proxy ubunta

/etc/environment : Is the correct place to specify system-wide environment variables that should be available to all processes.
/etc/profile : Is the system-wide proflie for shell
some other specific config fils are located under /etc/process/config file, like
/etc/apt/apt.conf : Is the apt config file
/etc/wgetrc

but no all processes use these seting, they own their own proxy profile

note :
/etc/profile.d/*.sh : same to /etc/profle, shell will read for it and /etc/profile.d/*.sh
if you start an application from desktop or other GUI, we should set proxy from network setting option, in contrast to Terminal, which is a container for shell applications

sudo ignores that environment variable by default
https://askubuntu.com/questions/158557/setting-proxy-from-terminal#158595
https://www.quora.com/How-do-I-set-the-proxy-setting-using-Linux-command-line-on-Ubuntu-14-04

https://www.howtogeek.com/247190/whats-the-difference-between-a-vpn-and-a-proxy/
VPNs are set up at the operating system level, and the VPN connection captures the entire network connection of the device it is configured on. This means that unlike a proxy server, which simply acts as a man-in-the-middle server for a single application (like your web browser or BitTorrent client), VPNs will capture the traffic of every single application on your computer, from your web browser to your online games to even Windows Update running in the background

Transparent proxy
https://en.wikipedia.org/wiki/Proxy_server#Transparent_proxy
a transparent proxy intercepts normal communication at the network layer without requiring any special client configuration. Clients need not be aware of the existence of the proxy. A transparent proxy is normally located between the client and the Internet, with the proxy performing some of the functions of a gateway or router.