The character device /dev/null always responds with an end-of-file if you read it.
If you write bytes to it, it will consume and forget them.
When you are using a command line, there is usually a character device that represents your "terminal", even if it is a synthetic device such as an xterm window. Writing to it prints out in your session. Reading from it allows you to type text in. You can see which one is yours with the ps command.
See also man mknod, man null, man pts, man zero, man console, man audio
A typical hard disk has one or more platters, with read/write heads zipping
along the surfaces. Floppy disks and zip disks have only one platter, made of flexible
mylar, and usually both surfaces are used. CDs and DVDs are a single platter; sometimes
DVDs use both surfaces, but CDs only use one surface.
Each surface has concentric circles called tracks. Because the head assembly moves as a unit, all of the heads use the same track on each surface, that is, if the top head is over track 47 on the top surface, all of the heads will be on track 47 on each of their surfaces. The collection of all of the track 47's is referred to as cylinder 47.
Cylinders are ; the head assembly reads and writes one cylinder at a time because there's one head for each disk surface. Typically a hard disk has hundreds or thousands of cylinders.
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