VMS Help PIPE, Description, Pipelines and TEEs, Using TEEs and SYS$PIPE  *Conan The Librarian (sorry for the slow response - running on an old VAX)

    In most cases, input from the pipe can be obtained by reading
    the data from SYS$INPUT; however, when a command procedure is
    invoked as a pipeline segment command, SYS$INPUT is redirected to
    the command procedure file. To obtain data from the pipe inside a
    command procedure, the logical SYS$PIPE can be used.

    The following is an example of a pipeline DCL application
    TEE.COM:

     $ ! TEE.COM - command procedure to display/log data flowing through
     $ !           a pipeline
     $ ! Usage: @TEE log-file
     $
     $ OPEN/WRITE  tee_file 'P1'
     $ LOOP:
     $  READ/END_OF_FILE=EXIT  SYS$PIPE LINE
     $  WRITE SYS$OUTPUT LINE ! Send it out to the next stage of the pipeline
     $  WRITE tee_file LINE  ! Log output to the log file
     $  GOTO LOOP
     $ EXIT:
     $  CLOSE tee_file
     $  EXIT

    The PIPE command to use TEE.COM can be:

    $ PIPE  SHOW SYSTEM | @TEE showsys.log | SEARCH SYS$INPUT LEF

    The command procedure TEE.COM is used to log the data flowing
    through the pipeline. It reads in the data from SYS$PIPE instead
    of SYS$INPUT.