How to create or remove symbolic link

What's symbolic link?
Symbolic link is a special kind of link that points to another file.
After created, a link target file is replaced as source file.

How to create symbolic link

ln -s <source file name> <link target file name>

e.g.
$ ln -s /usr/local/bin/ruby /usr/bin/ruby

$ ls -alt /usr/bin/ruby
/usr/bin/ruby -> /usr/local/bin/ruby

How to remove symbolic link

use unlink or rm

e.g.
unlink /usr/bin/ruby
rm /usr/bin/ruby

How to use telnet

What's telnet?
A simple CUI telnet client. 
Telnet itself is the abbreviation of "telecommunication network". 
It means a protocol originally.

Basic syntax

telnet <a IP address or a host name> <a port number> 

e.g.

# Connect to 189.0.0.3 by port 30
telnet 189.0.0.3 80
Trying 189.0.0.3...
Connected to 189.0.0.3.
Escape character is '^]'.

# After the connection is successful,  you can exit by running Ctrl + ] and Ctrl + d.