Working with pipes as devices

A powerful use of $DEVICE is to connect a KCML device address with a pipeline of Unix processes which can either supply characters to the device or take characters written to a device:

$DEVICE /290="|lp"

will send any output directed to the /290 device address, to the Unix spooler program lp. The spooler program runs in parallel with the KCML process. KCML knows that a device is an output pipe when the string on the right hand side starts with the pipe symbol '|' or '^'.

Pipes can also be used for input e.g.

$DEVICE /290="someprog|"

which will run the program someprog and make its output available to be read from device /290.

When using a pipe to output to a spooler it is necessary to explicitly close the device with a $REWIND or $CLOSE (if the device was previously $OPENed) before the printer daemon can spool the file to the printer, for example:

$DEVICE /290="|lp"
$OPEN /290
SELECT PRINT /290
REM Print Report
...
SELECT PRINT /005
$CLOSE /290

Alternatively $DEVICE /290 could be used to close the device permanantly by removing the device from the DET.

See also SELECT to see how the INPUT, PRINT, LIST streams ca be connected to pipes without the need for a $DEVICE.