How to set proxy configuration file in command line?
networksetup is a configuration tool for network settings in System Preferences. I don't really known what the ShadowsocksX project is,
but this command can help you up.
For instance, for a certain .pac config file, you could do:
networksetup -setautoproxyurl &quot;Wi-Fi&quot; &quot;http://somedomain.com/proxy.pac&quot;

PS: if you just need to set the proxy for something like wget, or curl, then you can use the fairly universal http_proxy and ftp_proxy ENV variables (e.g., export http_proxy=http://myproxy:port).