Is this the way to detect an Ubuntu Core host system?

snapd
1

I maintain a number of snaps whose default behaviour should different on an embedded, Ubuntu Core system and on a desktop “Classic” system.

It makes sense for them to run as a daemon on Core systems, but on Classic systems less so as they are more useful when started in a user session.

My latest iteration around this is to add an install hook that scans /proc/cmdline for snap_core= and sets a property accordingly:

snap_core=$(sed -n 's/.*snap_core=\([^ ]*\).*/\1/p' /proc/cmdline)

# On Ubuntu Core we default to daemon mode
if [ ! -z "${snap_core}" ]
then snapctl set daemon=true
else snapctl set daemon=false
fi

(I’m using a property as it is perfectly sensible for this default to be reconfigured by the user.)

That works on the systems I’ve tried, but I wondered whether this is reliable, or if there’s a better way to detect the type of host system.

2

it is definitely the most reliable thing to check for core today … but your code is pretty compilcated, why not just:

snapctl set daemon=false

if grep -q snap_core= /proc/cmdline; then
  snapctl set daemon=true
fi
3

True, that snippet could be simpler. But in the real install hook I have further logic:

# On Ubuntu Core18 we default to software cursor
if [[ "${snap_core}" =~ core18.*snap ]]
then snapctl set cursor=software
else snapctl set cursor=auto
fi
4 
snapctl set daemon=false
snapctl set cursor=auto

if grep -q snap_core= /proc/cmdline; then
  snapctl set daemon=true
  if grep -q snap_core=core18 /proc/cmdline; then
    snapctl set cursor=software
  fi
fi

:wink:

I’d also suggest to not use bash if avoidable but /bin/sh with POSIX compliant code. bash is usually slower and takes more ram (several MB vs a few kb). If there s a chance your snap gets installed on any low-end hardware this can have quite some impact (i.e. single-core/low-ram ARM CPUs running a digital-signage application).

5

Point taken. :wink:

However, translation may take some time.

6

Looking into kernel command lines no longer seems to be a solution. I’ve tested on core20-24 and the /proc/cmdline on neither of those contain snap_core=.

I’ve tried to detect by checking the value of ID inside /etc/os-release, but that also useless as ID is always set to ubuntu-core in the file available to the snap.

7

You could try check if the environment variable SNAP_SAVE_DATA is set. It’s supposed to be only set on Ubuntu Core:

SNAP_SAVE_DATA

This variable is only exposed on Ubuntu Core systems, and was introduced with snapd 2.57.

https://snapcraft.io/docs/environment-variables

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