REXX Sources of Information
Very popular programming language of the highly experienced mainframe techs

REXX

Recommendation for learning about REXX programming techniques and syntax:
  Internet search on any REXX programming verb with additional search words of REXX code example
  Example search for REXX code example substr

Introduction to REXX

REXX Reference

Understanding EXECIO read and write capabilitiy

Using SDSF with the REXX programming language

Using SDSF with REXX - Special variables reference

Using SDSF with REXX - Columns on the SDSF panels

Using SDSF with REXX - System panel (SYS) columns on the SDSF panels

z/OS REXX is more than just a scripting language. z/OS REXX can be compiled into machine code for best performance when creating high use code. REXX can be used to write business applications and software products. REXX has limitless flexibility in the z/OS environment.

REXX program logic flow