Why are bitwise shifts (<< and >>) used for cout and cin?

Question is in the title really; I'm sure there is something logical, but for now I'm stumped!

According to §8.3.1 of The Design and Evolution of C++:

The idea of providing an output operator rather than a named output function was suggested by Doug McIlroy by analogy with the I/O redirection operators in the UNIX shell (>, >>, |, etc.)

[...]

Several operators were considered for input and output operations: the assignment operator was a candidate for both input and output, but it binds the wrong way. That is cout=a=b would be interpreted as cout=(a=b), and most people seemed to prefer the input operator to be different from the output operator. The operators < and > were tried, but the meanings "less than" and "greater than" were so firmly implanted in people's minds that the new I/O statements were for all practical purposes unreadable (this does not appear to be the case for << and >>). Apart from that, '<' is just above ',' on most keyboards, and people were writing expressions like this:

cout < x , y, z;

It is not easy to give good error messages for this.

Maybe because it looks similar to the Unix append operation, as you are essentially appending to an input/output stream?

E.g.

Output

echo "foo" >> bar

Input

sendmail -f [email protected] << myemail.txt

(Stole input example from Zac Howland)

From "The C++ Programming language". Stroustrup's(language authors) words:

Overloading the operator << to mean ‘‘put to’’ gives a better notation and lets the programmer output a sequence of objects in a single statement.

But why <<? It is not possible to invent a new lexical token . The assignment operator was a candidate for both input and output, but most people seemed to prefer to use different operators for input and output. Furthermore, = binds the wrong way; that is, cout=a=b means cout=(a=b) rather than (cout=a)=b . I tried the operators < and >, but the mean ‘‘less than’’ and ‘‘greater than’’ were so firmly implanted in people’s minds that the new I/O statements were for all practical purposes unreadable.

So you remember that if you think cin as a keyboard and cout as a monitor, what you type goes into the variable

cin>>var;

Or the contents of your variable goes towards the screen

cout<<var;

>> and << are just operators and you can implement your own >> and << for your classes.

I suppose "somebody" selected them because: a) they are similar to shell file operations and b) to reuse existing operators because there are no need to create new ones