Contents

1 About this module

• Prerequisites:
• Objectives: This module introduces JCL.

2 What is JCL?

JCL is Job Control Language, it is similar to BASH in Linux or PowerShell in Windows, but not exactly the same. First, JCL is not interactive, there is no “console mode” to interact with a user. Interaction is done using TSO ("Time Share Option") in a mainframe environment. Second, JCL specifies exactly how a job is done in details, whereas BASH or PowerShell is merely a language to specify what programs needs to run, but the operating system figures out what resources are needed and manages the resources without user specification.

It is crucial to understand the background of JCL. JCL is a language based on punched cards. A punch card has 80 columns to store what we now call 80 characters. Furthermore, computers (even “mainframe computers”) were significantly simpler than what we have today.

3 JCL line format

Because computers did not have a lot of resources for parsing, JCL was invented to specify parts based on position on a line.

All JCL statements start with two slashes (in column 1 and 2), like

//

This is important because the content of a job is interleaved with JCL statements. As a result, this JCL prefix is crucial to inform a mainframe computer whether a line (or punch card) is a part of the JCL or a part of the data.

If a line is to be treated as comment, then column 3 must be an asterisk *. Otherwise the rest of the line is interpreted as a JCL construct.

From column 3 (assuming it is not a *) to column 10 is a name. It can be the name of various constructs as we will explore in later sections. Note that this is contrary to most modern programming languages where the kind of construct being named precedes the name.

Starting exactly on column 12 is the operation. An operation specifier can terminate by a space, but it cannot extend past column 15. This means an operation can only be up to 4 characters long, but it can be shorter and be terminated by a space.

After the operation specifier, operands (additional information needed to specify the operation) can use up to column 71.

If a line is not long enough, then it can be continued under certain conditions.

• if a parameter or comment is continued, then the next line starts with two slashes and one space in column 3.
• if a quoted content is continued, then the next line start with two slashes and the rest of the content start in column 16.

4 JCL structure

• Job statement: identifies the job for references, also include additional parameters.
  • job name: this is a name from 1 to 8 characters
  • JOB: this is a keyword to identify this is a job card
  • positional parameters: this is a list of parameters where the meaning is implied by ordering, comma separated
    • (account): the person or group to have CPU time billed
    • ’programming name’: this is the name of the programmer
  • keyword parameters: this is a list of parameters where the meaning is explicitly stated, comma separated
    • CLASS=alphanumeric character: scheduling of the job, A means immediately
    • MSGCLASS=alphanumeric character: output destination
    • MSGLEVEL=(ST,MSG):
      • ST: statements written to output log: 0 for job statements only, 1 for parameters expanded, 2 for input JCL only
      • MSG: messages written to output log: 0 for allocation and termination messages for abnormal completion, 1 for allocation and termination for all completion
• EXEC statement: specifies the action of the job
• DD statement: specifies the dataset (file) resources for this job