The legacy program, provided by CUPS for sending files to the printer queues on the command line is:

`lpd` (Line Printer Daemon) is the print server daemon itself, not a command for sending print jobs.

`lpp` is not a standard command for printing on Linux systems.

`lpq` is used to examine the contents of the printer spool queue, not to send print jobs.

The `lpr` command is the standard legacy command-line utility used to submit print jobs to the print spooler or queue. CUPS provides its own implementation of `lpr` that is compatible with the System V and BSD printing systems, allowing users to send files to CUPS-managed printers.