Detect if current connection is metered with NetworkManager

With the nmcli utility, the necessary steps are:

1. verify NetworkManager is version 1.0.6+:

   $ nmcli -v nmcli tool, version 1.9.0

2. check GENERAL.METERED on an interface:

   $ nmcli -t -f GENERAL.METERED dev show eth1 GENERAL.METERED:unknown

3. values are: unknown, yes, no, yes (guessed), no (guessed)

4. Forcing the value is done like this:

   $ nmcli dev modify wlan1 connection.METERED yes Connection successfully reapplied to device 'wlan1' $ nmcli -t -f GENERAL.METERED dev show wlan1 GENERAL.METERED:yes

And, to get a list grouped by device:

  $ nmcli -t -f GENERAL.DEVICE,GENERAL.METERED dev show

  GENERAL.DEVICE:wlan1
  GENERAL.METERED:yes

  GENERAL.DEVICE:eth1
  GENERAL.METERED:unknown

  GENERAL.DEVICE:lo
  GENERAL.METERED:unknown

Trying to cut this down to info on just the default route would still require a call to another command as NetworkManager doesn't try to distinguish between multiple devices in a connected state:

  $ nmcli -t -f GENERAL.DEVICE,GENERAL.METERED dev show `ip route list 0/0 | sed -r 's/.*dev (\S*).*/\1/g'`

You can also get the metered status of the current connection via D-Bus. From a shell, you can use busctl:

busctl get-property org.freedesktop.NetworkManager /org/freedesktop/NetworkManager org.freedesktop.NetworkManager Metered

which is only one command, in contrast to the nmcli solution, and in other programming languages it can be more efficient to use D-Bus directly instead of having to call nmcli.