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Rexx - Basic Syntax
Rexx - Basic Syntax
In order to understand the basic syntax of Rexx, let us first look at a simple Hello World program.
Example
/* Main program */ say "Hello World"
One can see how simple the hello world program is. It is a simple script pne which is used to execute the Hello World program.
The following things need to be noted about the above program −
The say command is used to output a value to the console.
The /* */ is used for comments in Rexx.
The output of the above program will be −
Hello World
General Form of a Statement
In Rexx, let’s see a general form of a program. Take a look at the following example.
/* Main program */ say add(5,6) exit add: parse arg a,b return a + b
The output of the above program will be −
11
Let’s go through what we have understood from the above program −
Add is a function defined to add 2 numbers.
In the main program, the values of 5 and 6 is used as parameters to the add function.
The exit keyword is used to exit from the main program. This is used to differentiate the main program from the add function.
The add function is differentiated with the ‘:’ symbol.
The parse statement is used to parse the incoming arguments.
Finally, the return statement is used to return the sum of the numeric values.
Subroutines and Functions
In Rexx, the code is normally spanided into subroutines and functions. Subroutines and functions are used to differentiate the code into different logical units. The key difference between subroutines and functions is that functions return a value whereas subroutines don’t.
Below is a key difference example between a subroutine and a function for an addition implementation −
Function Implementation
/* Main program */ say add(5,6) exit add: parse arg a,b return a + b
Subroutine Implementation
/* Main program */ add(5,6) exit add: parse arg a,b say a + b
The output of both the programs will be the value 11.
Executing Commands
Rexx can be used as a control language for a variety of command-based systems. The way that Rexx executes commands in these systems is as follows. When Rexx encounters a program pne which is neither an instruction nor an assignment, it treats that pne as a string expression which is to be evaluated and then passed to the environment.
An example is as follows −
Example
/* Main program */ parse arg command command "file1" command "file2" command "file3" exit
Each of the three similar pnes in this program is a string expression which adds the name of a file (contained in the string constants) to the name of a command (given as a parameter). The resulting string is passed to the environment to be executed as a command. When the command has finished, the variable "rc" is set to the exit code of the command.
The output of the above program is as follows −
sh: file1: command not found
3 *-* command "file1"
>>> " file1"
+++ "RC(127)"
sh: file2: command not found
4 *-* command "file2"
>>> " file2"
+++ "RC(127)"
sh: file3: command not found
5 *-* command "file3"
>>> " file3"
+++ "RC(127)"
Keywords in Rexx
The free syntax of REXX imppes that some symbols are reserved for the language processor s use in certain contexts.
Within particular instructions, some symbols may be reserved to separate the parts of the instruction. These symbols are referred to as keywords. Examples of REXX keywords are the WHILE in a DO instruction, and the THEN (which acts as a clause terminator in this case) following an IF or WHEN clause.
Apart from these cases, only simple symbols that are the first token in a clause and that are not followed by an "=" or ":" are checked to see if they are instruction keywords. You can use the symbols freely elsewhere in clauses without their being taken to be keywords.
Comments in Rexx
Comments are used to document your code. Single pne comments are identified by using the /* */ at any position in the pne.
An example is as follows −
/* Main program */ /* Call the add function */ add(5,6) /* Exit the main program */ exit add: /* Parse the arguments passed to the add function */ parse arg a,b /* Display the added numeric values */ say a + b
Comments can also be written in between a code pne as shown in the following program −
/* Main program */ /* Call the add function */ add(5,6) /* Exit the main program */ exit add: parse /* Parse the arguments passed to the add function */ arg a,b /* Display the added numeric values */ say a + b
The output of the above program will be −
11
You can also have multiple pnes in a comment as shown in the following program −
/* Main program The below program is used to add numbers Call the add function */ add(5,6) exit add: parse arg a,b say a + b
The output of the above program will be −
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