How to use a proxy for common applications

Global environment
Set the HTTP_PROXY environment variable

export HTTP_PROXY=http://username:password@proxy.server:port/  

apt
Create the file /etc/apt/apt.conf.d/43proxy and place the line below (be sure to provide the appropriate proxy information):

Acquire::http::Proxy "http://username:password@proxy.server:port/";

wget
Set the proxy settings in /etc/wgetrc

http_proxy=http://server_ip:port

git
Use the command below:

git config --global http.proxy http://username:password@proxy.server:port/