What operating system and software is this HP computer running?

HP-UX is technically possible, since graphical desktop environments can run on it, but it would be an unusual choice for a restaurant terminal. A normal Linux installation with KDE, plus an HP utility and a specialized point-of-sale program, is much more likely. The dated appearance may come from the theme rather than the actual age of the system.

This most likely isn’t an HP operating system at all. The desktop looks like Linux running KDE or Plasma with an old-looking theme and icon set. The HP icon is probably a printer or imaging utility, such as HP’s Linux printing software, rather than the operating system itself.

The icons and taskbar resemble KDE, although the customized appearance makes the exact version difficult to identify. The HP logo in the corner could simply be a background printer-management or firmware utility. Without terminal access or system information, it isn’t possible to determine the exact Linux distribution with confidence.

The foreground application appears to be a Hungarian hospitality or catering POS system. The visible fields seem to involve orders, customer information, discounts, and payment or credit balances. It was probably built by a local software vendor, while the computer itself is just an HP workstation running Linux and the application.