Command of the day: ^]

You may have seen it hundreds of times but few know what it means.

Escape character is '^]'.
It is actually quite obvious is you know that the caret ^ is a shorthand notation for 'Control' (or <Ctrl> as would be more familiar to windows users).

So what is it? It is not actually a command but a keystroke for the telnet command. It opens a command prompt from where you can enter a variety of commands.

Possibly the most exciting thing you can do is then press ^Z ( <Ctrl>+z ) and suspend your telnet session, returning you to your local shell. You can then resume later by using your shell job control (ie. 'fg').

Some other useful commands are

• quit (or press ^C)
• send brk (which if supported will send a break)
• speed 115200 (which if supported will switch the serial line speed)
• close (disconnect the session)
• open (establish a new session)
• help (list the available commands)

And there are many more documented in the telnet man page.

In this day and age, any server shell access should be via a secure protocol like ssh. telnet remains however as a handy tool for testing TCP servers and accessing specialised equipment like serial concentrators.

It is also worth noting that telnet is actually a protocol and not just a raw TCP stream. There is a lot of negotiation which is done to ensure that your terminal works correctly. Often this only becomes obvious when your terminal does not work correctly.

Command of the day: column

Often I come across a new command which I think Wow! I always wanted to know how to do that.

I decided that I will blog about them and that way I will have a record for next time when I forget how to do it.

So my first command of the day I is one I saw on a Red Hat post. It can show the contents of a file tabulated into easy to read columns.

Example:
cat /etc/passwd | column -s : -t


Now you are probably aware of the <tab> character, often entered as \t or in bash as $'\t'. It will advance the cursor to the next tab stop which is traditionally every 8 characters. Well what if you have a tab delimited file but some data is longer than 8 characters and some it less. Well when you look at it, the output will be all over the place.

Using column you can get all your tab columns to line up like this:
cat data | column -s $'\t' -t
Note: for this to work correctly you must have a value in every column (perhaps a bug?)