PARSE ARG default fallback to stdin in non‑interactive scripts

Question

Behavior overview
The PARSE ARG built‑in extracts command‑line arguments into variables. When arguments are omitted the variables retain their previous values unless a default is supplied via the PULL option.
If no PULL default is provided, the language specification allows PARSE ARG to read from the input stream, which can cause the script to block in pipelines or scheduled jobs where stdin is not connected.
Implementations differ on whether the UPPER/LOWER options affect the fallback behavior, and the documentation does not state a portable way to disable the stdin read entirely.
Goal: configure a Rexx script so that missing arguments receive explicit defaults without ever waiting for stdin.
Is there a standard option or coding pattern that guarantees PARSE ARG never reads from stdin? How do the major Rexx interpreters (Regina, ooRexx, NetRexx) handle the PULL default when stdin is closed? Can environment variables be used as a reliable fallback without modifying the calling process?

Tasadduq Burney · Answer

Answer
PARSE ARG never reads from stdin; it only receives the argument string that the Rexx program was invoked with. Therefore, to guarantee that no stdin read occurs, simply avoid the PULL option and supply defaults after parsing.
Confirmed facts

PARSE ARG receives the whole command‑line argument string; any missing placeholders become null strings, not input from the console.
Major interpreters (Regina, ooRexx, NetRexx) follow this rule; with stdin closed or redirected they do not block or wait for input.
Environment variables can be read safely with the VALUE() function and used as defaults without altering the calling process.

Recommended pattern

Parse the arguments:
PARSE ARG arg1 arg2 arg3
Assign defaults for any empty placeholders:
IF arg1='' THEN arg1='default1'
IF arg2='' THEN arg2='default2'
IF arg3='' THEN arg3='default3'
Or use the ARG() function:
arg1 = ARG(1); IF arg1='' THEN arg1='default1'
arg2 = ARG(2); IF arg2='' THEN arg2='default2'
arg3 = ARG(3); IF arg3='' THEN arg3='default3'

Verification steps

Create a test script testrx containing the pattern above.
Run echo '' | rexx testrx (pipe empty stdin) and observe that the script exits immediately.
Run rexx testrx  and confirm the exit status is zero and no input is consumed.
Check that when no command‑line arguments are supplied, the variables receive the supplied defaults.

PARSE ARG default fallback to stdin in non‑interactive scripts

Behavior overview

1 answer

Answer

Confirmed facts

Recommended pattern

Verification steps

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