Using_TEEs_and_SYS$PIPE

   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.