Table Of ContentControl Language Programming for IBM i
Jim Buck, Bryan Meyers, and Dan Riehl
First Edition
First MC Press Printing—August 2011
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Acknowledgements
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Chapter 1
What Is CL?
Chapter Overview
This chapter examines the unique role of the IBM i operating system’s
Control Language (CL) in relation to other computer languages and
describes CL’s strengths and weaknesses. Emphasis is placed on the
use of CL to control workflow and to define job streams to the System i
computer. You will become familiar with the contrast between CL and
typical high-level languages (HLLs), such as Cobol and RPG. The
chapter also contains a brief discussion of compiled and interpreted CL,
along with an explanation of the execution environment restrictions in
effect when you use CL commands.
What CL Is
Control Language (CL) is a set of commands that you use to control
operations and to request system-related functions on the IBM System i
computer. IBM provides CL as an integrated part of the IBM i operating
system (formerly known as i5/OS, and before that as OS/400), not as a
separate product.
As a means of controlling workflow, CL serves purposes comparable to
Job Control Language (JCL) on mainframe computers. CL, however,
provides much more functionality than JCL. In some respects, CL is similar
to the PC-DOS, Windows, or Linux commands you may use on a PC.
Operations such as copying files, redirecting output, and making directories
in those operating systems have direct CL counterparts.
CL originated on the IBM System/38 computer in the late 1970s and
continued as the principal system-control language when IBM introduced
the AS/400 in 1988. The AS/400 version of CL provided many
enhancements over the S/38 version, including new commands and other
added functionality. As the AS/400 and its OS/400 operating system
evolved into the System i and its i5/OS, now IBM i, operating system, the
system retained CL as its system-control language, making the necessary
enhancements and changes to support new features and capabilities.
Because of similarities among the various versions of CL, many S/38 and
AS/400 CL commands and programs can be used on the System i, and vice
versa.
A single CL statement is called a command. A CL command is the
primary means of interacting with the System i. Nearly everything the
computer does is requested by a command. There are commands to create
and delete objects, commands to start programs, a command to power
down the system, and even a command to create a command.
You can think of a command as an instruction to the computer to
perform some function. For example, the DLTF (Delete File) command
instructs the System i to delete a file. This command would be comparable
to the DEL command, which is a part of DOS on personal computers. Most
CL commands are supplied by IBM as part of the operating system, but you
also can create your own commands, customized to your own installation’s
particular needs.
CL, as shipped by IBM, consists of thousands of commands but still
maintains a useful consistency that makes the language easy to learn and
to understand. You can use most CL commands in at least two ways: by
typing them individually onto a command line and by grouping them
together into a CL program. CL programs typically consist of multiple CL
commands that define procedures or operations on the computer. By
combining multiple commands within a CL program, you can automate most
System i operations.
You should write CL programs to perform repetitive processing, thereby
reducing errors and operator/user intervention. CL also is a natural choice
for many utility programs that control or monitor system-related activities on
the System i. Here are a few of the many functions CL can perform:
Start jobs by calling programs or by submitting jobs for batch
processing
Control the sequence of processing within a program and among
different programs
Check for the existence of objects in the system and check for security
authorizations to objects
Create and manage system objects (e.g., files, programs, commands)
Handle error conditions caused by commands or programs
Control the operation of an application by establishing the values of
variables used in the application, such as date, time, and other external
information
Control communications
Send messages between programs and users or between programs
and other programs
Manage work on the system
Create the environment in which a job will execute, controlling
attributes such as which objects the job will use, what its execution
priority will be, and how it will respond to unexpected errors
Manipulate variable information, byte strings, and date formats
Change the configuration of your server and define the devices
attached to it
