Commands and Programs
Parameters (command-line arguments) defined in the main procedures of all traditional AS/400 programs are hard coded lists that are made up of parameters that can be numeric, alphanumeric, boolean, etc. and the order in which parameters are passed is important. This is a stark difference from the Unix and DOS worlds where the parameter list in Unix shell scripts and C programs is a set or array of character pointers and more often than not the parameters are not positionally dependent.
The AS/400 developer's solution to this problem was the command object (*CMD). While the parameters on the command can be specified in any order, each parameter is defined to be passed in a specific order to the program. The programmer can also define, among other things, the parameter's data type, unique parameter name, descriptive text (for prompting), default value (used only if the parameter isn't specified during execution), if the values are restricted to a certain set or range, if the data entered should be changed to another value before calling the program, etc.
At its most basic a command names a single program to call when the user types or prompts the command and presses the Enter key. The command takes all of the parameters typed by the user, and those not typed by the user, and builds a parameter list that it passes to the program when it's called.
Read more about this topic: AS/400 Control Language
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