It isn't a file, actually, though you can use it like one. /dev/null is a UNIX device.  It's not a physical device. /dev/null is a special device that "eats" any text written to it and returns "end-of-file" (a file of length 0) when you read from it. So what the heck can you use it for?
 Well, okay. It's a device file.
Make another program "quiet" by redirecting its output there. For instance, if you're putting a program into the background and you don't want it to bother you, type:
> /dev/null &
Thatstandard output but leaves standard error hooked to your terminal, in case there is an error.
Answer a program that asks a lot of questions-you know you'll just press RETURN at each prompt. In a lot of cases, you can redirect the program's standard input from /dev/null:
< /dev/nullWant the default setup? If yes, press RETURN: Enter filename or press RETURN for default: ...
You should test that with each program, though, before you assume this trick will work. (If it doesn't work, try.)
Where a program needs an extra filename but you don't want it to read or write an actual file. For instance, theprograms won't give the name of the file where they find a match unless there are at least two filenames on the command line. When you use a wildcard in a directory where maybe only one file will match,
" * /dev/null
You're guaranteed that grep won't match its regular expression in
Article 24.2 shows even more uses for /dev/null.
Another interesting device (mostly for programmers) is /dev/zero. When you read it, you'll get forever. There are no newlines either. For both of those reasons, many UNIX commands have trouble reading it. If you want to play, the command below will give you a start (and will give you a stop!): 
 On some UNIX versions, the head program may not terminate after it's printed the first ten lines. In that case, use
sed 10qinstead of