Welcome to the first step of your journey into CL programming! In this article, we will provide an in-depth introduction to IBM i, often known as AS400, and the role of CL (Control Language) within the system. By understanding the foundational concepts of IBM i and the role that CL plays in it, you’ll be well-equipped to begin writing your own CL programs.
What is IBM i?
IBM i is an operating system developed by IBM for its family of Power Systems hardware (formerly known as AS400). It is a robust, integrated, and highly secure platform widely used in enterprise environments for managing business-critical applications and data. The system has evolved over decades from its origins in the 1980s as the AS400, to becoming IBM iSeries, and is now referred to simply as IBM i.
IBM i is designed for both transactional and batch processing, supporting thousands of users simultaneously while ensuring high availability, reliability, and data integrity. It integrates a range of technologies, including databases, user interfaces, job scheduling, and networking, into one cohesive system. Companies across industries use it for enterprise resource planning (ERP), financial systems, and other critical applications.
Key Features of IBM i:
- Integrated Relational Database (DB2 for i): The database is tightly integrated into the system, offering powerful data processing capabilities.
- Object-Oriented Architecture: Everything in IBM i is an object, from files and programs to devices and jobs.
- Security and Reliability: IBM i provides a high level of security, auditability, and operational stability, which makes it ideal for mission-critical applications.
- Job Processing: The system is optimized for batch processing, job control, and multitasking operations.
- Application Development Support: In addition to CL, IBM i supports a variety of programming languages, such as RPG, COBOL, Java, C, and C++.
What is CL (Control Language)?
CL, or Control Language, is a procedural language used primarily for interacting with the IBM i operating system. It is used to:
- Automate repetitive tasks,
- Control job flows,
- Schedule batch jobs,
- Manage system resources,
- Execute commands that would otherwise be performed manually through the system’s command line interface.
CL scripts (also known as CL programs) are fundamental in managing the system’s behavior, controlling system jobs, and interacting with other system objects such as files, devices, and data queues.
The Role of CL in IBM i
- Automation: Many tasks in IBM i, such as backups, reports, and data processing, can be automated through CL scripts.
- Job Management: CL commands help manage jobs on the system, including starting, stopping, and monitoring them.
- System Interaction: CL allows users and administrators to control the system programmatically rather than manually typing commands.
- File and Database Handling: CL programs interact with the system’s database files, handling records, data queues, and objects efficiently.
In essence, CL acts as a glue between the user (or other programs) and the core functionality of the IBM i system, enabling robust automation and system control.
Understanding IBM i Objects
Before we dive into CL itself, it’s important to understand the concept of objects in IBM i. IBM i is built on an object-based architecture, where every component of the system is treated as an object.
Some common types of objects in IBM i include:
- Programs (
*PGM): Executable code, including CL programs and RPG applications. - Files (
*FILE): Objects that hold data, including database files, source files, and device files. - Libraries (
*LIB): Containers for grouping related objects. - Messages (
*MSG): Messages sent between programs, jobs, or users. - Devices (
*DEVD): Hardware resources like printers and disk drives. - User Profiles (
*USRPRF): Objects representing system users and their permissions.
Libraries and Object Storage
In IBM i, objects are stored in libraries, which serve as logical containers for organizing resources. Libraries themselves are also objects, and there is a system library list that the operating system uses to search for objects. When working with CL, you’ll interact with libraries frequently as you manage the location and accessibility of various objects.
The Structure of a CL Program
A CL program is a sequence of CL commands that execute specific tasks. The structure is relatively simple, making it easy for both beginners and advanced users to pick up and automate tasks.
Here’s a basic structure of a CL program:
PGM /* Start of the CL program */ /* Variable declarations and initializations */ /* CL commands for processing */ ENDPGM /* End of the CL program */
Each line in a CL program typically begins with a CL command, followed by parameters. For example, a simple CL program to display a message on the screen might look like this:
PGM SNGMSGPGM PGM('Hello, World!') ENDPGM
This program uses the DSPLY command to display a message. We’ll explore such commands in more detail later in the series.
Command Entry and Prompting
IBM i provides several ways to execute CL commands:
- Command Line: You can manually type commands at the command line (accessible via a terminal or emulator).
- Programs: You can group multiple commands into a CL program, which can be executed as a single entity.
- Batch Jobs: CL commands can be executed as part of a batch job, running in the background without user interaction.
One of the great features of CL is command prompting. When you type a command but aren't sure of its parameters, you can press F4, and IBM i will display a prompt with fields for all available parameters, helping you construct the command correctly.
For example, typing DSPLIB and pressing F4 will bring up a prompt for the Display Library command, where you can enter or select the parameters.
The Importance of CL in Daily Operations
For IBM i administrators, CL is indispensable. Here are some typical tasks that can be automated or controlled through CL:
- Job Scheduling and Monitoring: Automating the execution of batch jobs using CL commands like
SBMJOB(Submit Job). - Backup and Recovery: Automating system backups and restoring data using CL scripts.
- File Management: Managing system files, moving data between environments, and processing files automatically.
- Security and User Management: Managing user profiles, setting authorities, and auditing system activities through CL programs.
Many organizations with IBM i systems rely on CL for their critical processes. For instance, daily financial reports, system monitoring, and large-scale batch processing are often done using CL scripts.
Tools for Writing and Executing CL Programs
SEU (Source Entry Utility)
Historically, the Source Entry Utility (SEU) was used for writing CL programs. SEU is an editor for creating and maintaining source files on IBM i. However, it is now considered legacy, and most developers use more modern alternatives like RDi (Rational Developer for i).
RDi (Rational Developer for i)
Rational Developer for i is an Eclipse-based integrated development environment (IDE) that offers more robust features like code syntax highlighting, debugging tools, and an enhanced user experience compared to SEU.
PDM (Program Development Manager)
The Program Development Manager (PDM) is a tool used to manage source files, objects, and libraries. It allows developers to compile, run, and manage programs from a single interface.
Running CL Programs
Once written, a CL program can be compiled into an executable object using the CRTCLPGM command (Create CL Program). After compilation, it can be executed directly or scheduled to run in the background using commands like SBMJOB.
Conclusion
With this introduction to IBM i and Control Language (CL), you now have a solid understanding of the system and the role that CL plays. From managing jobs and automating tasks to controlling system resources, CL is an invaluable tool in the IBM i environment. As we progress, we’ll delve deeper into specific topics, starting with how to create your first CL program and exploring more advanced concepts.
In the next article, we’ll take our first steps by setting up the development environment and writing a basic CL program. Stay tuned as we start building your CL expertise from the ground up!
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