How to change Gsettings via remote shell?

The key is to set the DBUS_SESSION_BUS_ADDRESS environment variable.

On this thread I found the following script, that helps to get the correct value of that variable. It requires name of the process, that is running on the desktop, on which we want to change the dbus settings. (There can be more than 1 running graphical sessions in parallel). Lets call it discover_session_bus_address.sh

#!/bin/bash

# Remember to run this script using the command "source ./filename.sh"

# Search these processes for the session variable 
# (they are run as the current user and have the DBUS session variable set)
compatiblePrograms=( nautilus kdeinit kded4 pulseaudio trackerd )

# Attempt to get a program pid
for index in ${compatiblePrograms[@]}; do
    PID=$(pidof -s ${index})
    if [[ "${PID}" != "" ]]; then
        break
    fi
done
if [[ "${PID}" == "" ]]; then
    echo "Could not detect active login session"
    return 1
fi

QUERY_ENVIRON="$(tr '\0' '\n' < /proc/${PID}/environ | grep "DBUS_SESSION_BUS_ADDRESS" | cut -d "=" -f 2-)"
if [[ "${QUERY_ENVIRON}" != "" ]]; then
    export DBUS_SESSION_BUS_ADDRESS="${QUERY_ENVIRON}"
    echo "Connected to session:"
    echo "DBUS_SESSION_BUS_ADDRESS=${DBUS_SESSION_BUS_ADDRESS}"
else
    echo "Could not find dbus session ID in user environment."
    return 1
fi

return 0

With this script we can do the following, assuming that unity process is running on the desktop, on which we want to apply our settings:

. ./discover_session_bus_address.sh unity
gsettings set org.compiz.core:/org/compiz/profiles/unity/plugins/core/ hsize "4"

And things should work OK.

I was having this same issue while trying to make gsettings changes for a vagrant image via SSH during provisioning.

This solution https://askubuntu.com/a/326773 did the trick for me w/o all the fishing for the active connection and trying to spoof the environment. YMMV...