When a REXX program runs in a z/OS shell environment or is called from a program, the PARSE SOURCE instruction returns nine tokens, in this order:

1. The string TSO
2. The string COMMAND, FUNCTION, or SUBROUTINE, depending on whether the program was invoked as a host command, from a function call in an expression, or with the CALL instruction
3. The path name of the REXX program
4. The string PATH
5. The path name of the REXX program
6. ? (question mark)
7. The name of the initial host command environment in uppercase: SH
8. The name of the address space in uppercase: OMVS
9. An 8-character user token: OpenMVS

To determine whether the REXX program was run from a z/OS shell, use token 8 or 9.