For a REXX program that is run from a z/OS shell or from a program, file descriptors 0, 1, and 2 (conventionally, standard input, standard output, and standard error files) are typically inherited.

Attention: A read or write error on file descriptors 0, 1, or 2 results in a halt interruption if the read or write was from a PARSE EXTERNAL instruction, SAY instruction, or EXECIO.

If the REXX program issues a PARSE EXTERNAL instruction, either explicitly or implicitly (such as from a PARSE PULL instruction with an empty stack), it reads standard input for a single text record. The newline character is stripped from the record before it is returned to the REXX program. Standard input is assumed to be a text file, such as your terminal input.

If the REXX program issues a SAY instruction, the text is directed to standard output, and a newline character is appended to the end of the text. Messages issued by REXX, including error and trace messages, are similarly directed to standard output.

If PARSE EXTERNAL is used after standard input has reached the end of the file, null lines are returned. The end-of-file condition can be detected by EXECIO. For more information, see Using EXECIO.