If you are trying to troubleshoot a connection issue, you have probably used the telnet tool. Telnet is an old socket protocol which, for the intents of our explanation, simply opens a network socket to the other server and passes data through plain.
For example, if you want to see if a SMTP server is running on a remote server 188.8.131.52 you would telnet to it on port 25:
telnet 184.108.40.206 25
If you wanted to check if an FTP server was running, you would instead run it against port 21, and so on – lookup different protocols online and find out their “default port” for “plain text” traffic (note that sending passwords via plaintext is a bad idea in general – but some services still allow you to do it. Tsk tsk.)
On Mac OS X, Linux and BSD, when you launch telnet, your session will probably open with the following statement
tai@demoserver:~/$ telnet localhost 25
Connected to localhost.
Escape character is ‘^]’.
220 demoserver.local ESMTP Postfix (Ubuntu)
The topic of this article is: what’s that escape character??
You can in fact come out of the telnet stream. In the above example, if you simply typed the normal interrupt ( Ctrl+D ) it would just send the interrupt byte along the wire – and not get caught by the application.
To do that we need to first invoke the escape sequence. So how to type the escape character?
There are two sequences needed to type it:
( Ctrl + “v” ) this causes the input to wait for a special character
( Ctrl + “]” ) this provides the special character
This generates a single character, denoted as “^]”. Send it by pressing return. This returns you to the local telnet prompt. You can now issue a ( Ctrl + D ) command to exit.
This technique also works elsewhere.
For example, you can display text in colour:
$> echo “^[[1;31mhello^[[0m”
Where the ” ^[ ” sequence is actually a special sequence as described above. Note this is using “[” and not “]”
^[ — special character for output stream control
[ — formatting code follows
1 — bold true (can be “0” to turn off bold)
;31 — red ; 32 is green, 33 yello, 34 blue. Try other combinations.
m — end modification
Text entry in this form also works when editing text in vi for example – when the resulting file is output via `cat` or `less -R`, you get colours and bolds!
Note that unless you include the code to turn off custom colours (^[[0m = “terminal default”) then the rest of your command line will keep the last selected colour mode.