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Usual syntax of a command is dictated by the external host for which it is intended, but normally will follow the form of a name or letter followed by parameter data.
Command names can be given as either a symbol or a string, but it is safer to use a string to prevent being mistaken as a keyword.
The interpretation of the received commands depends entirely on the host application.
The destination for a command depends upon a host address, which is the name of the public message port managed by an external program.
ARexx maintains two implicit host addresses: a "current" and a "previous" value. These can be changed at any time using the ADDRESS instruction or its synonym SHELL . The current address can be inspected using the ADDRESS() built-in function.
The default host address is REXX, but the Interpreter also maintains a host address called AREXX which is an asynchronous port, ie.- calls or commands to this address return immediately without indicating an error. This is equivalent to "run rx <program>" but without the overhead of an extra shell
process.
One special host address is recognized. The string COMMAND indicates that the command should be issued directly to the underlying DOS. All other host addresses are assumed to refer to a public message port.