13. 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 line which is nether
  an instruction nor an assignment, it treats that line as a string
  expression which is to be evaluated and passed to the environment.
  For example:

      /* Execute a given command upon a set of files */
      parse arg command
      command "file1"
      command "file2"
      command "file3"
      exit

  Each of the three similar lines 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 environment to which a command is given varies widely between systems,
  but in most systems you can select from a set of possible environments by
  using the "address" instruction.