Rexx I/O processing (a). Read routine  and (b). Write routine (c). Close the file...
READ_CNTL_LIST:
   "EXECIO * DISKR INPLIST (FINIS"     /* Reads entire Inputfile & Close */
   DISKR_RC = RC
IF DISKR_RC > 0 THEN DO
     SAY ERROR_MESSAGE_READING IN_LIST ' RC = ' DISKR_RC
     EXIT DISKR_RC
     END
IN_CNT = QUEUED() /* The Queued function returns total rows read from input file */
RETURN 0

WRITE_OUT_JCL:
   "EXECIO 1 DISKW OUTPUT1 " /* Write one line to the file */
   IF RC > 1 THEN DO
      SAY ERROR_MESSAGE_WRITING CNTL_FILE ' RC = ' RC
      EXIT RC
   END
RETURN 0

/* To close the output file. */
FREE_OUTPUT:
"EXECIO 0 DISKW OUTPUT1 (FINIS "
"FREE F(OUTPUT1)"
RETURN 0

Note: "EXECIO * DISKW JCLINP (FINIS"     - Writes the buffer/queue to a file in one shot.

A Rexx exec to displ ay a n ISPF screen/panel for data entry...
/*** REXX ****/
DISPLAY_PANEL:
ADDRESS ISPEXEC
"LIBDEF ISPPLIB DATASET ID('LCA8455.UTIL.PANELS')"
CLRSCRN /* Clears the Screen */
"DISPLAY PANEL(UTILMENP)"
EXIT 0 /* END, and Return to calling Routine */

Rexx Notes:
PUSH stacks the string resulting from the evaluation of expression LIFO (Last In, First Out) onto the external data queue.
QUEUE appends the string resulting from expression to the tail of the external data queue. That is, it is added FIFO (First In, First Out).
PULL reads a string from the head of the external data queue. It is just a short form of the instruction:
Say 'Do you want to erase the file? Answer Yes or No:'
Pull answer .
if answer='NO' then say 'The file will not be erased.'
Here the dummy placeholder, a period (.), is used on the template to isolate the first word the user enters.
The QUEUED built-in function, QUEUED() , returns the number of lines currently in the external data queue.

Example of a Loop...
DO J = 1 TO D_LINES_CNT
   INLINE = DET_LINES.J
   CALL PROCESS_DETAIL_LINES
END /* End Detail Lines */

LCAXXXX.UTIL.PANELS(CHKDSNPN) *** This is an ISPF panel ***
------------------------------------------------------------------------------------------------------
)ATTR
+ TYPE(TEXT)   INTENS(LOW) SKIP(ON)
% TYPE(TEXT)   INTENS(HIGH) SKIP(ON)
_ TYPE(INPUT) INTENS(HIGH) CAPS(ON) HILITE(USCORE) COLOR(WHITE)
\ TYPE(OUTPUT) INTENS(HIGH)
$ TYPE(TEXT)   INTENS(HIGH) SKIP(ON)
~ TYPE(INPUT) INTENS(HIGH) CAPS(ON)
? TYPE(TEXT)   INTENS(LOW) COLOR(BLUE)
)BODY
%COMMAND ===>_ZCMD
+                === REXX AUTOMATED UTILITY SYSTEM (RAUS) ===
+ *** DB2 IMAGE COPY DATASET/GDGs VERIFICATION REPORT ***
+ (Checks The DB2 Catalog (Syscopy) Entries against the ICF Catalog)
+
+Please Enter DB2 SUB-SYSTEM NAME ==>_ENV + %(e.g. EGP1,EGQ1,EGT1)
+                 DATABASE NAME ? ==>_DBNAME +%(e.g. LPAHCA,LQAQA1)
+
+TABLESPACES Like ?==>_TSPREFX +%(Blank for All, OR use a prefix String)
+
+ *** Enter ImageCopy Date & Time for Verification/Validation Report ***
+
+ImageCopy DATE ? ==>_ICDATE+ %(Enter ImageCopy Date (YYMMDD) Ex: 031117
+ImageCopy Time Range:
+    Begin Time ==>_ICTIME1+ End Time ==>_ICTIME2+ %(HHMMSS) Ex: 235959
)MODEL
+*** Please Read ***
+ImageCopy Date (ICDATE) and Time Range (ICTIME1 & ICTIME2) are required
+
)INIT
&KEYNUM   = .PFKEY
&ENV      = ''
&DBNAME   = ''
&TSPREFX = ''
&ICDATE   = ''
&ICTIME1 = ''
&ICTIME2 = ''
&ZTDMARK = '---------------------------------------------+
               ----------------------------------'
)PROC
VER (&ENV, NB)
VER (&DBNAME, NB)
VER (&ICDATE, NB)
VER (&ICTIME1, NB)
VER (&ICTIME2, NB)
IF (&KEYNUM = 'PF03')
   &NOMORE = 'Y'
)END
**************************** Bottom of Data ***********

*** This is the Rexx code that calls the ISPF panel and process the input ***

* ==== REXX ===================================================*/
* REXX EXEC NAME: RXCHKDSN -                                   */
* DESCRIPTION:    THIS REXX PGM WILL CHECK AND VERIFY THE DB2 */
*                 IMAGE COPY DATASETS (GDGS) AGAINST THE MVS   */
*                 CATALOG, TO VERIFY ALL DB2 BACKUPS ARE GOOD, */
*                 AND VERIFIES THE OFFSITE DATASETS ARE ON THE */
*                 CORRECT VOLUMES (TAPES).                     */
*   *** DB2 IMAGE COPY DATASET/GDGs VERIFICATION REPORT ***   */

(BELOW IS SOME OF THE REXX CODE TO CREATE THE VERIFICATION REPORT, USING USER INPUT FROM PANEL)

PGM_MAINLINE:
ADDRESS TSO
SYSUID = sysvar(SYSUID)   /* THE USERID */
KEYNUM = 0
NOMORE = ''
DO UNTIL (NOMORE = 'Y')
     CLRSCRN /* Clear the Screen */
     CALL PGM_INIT
     CALL DISPLAY_PANEL
     CALL PROCESS_INPUT_LIST
END /*End of process loop*/
EXIT 0
RETURN 0

DISPLAY_PANEL:
ADDRESS ISPEXEC
"LIBDEF ISPPLIB DATASET ID('LCAXXXX.UTIL.PANELS')"
ENV=''; DBNAME=''; TSPREFX=''; ICDATE=''; ICTIME1=''; ICTIME2='';
"DISPLAY PANEL(CHKDSNPN)"
ENV     = STRIP(ENV,B)
DBNAME = STRIP(DBNAME,B)
TSPREFX = STRIP(TSPREFX,B)
ICDATE = STRIP(ICDATE,B)
ICTIME1 = STRIP(ICTIME1,B)
ICTIME2 = STRIP(ICTIME2,B)
SELECT
   WHEN ENV      = ''    THEN NOMORE = 'Y'
   WHEN DBNAME   = ''    THEN NOMORE = 'Y'
   WHEN KEYNUM = 'PF03' THEN NOMORE = 'Y'
   OTHERWISE
END
IF (NOMORE = 'Y')    THEN DO
    CALL EXIT_MENU /*EXIT PGM*/
    END
RETURN 0

On the right side is the ISPF panel for data input/entry to the RXPARSER.
Below is the main routine of the rexx exit (pgm) to parse the input string in
the in file and create a new output ... there's an option to modify the line where
the input string is found, or to delete the line containing the input string.

Note: this is only a portion of the rexx code ... not shown here is the call to the
ISPF panel, the Input/Ouput routines, where we read in of the original file and
writing of the modified output file, error messages, etc.

PROCESS_IN_LINES:
LEN_INLINE = LENGTH(INLINE)
MATCH_FOUND = 'N'
END_OF_LINE = 'N'
DO UNTIL (END_OF_LINE = 'Y')
     CALL CHECK_IN_LINE_FOR_STR
     POS_POINTER = POS_POINTER +1
     IF POS_POINTER > LEN_INLINE | MATCH_FOUND ='N' THEN
        END_OF_LINE = 'Y'
END
/*SAY 'EDT_OPT=' EDT_OPT ' AND MATCH_FOUND=' MATCH_FOUND*/
IF (EDT_OPT = 'D') & (MATCH_FOUND = 'Y') THEN
     LNDEL_CNT = LNDEL_CNT +1
ELSE DO
     CALL WRITE_OUT_FILE /*Write the Output FILE */
     /* IF Match, Check for posible Truncation of the line */
     IF MATCH_FOUND = 'Y' THEN DO
        IF LEN_NL > LEN_INLINE THEN DO
           TRUNCATE_CNT = TRUNCATE_CNT +1
           /*SAY 'Out line > In Line by 'LEN_NL - LEN_INLINE */
        END /*End-of-If Truncate*/
     END /*End-of-If Match*/
END /*End-Else-DO-Write*/
RETURN 0
CHECK_IN_LINE_FOR_STR:
POS1 = POS(IN_STR1,INLINE)
IF POS1 > 0 THEN DO
     MATCH_FOUND = 'Y'
     FOUND_CNT = FOUND_CNT + 1
     POS_POINTER = POS1 + LEN_IN1
     LN1 = SUBSTR(INLINE,1,POS1 -1)
     LN2 = SUBSTR(INLINE,POS_POINTER)
     INLINE = LN1||OUT_STR1||LN2
     LEN_NL = LENGTH(LN1) + LENGTH(LN2) + LEN_OUT1
END
Return 0

The ISPF panel for screen Input Entry:

)ATTR
+ TYPE(TEXT) INTENS(LOW) SKIP(ON)

% TYPE(TEXT) INTENS(HIGH) SKIP(ON)

_ TYPE(INPUT) INTENS(HIGH) CAPS(ON) HILITE(USCORE) COLOR(WHITE)
\ TYPE(OUTPUT) INTENS(HIGH)
# TYPE(OUTPUT) INTENS(HIGH) JUST(RIGHT)
       TYPE(TEXT)   INTENS(LOW) SKIP(ON)
$ TYPE(TEXT)   INTENS(HIGH) SKIP(ON)
~ TYPE(INPUT) INTENS(HIGH) CAPS(ON)
)BODY EXPAND(::)
%COMMAND ===>_ZCMD
+=== REXX AUTOMATED UTILITY SYSTEM (RAUS) ===
+    ==== ALTER A DATASET OR FILE Using an input and output string ====
+
     Enter INPUT FILE NAME ==>_INF                                        +%
          OUTPUT FILE NAME ==>_OUTF                                       +%
+
     Enter INPUT STRING ==>_INSTR                                         +%
          INPUT STRING ==>_OUTSTR                                        +%
+
     Edit Option ==>_OPT    + % [ REPLACE string (R) or DELETE line (D) ]
+
+     The Replace (R) option Replaces the In string with the Out string.
+     The Delete (D) option Deletes all lines containing the In string.
+ *The Output file must be New, Will Not over-writing an existing file*
)MODEL
+
)INIT
&INF      = ''
&OUTF     = ''
&INSTR    = ''
&OUTSTR   = ''
&OPT      = 'REPLACE'
&KEYNUM   = .PFKEY
&ZTDMARK = '---------------------------------------------+
               ----------------------------------'
)PROC
VER (&INF, NB)
VER (&OUTF, NB)
VER (&INSTR, NB)
VER (&OPT, NB, LIST 'R','REPLACE','REP','D','DELETE','DEL')
IF (&OUTF = &INF)
   &OUTF = '***Error: Input name = Output name ***'
IF (&KEYNUM = 'PF03')
   &NOMORE = 'Y'
)END

(sse display of screen panel below)

    REXX AUTOMATED UTILITY SYSTEM (RAUS) ===
    ==== ALTER A DATASET OR FILE Using an input and output string ====

Enter INPUT FILE NAME ==>
     OUTPUT FILE NAME ==>

Enter INPUT STRING ==>
      INPUT STRING ==>

Edit Option ==>     REPLACE            [ REPLACE string (R) or DELETE line (D) ]

The Replace (R) option Replaces the In string with the Out string. The Delete (D) option Deletes all lines containing the In string.
*The Output file must be New, Will Not over-writing an existing file*

Samples Usage Policy: All samples (on this site) are free! You may use as is or change/enhance these samples as needed to accomplish your objectives.

Rexx for OS390/zOS; MVS; TSO; JCL; and Generating DB2 Utilities