The OpenPrinting CUPS Snap

Complete CUPS printing stack in a Snap

Introduction

This is a complete printing stack in a Snap. It contains not only CUPS but also cups-filters, Ghostscript, and Poppler (the two latter as PostScript and PDF interpreters). This is everything (except printer-model-specific drivers) which is needed for printing.

This Snap is designed for the following two use cases:

Providing a printing stack for a purely Snap-based operating system, like Ubuntu Core.
Providing a printing stack for a classic Linux system, installing the Snap instead of the system's usual printing packages. It is planned that Ubuntu Desktop switches over to use CUPS from this Snap.

Note that this Snap is still under development and therefore there are probably still many bugs.

Installation and Usage

The CUPS Snap works on both classic systems (standard Linux distributions like Ubuntu Desktop) and purely snap-based systems (like Ubuntu Core).

If on your classic system there is already CUPS running, you do not neccessarily need to stop or remove this CUPS for using the Snap. The Snap's CUPS will run on port 10631 (instead of port 631) then and it will use the domain socket /var/snap/cups/common/run/cups.sock (instead of the standard (/var)/run/cups/cups.sock). So you can have two CUPS instances (each with their own cups-browsed) on one system, for example for development.

If the Snap's CUPS runs alone, the standard resources, port 631 and (/var)/run/cups/cups.sock are used to assure maximum compatibility with both snapped and classically installed client applications.

Note that running two CUPS instances on one system is not recommended on production systems. Disable or remove your system's CUPS if you want to use the Snap's CUPS.

The Snap is currently being prepared to get put into the Edge channel of the Snap Store and there one will be able to install it via

snap install --edge cups

If you want to install from source, you have to build it via

snapcraft snap

or if this does not work on your system with

snapcraft cleanbuild

and install it with the command

sudo snap install --dangerous <file>.snap

with <file>.snap being the name of the snap file.

You also need to manually connect the snap to the Avahi interface. On classic systems run:

sudo snap connect cups:avahi-control

On snap-based systems install the Avahi snap at first

sudo snap install avahi

and connect to the avahi snap:

sudo snap connect cups:avahi-control avahi

This allows sharing of printers between your system's CUPS and the snap's CUPS. cups-browsed (one instance on the system, one in the snap) automatically creates appropriate queues.

For USB printer access you need to connect to the raw-usb interface on both classic and snap-based systems:

sudo snap connect cups:raw-usb

You will also need to do the following manual connections:

sudo snap connect cups:cups-control
sudo snap connect cups:cups-domain-socket
sudo snap connect cups:system-config
sudo snap connect cups:snapd-control

We are working on making these manual connections not needed any more in the future.

If there is already a CUPS instance running on your system, The snap's CUPS will run on port 10631 and on the /var/snap/cups/common/run/cups.sock domain socket.

To use use the snap's command line utilities acting on the snap's CUPS, preceed the commands with cups.:

cuos,lpstat -H
cups.lpstat -v
cups.cupsctl
cups.lpadmin -p printer -E -v file:/dev/null

You can run administrative commands without sudo and without getting asked for a password if you are member of the "lpadmin" group if it exists on your system, otherwise if you are member of the "adm" group (this is the case for the first user created on a classic Ubuntu system). This works on classic systems (you can also add a user to the "lpadmin" or "adm" group) but not on snap-based systems (the standard user is not in the "adm" group and you cannot add users to the "adm" group). You can always run administrative programs as root (for example running them with sudo).

The snap's command line utilities can only access files in the calling user's home directory if they are not hidden (name begins with a dot '.'). So you can usually print with a command like

cups.lp -d <printer> <file>

For hidden files you have to pipe the file into the command, like with

cat <file> | cups.lp -d <printer>

or copy or rename the file into a standard file.

The web interface can be accessed under

http://localhost:631/

http://localhost:10631/

To make administrative tasks working, you have to enter user name and password of a user in the "lpadmin" or "adm" group, or "root" and the root password if your system is configured appropriately.

You can also access the snap's CUPS with the system's utilities by specifying the server (example if the snap's CUPS runs in parallel with a system's one, on port 10631 and /var/snap/cups/common/run/cups.sock):

lpstat -h localhost:10631 -v
lpstat -h /var/snap/cups/common/run/cups.sock -v

What is planned/still missing?

Auto-connect to all interfaces (avahi, raw-usb, home, system-files).

Change on design goals: Printer drivers deprecated -> Printer Applications

Printer drivers (printer-model-specific software and/or data) in the form of filters and PPD files added to CUPS are deprecated. They get replaced by Printer Applications, simple daemons which emulate a driverless IPP printer on localhost and do the filtering of the incoming jobs and connection to the printer. This daemons will also be packaged in Snaps, to make the driver packages distribution-independent.

Therefore we will not add a printer driver interface as it is not needed any more.

Note that this Snap DOES NOT support classic printer drivers!

See "Printer Applications" below.

Discussion

The development of this snap is discussed on the Snapcraft forum:

capruro/cups-snap