HP 3000 Manuals

Command Interpreter Access and Variables Programmer's Guide

Determining Redirection:  HPSTDIN and HPSTDLIST 

CIOR provides two predefined MPE/iX string variables to reflect the state
of I/O redirection.  These variables can be used within User Commands or
programs that need to detect if their input or output has been
redirected.  HPSTDIN will default to the string "$STDIN".  HPSTDLIST will
default to the string value "$STDLIST".  These values also indicate
whether or not input or output is currently redirected.  If input is
redirected, HPSTDIN will contain the name of the file to which input has
been redirected.  If input has been redirected through a file equation
backreference, HPSTDIN will contain the formal file designator of the
file equation.  Similarly, HPSTDLIST will contain the filename or formal
file designator of the file or file equation to which output has been
redirected.

MPE/iX 5.0 Documentation