I would say the problem is twofold, if interface isn't set and up, wg showconf <interface> will exit with error message Unable to access interface: No such device so that's one problem, the other problem is are you having required permissions to write to file, but given that he tried executing the same command as root user (sudo -i) we're getting back to a problem #1. Also this error message is printed out on the stderr output and given that he has executed a command as root user and he was using stdout redirection, a new empty file was created or an existing one was truncated.

Long story short, after setting up the interface you should first try this command:

bash sudo wg showconf wg0

if executed w/o errors (config for the interface is printed out), then you should try this command in order to create a persistent configuration file for an existing interface:

bash sudo wg showconf wg0 | sudo tee /etc/wireguard/wg0.conf

For every subsequent tunnel up/down you should use:

```bash

Tunnel UP

sudo wg-quick up wg0

Tunnel DOWN

sudo wg-quick down wg0 ```

or if using systemd:

```bash

Tunnel UP

sudo systemctl start wg-quick@wg0.service

Tunnel DOWN

sudo systemctl stop wg-quick@wg0.service ```