In this article, we will explore the fundamental syntax and structure of CLP (Control Language Programming) on IBM i (formerly AS400). Understanding the syntax is critical to writing CL programs that automate tasks, manage jobs, and interact with the system. This guide will walk you through the essential elements of CLP, including the structure of a CL program, the rules of syntax, and the core components you’ll use when writing your own CL programs.
Basic Structure of a CL Program
A CL program is a sequence of commands executed by the IBM i operating system to perform specific tasks. These tasks can range from managing files and data to submitting jobs and sending messages.
Here’s a simple overview of a CL program structure:
PGM /* Start of the program */ /* Declarations and initializations */ /* CL commands */ ENDPGM /* End of the program */
The two most important parts of every CL program are:
PGM: Marks the beginning of the program.ENDPGM: Marks the end of the program.
Between these two statements, you will place your CL commands, variable declarations, and logic flow.
Key Components of CLP Syntax
Commands: CL programs are driven by commands, which perform specific actions. Commands like SNDPGMMSG (Display),
SNDMSG(Send Message), andSBMJOB(Submit Job) are the building blocks of a CL program.For example, here’s a simple command that displays a message on the screen:
SNDPGMMSG MSG('Hello World!')Parameters: Many commands take one or more parameters that provide additional instructions. Parameters follow the command and are separated by spaces.
For example, in the
DSPLYcommand, the message'Hello, World!'is the parameter being passed to theDSPLYcommand:SNDPGMMSG MSG('Hello World!')Some commands may have multiple parameters, which are often separated by commas:
SNDMSG MSG('Processing completed') TOUSR(USER1)Variables: Variables are used in CL programs to store data for later use, such as user inputs, system values, or intermediate results.
You can declare variables using the
DCL(Declare) command. Variables can hold different types of data, including characters, numbers, and dates.Here’s an example of declaring a character variable:
DCL VAR(&USERNAME) TYPE(*CHAR) LEN(10)Comments: Comments in CL programs are indicated by double slashes
/* ... */. They can be placed on any line to describe what the program is doing. Comments are ignored by the system but are useful for developers to understand the code.Here’s an example of a comment:
SNDPGMMSG MSG('Starting the program') /* Display start message */
CLP Command Syntax
Let’s break down the structure of a CL command. Each CL command generally follows this pattern:
COMMAND_NAME [parameters]
- COMMAND_NAME: The name of the command (e.g.,
DSPLY,CRTCLPGM,SBMJOB). - parameters: The values or options passed to the command to define its behavior.
Commands in CL programs are written one per line, and their parameters can include strings, variables, or other system values.
Example 1: The SNMSGPGM Command
The DSPLY command is used to display a message on the screen. Here’s the basic syntax:
SNGMSGPGM MSG('Your message here')
- message: This parameter is the text you want to display.
Example:
SNDMSGPGM MSG('Hello, welcome to the system!')
This will display the message: "Hello, welcome to the system!".
Example 2: The SBMJOB Command
The SBMJOB command is used to submit a job to the job queue for batch processing. This command can have several parameters to define the job name, user, job queue, etc.
SBMJOB CMD(CALL PGM(MYPROGRAM)) JOB(MYJOB) JOBQ(QBATCH)
In this example:
CMD(CALL PGM(MYPROGRAM)): Submits the job to call a program namedMYPROGRAM.JOB(MYJOB): Names the jobMYJOB.JOBQ(QBATCH): Specifies that the job should be submitted to theQBATCHjob queue.
Declaring and Using Variables
Variables are crucial for holding values that can change during program execution. The DCL (Declare) command is used to declare variables in CL programs. Variables are typically named with an & symbol (e.g., &USERNAME, &RESULT).
Syntax for Declaring a Variable:
DCL VAR(&variable-name) TYPE(type) LEN(length)
VAR(&variable-name): The name of the variable.TYPE(type): The data type, which can be:*CHAR: Character string.*DEC: Decimal number.*LGL: Logical (true/false).*INT: Integer number.
LEN(length): The length of the variable (relevant for character and decimal types).
Example:
DCL VAR(&USERNAME) TYPE(*CHAR) LEN(10) DCL VAR(&COUNTER) TYPE(*DEC) LEN(3 0)
In this example:
&USERNAMEis a character variable with a length of 10.&COUNTERis a decimal variable that can store values up to 999.
Assigning Values to Variables
You can assign values to variables using the CHGVAR (Change Variable) command.
CHGVAR VAR(&USERNAME) VALUE('JohnDoe') CHGVAR VAR(&COUNTER) VALUE(5)
CHGVAR VAR(&USERNAME) VALUE('JohnDoe'): Assigns the value'JohnDoe'to the&USERNAMEvariable.CHGVAR VAR(&COUNTER) VALUE(5): Sets the&COUNTERvariable to5.
Conditional Statements and Control Flow
In CL programs, you can control the flow of execution using conditional statements and loops. These are important for implementing logic in your programs.
IF Statement
The IF statement allows you to execute commands based on a condition.
IF COND(&COUNTER *EQ 5) THEN(SNDMSGPGM MSG('Counter is 5'))
COND(&COUNTER *EQ 5): The condition being evaluated (if&COUNTERequals 5).THEN(SNDMSGPGM MSG('Counter is 5')): The command to execute if the condition is true.
DO and ENDDO Blocks
The DO block allows you to group multiple commands that should be executed together if a condition is met. The block ends with ENDDO.
IF COND(&COUNTER *LT 10) THEN(DO) CHGVAR VAR(&COUNTER) VALUE(&COUNTER + 1) SNDMSGPGM MSG('Counter incremented') ENDDO
In this example, if &COUNTER is less than 10, the program will increment the counter and display a message.
Handling Loops in CLP
CL programs support looping structures to repeatedly execute commands. Two common types of loops are DOWHILE and FOR.
DOWHILE Loop
A DOWHILE loop continues executing a block of commands as long as a condition remains true.
DOWHILE COND(&COUNTER *LT 5) CHGVAR VAR(&COUNTER) VALUE(&COUNTER + 1) SNGMSGPGM MSG('Loop iteration') ENDDO
This loop will continue until &COUNTER reaches 5.
FOR Loop
The FOR loop is another method of controlling repetitive execution, commonly used when you know the number of iterations.
FOR VAR(&I) FROM(1) TO(10) SNGMSGPGM MSG('Iteration') ENDFOR
This loop will iterate 10 times, displaying a message on each iteration.
More control flow statements can be found here: https://www.ibm.com/docs/en/i/7.5?topic=programming-controlling-processing-within-cl-program-cl-procedure
Monitoring for Errors: MONMSG
CL programs can monitor for errors using the MONMSG (Monitor Message) command. This allows you to handle exceptions gracefully instead of letting the program terminate unexpectedly.
PGMMONMSG MSGID(CPF0000) EXEC(DO) SNGMSGPGM MSG('An error occurred') ENDDO /* Normal commands here */ ENDPGM
MONMSG MSGID(CPF0000): Monitors for any system message starting withCPF.EXEC(DO): Executes the commands inside the block if an error occurs.
Best Practices for CLP Syntax and Structure
- Use Meaningful Variable Names: Choose variable names that reflect their purpose (e.g.,
&CUSTOMERNAME,&ORDERCOUNT). - Indentation for Readability: Indent blocks of code inside
DO/ENDDOand conditional statements for better readability. - Comment Your Code: Add comments to explain the purpose of complex commands or logic.
- Monitor for Errors: Use
MONMSGto handle potential errors and prevent program crashes.
Conclusion
The syntax and structure of CL programming are straightforward but powerful. With commands, variables, conditional logic, and loops, CL programs can automate complex tasks on IBM i. By following the guidelines and best practices outlined in this article, you are now equipped to start writing well-structured CL programs.
In the next article, we’ll dive deeper into Data Types and Variables in CL programming, covering how to use them effectively to manipulate data in your programs. Stay tuned for more!
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