The SYS$INPUT stream, which is equivalent to the UN*X standard input (stdin) stream, usually points to your terminal.
When a DCL procedure is invoked, SYS$INPUT points to the current procedure. Similarly, when a DCL procedure is SUBMITted to batch, SYS$INPUT points to the current procedure.
SYS$INPUT can be redirected using DEFINE (or ASSIGN). If DEFINEd in /USER mode, it will be DEASSIGNed when a program is run and that program terminates. If DEFINEd in /SUPERVISOR mode (the default if no access mode is specified in the DEFINE (or ASSIGN) command) or an “inner” (more privileged) mode, it remains in effect until explicitly DEASSIGNed.
