),DCB=(RECFM=FB,LRECL=80,BLKSIZE=800)<br>//SYSOUT   DD  SYSOUT=*<br>//SYSIN    DD  *<br>   Parameters for MYPROGRAM<br>/*<br>

Explanation of the Script

• JOB Statement: Defines the job, specifying the accounting information and the message class.
• EXEC Statement: Calls the program MYPROGRAM to be executed.
• DD Statements:
  • INPUT: Specifies the input data set, with DISP=SHR indicating it is shared.
  • OUTPUT: Defines the output data set, with parameters for allocation and data control block (DCB).
  • SYSOUT: Directs system output messages to the specified class.
  • SYSIN: Provides in-stream data or parameters to the program.

Advanced JCL Concepts

Procedures and Symbolic Parameters

Using procedures and symbolic parameters enhances JCL’s flexibility. Procedures allow for reusable code segments, and symbolic parameters enable dynamic parameterization.

Example of a Procedure

//MYPROC  PROC<br>//STEP1   EXEC PGM=&PGMNAME<br>//INPUT   DD  DSN=&DSNIN,DISP=SHR<br>//OUTPUT  DD  DSN=&DSNOUT,DISP=(NEW,CATLG,DELETE),<br>//             UNIT=SYSDA,SPACE=(CYL,(5,5)),DCB=(RECFM=FB,LRECL=80,BLKSIZE=800)<br>//SYSOUT  DD  SYSOUT=*<br>//SYSIN   DD  *<br>   &PARAMS<br>/*<br>//         PEND<br>

To use this procedure:

//JOBNAME  JOB  (ACCT#),'PROGRAMMER NAME',CLASS=A,MSGCLASS=A<br>//STEP1    EXEC MYPROC,PGMNAME=MYPROGRAM,DSNIN=MY.INPUT.DATASET,<br>//         DSNOUT=MY.OUTPUT.DATASET,PARAMS='Parameters for MYPROGRAM'<br>

Conditional Execution

JCL supports conditional execution through the use of the COND parameter. This parameter allows job steps to execute based on the return codes of previous steps.

Example of Conditional Execution

//JOBNAME  JOB  (ACCT#),'PROGRAMMER NAME',CLASS=A,MSGCLASS=A<br>//STEP1    EXEC PGM=MYPROGRAM<br>//STEP2    EXEC PGM=ANOTHERPGM,COND=(4,LT,STEP1)<br>//STEP3    EXEC PGM=FINALPGM,COND=(0,EQ,STEP2)<br>

In this script:

• STEP2 will execute only if STEP1 returns a code less than 4.
• STEP3 will execute only if STEP2 returns a code of 0.

Frequently Asked Questions Related to JCL (Job Control Language)