Ruby one-liner study

In that presentation, there is a funny ruby one-liner. Here is the explanation:

yes|ruby -ne '$><<"\r\x1b[#{31+($.+=1)%7}mZOMG PREFORKING"; sleep 0.1;'

How does it work?

The -n option means assume 'while gets(); ... end' loop around your script. $> is the default output stream (kind of an alias of $stdout) and the << operator of the IO class writes an object to a stream.

So basically each time the program receives an input (with the yes command), it outputs the "\r\x1b[#{31+($.+=1)%7}mZOMG PREFORKING" string and sleeps 100 milliseconds.

So now, what does that weird string mean?

The \r character is the carriage return so that the program write the string at the beginning of the line and erase what was previously written.

The \x1b character is the escape character. It is the same as \033 and \e and corresponds to the 27th character in the ASCII table. The character which is escaped is [ and that means that here we start a sequence of non-printing characters. In our case, the sequence will define some foreground colour.

$. is a special variable which contains the line number last read by interpreter. As a small example, the following command will print the number of time you pressed the <Enter> key each time you press it:

ruby -ne '$> << $.'

The value computed by the expression 31+($.+=1)%7 will always be between 31 and 37.

Maybe one day I’ll finish this explanation.