CLIST Basics: Write, Run, Debug CLISTs from ISPF

CLIST (Command List) is a mainframe scripting language for automating TSO/ISPF operations. CLISTs allow you to create reusable procedures that execute sequences of commands, making repetitive tasks more efficient. Understanding CLIST basics helps you automate operations and create custom commands. This tutorial covers writing CLISTs, CLIST syntax, running CLISTs from ISPF, debugging techniques, and best practices.

CLISTs are powerful tools for mainframe automation. They can execute TSO commands, perform logic operations, handle user input, and call other CLISTs. Learning CLIST basics enables you to create automation scripts that save time and reduce errors. This tutorial provides practical guidance for CLIST development.

Understanding CLIST

CLIST is a scripting language for TSO/ISPF automation.

What is CLIST?

CLIST (Command List) is:

• A scripting language for TSO/ISPF
• Stored as members in PDS libraries
• Executed like commands
• Used for automating operations
• Supports variables, logic, and control structures

CLIST Uses

CLISTs are used for:

• Automating repetitive tasks
• Creating custom commands
• Building interactive procedures
• Simplifying complex operations
• Standardizing processes

Writing CLISTs

CLISTs are written as text members in PDS libraries.

Creating a CLIST Member

To create a CLIST:

• Use ISPF Edit to create a new member
• Store in a PDS library (typically CLIST library)
• Write CLIST statements
• Save the member
• CLIST is ready to execute

Basic CLIST Structure

A basic CLIST structure:

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PROC 0
  /* CLIST comments */
  WRITE Hello from CLIST
  EXIT

This shows a simple CLIST with PROC statement, comment, WRITE command, and EXIT.

PROC Statement

The PROC statement defines the CLIST:

• PROC 0: No parameters
• PROC 1 PARM1: One parameter
• PROC 2 PARM1 PARM2: Two parameters
• Parameters are accessed as &PARM1, &PARM2, etc.

Example with parameters:

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PROC 1 DSN
  WRITE Dataset name is &DSN
  LISTCAT ENTRIES(&DSN)
  EXIT

CLIST Syntax

Understanding CLIST syntax is essential.

Variables

Variables in CLIST:

• Referenced with & prefix (e.g., &VAR)
• Assigned with SET command
• Can contain strings or numbers
• Used in commands and expressions

Example variable usage:

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PROC 0
  SET &NAME = USERID
  SET &DSN = &NAME..SOURCE.COBOL
  WRITE Dataset is &DSN
  EXIT

Control Structures

CLIST supports control structures:

• IF/THEN/ELSE: Conditional execution
• DO/END: Block statements
• DO WHILE: Loops
• DO UNTIL: Loops

Example with IF/THEN:

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PROC 1 DSN
  IF &DSN = '' THEN +
    DO
      WRITE Error: Dataset name required
      EXIT CODE(8)
    END
  ELSE +
    LISTCAT ENTRIES(&DSN)
  EXIT

Comments

Comments in CLIST:

• Use /* */ for comments
• Comments can span multiple lines
• Useful for documentation

Running CLISTs

CLISTs are executed using the EXEC command.

Basic Execution

To run a CLIST:

• Use EXEC command followed by CLIST name
• Specify PDS and member name
• Pass parameters if needed
• CLIST executes and returns

Example execution:

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EXEC 'USERID.CLIST(MYCLIST)'
EXEC 'USERID.CLIST(MYCLIST)' 'PARM1' 'PARM2'

Running from ISPF

To run from ISPF:

• Use Option 6 (Command) for TSO commands
• Type EXEC command with CLIST name
• Or run from ISPF panels
• Or call from other CLISTs

Passing Parameters

To pass parameters:

• List parameters after CLIST name
• Parameters are positional
• Access in CLIST as &PARM1, &PARM2, etc.
• Quote parameters with spaces

Debugging CLISTs

Debugging helps identify and fix CLIST issues.

TRACE Command

TRACE shows execution flow:

• Use TRACE ON to enable tracing
• Shows each statement as it executes
• Helps identify execution path
• Use TRACE OFF to disable

Example with TRACE:

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PROC 0
  TRACE ON
  SET &VAR = TEST
  WRITE Variable is &VAR
  TRACE OFF
  EXIT

LIST Command

LIST displays variable values:

• Use LIST to see variable contents
• Shows all variables or specific ones
• Useful for debugging

WRITE for Debugging

Use WRITE for diagnostic output:

• Add WRITE statements to show values
• Display intermediate results
• Track execution flow
• Remove or comment out when done

Error Handling

For error handling:

• Check return codes from commands
• Use IF statements to test conditions
• Provide meaningful error messages
• Use EXIT CODE for error returns

Common CLIST Patterns

Common patterns for CLIST development.

Parameter Validation

Validate parameters:

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PROC 1 DSN
  IF &DSN = '' THEN +
    DO
      WRITE Error: Dataset name required
      EXIT CODE(8)
    END
  /* Continue processing */
  EXIT

Looping Through Members

Process multiple items:

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PROC 0
  SET &I = 1
  DO WHILE &I <= 10
    WRITE Processing item &I
    SET &I = &I + 1
  END
  EXIT

Calling Other CLISTs

Modular CLIST design:

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PROC 1 DSN
  EXEC 'USERID.CLIST(VALIDATE)' &DSN
  IF &LASTCC > 0 THEN EXIT CODE(8)
  EXEC 'USERID.CLIST(PROCESS)' &DSN
  EXIT

Best Practices

Following best practices improves CLIST quality:

• Use Meaningful Names: Use descriptive CLIST and variable names
• Validate Input: Always validate parameters and input
• Add Comments: Document CLIST purpose and logic
• Handle Errors: Check return codes and handle errors
• Test Incrementally: Test CLISTs as you develop them
• Use Modular Design: Break complex CLISTs into smaller ones
• Provide Messages: Give users feedback on progress
• Document Usage: Document how to use the CLIST

Explain Like I'm 5: CLIST

Think of CLIST like a recipe for the computer:

• CLIST is like a recipe that tells the computer what to do step by step. Instead of telling the computer each command one at a time, you write down all the commands in a recipe (CLIST), and the computer follows the recipe automatically. It's like having a cooking recipe that tells you all the steps to make a cake!
• Writing a CLIST is like writing down your recipe. You write the steps (commands) the computer should follow, in the right order. You can include instructions like "if this happens, do that" (IF/THEN) and "do this 10 times" (loops). It's like writing a detailed recipe with all the steps!
• Running a CLIST is like following the recipe. You tell the computer "follow this recipe" (EXEC command), and it does all the steps automatically. You don't have to tell it each step - it reads the recipe and does everything. It's like giving someone your recipe and they make the cake for you!
• Debugging is like checking if your recipe works. If something goes wrong, you use special tools (like TRACE) to see what step the computer is on and what it's doing. You can add notes (WRITE statements) to see what's happening. It's like watching someone follow your recipe to see where they might make a mistake!

So CLIST is like writing a recipe for the computer that tells it exactly what to do, and the computer follows the recipe automatically!

Practice Exercises

Complete these exercises to reinforce your CLIST skills:

Exercise 1: Write Simple CLIST

Practice basics: write a simple CLIST that displays a message, understand CLIST structure, save and test the CLIST, and learn basic CLIST writing. Master basic CLIST writing.

Exercise 2: CLIST with Parameters

Practice parameters: write a CLIST that accepts parameters, use parameters in the CLIST, test with different parameters, and learn parameter handling. Master CLIST parameters.

Exercise 3: CLIST with Logic

Practice logic: write a CLIST with IF/THEN logic, use control structures, test different conditions, and learn CLIST logic. Master CLIST control structures.

Exercise 4: Run and Test CLIST

Practice execution: run CLISTs from ISPF, pass parameters, verify execution, and learn CLIST execution. Master CLIST execution.

Exercise 5: Debug CLIST

Practice debugging: use TRACE to debug a CLIST, use LIST to view variables, add WRITE statements, fix issues, and learn CLIST debugging. Master CLIST debugging.

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