Understanding the Printing Process

When you print from an application in the Mac OS X graphical interface, the application uses the Mac OS X imaging frameworks to create a PDF spool file that is passed off to the Common UNIX Printing System (CUPS), a cross-platform open-source printing solution. When you print from the command line, the spool file might be in a different format, such as text or PostScript. CUPS allows you to print to PostScript and raster printers, whether they are connected directly to your computer or shared over a network.

Spool files created by Mac OS X applications or by the command-line utilities are placed in the directory /var/spool/cups. The CUPS daemon then finds each spool file and passes it through a set of filter processes known as the print chain. These processes convert the file to a format that the printer understands, and then send the output file to the printer.

CUPS contains many features that are beyond the scope of this book. You can also learn more about CUPS by accessing online help in the built-in CUPS web server by entering the following URL in a web browser running on Mac OS X: http://127.0.0.1:631.