What happens to things in /dev/null?

[swoll1980]

I'm reading a book, and I just don't get it. Why would you use it? What happens to stuff once it's in there? Does it get deleted when you restart? Can things be recovered from there?

[tica vun]

You can't put files into /dev/null. You can for example direct output of programs you don't want to see into it.

[CptPicard]

It's just something that you can redirect output to if you want it to be discarded. The whole point is that anything that goes there, is just ignored.

[JSeymour]

I'm reading a book, and I just don't get it. Why would you use it?Suppose, for example, you wanted to run a "find" against a directory that you knew contained directories you weren't allowed to read. Normally, "find" would grumble about the unreadable directories. But if you do "2>/dev/null" (sending STDERR output to the bit-bucket), voila': No error messages cluttering things up. There are many uses like that.


What happens to stuff once it's in there?It does into a kind of "data purgatory," where daemons poke at it with pitchforks for all eternity.


Does it get deleted when you restart?Actually, there's nothing to delete. The /dev/null device simply discards it upon receipt :)

Jim

[Simian Man]

No, the point of it is that everything you send there is never written at all anywhere. That's useful only when you just don't care about the stuff you send. For example, the Kate text editor prints out a bunch of garbage on the command line. So if you still want your terminal to be clean, you can start it like so:



kate 1>/dev/null 2>/dev/null &


The "1>/dev/null" redirects the stdout output to /dev/null and the "2>/dev/null" redirects stderr there. The "&" of course runs it as a background process. When I was trying out kate I aliased the kate command to do that automatically.

I'm sure others can think of other examples where having a dummy output location is helpful.

[swoll1980]

so if I did something like


cat /etc/resolv.conf > /dev/null

Would it delete the contents of resolv.conf or just not show me the contents?

[tica vun]

so if I did something like


cat /etc/resolv.conf > /dev/nullWould it delete the contents of resolv.conf or just not show me the contents?

Think, does cat manipulate the file in any other way than printing its contents on the designated output? It doesn't, so using cat can't delete the file, no matter what the output is.

[schauerlich]

so if I did something like


cat /etc/resolv.conf > /dev/null

Would it delete the contents of resolv.conf or just not show me the contents?

It would not delete it.

[Simian Man]

It would essentially do nothing at all.

[lykwydchykyn]

You can redirect /dev/null INTO a file, which is an effective way of emptying a file without deleting it.
e.g.


cat /dev/null > file_I_dont_care_about.really


I use that to empty log file sometimes, since deleting them and recreating them causes some programs to quit logging.

[_h_]

It's the blackhole of *nix based systems.

[diesch]

You can redirect /dev/null INTO a file, which is an effective way of emptying a file without deleting it.
e.g.


cat /dev/null > file_I_dont_care_about.really
I use that to empty log file sometimes, since deleting them and recreating them causes some programs to quit logging.

You can do that with any other empty file, too.

[swoll1980]

so if I accidentally did it backwards;

cat /dev/null > /etc/resolv.conf
It would make /etc/resolv.conf a blank document?

[Simian Man]

so if I accidentally did it backwards;

cat /dev/null > /etc/resolv.conf
It would make /etc/resolv.conf a blank document?

Yes. For even more fun, try it with /dev/random :).

[swoll1980]

Well I guess I should be careful with the things I'm trying to do then.

[donkyhotay]

so if I accidentally did it backwards;

cat /dev/null > /etc/resolv.conf
It would make /etc/resolv.conf a blank document?Yes. For even more fun, try it with /dev/random :).

Please don't do either... your going to make some poor ubuntuforum user cry when you ask them to fix your system without reinstalling.

[swoll1980]

Yes. For even more fun, try it with /dev/random :).

Is there a set number of characters in the string it produces, or will it go on forever.

[Penguin Guy]

Putting something in /dev/null is like talking to a brick wall - since it can't hear you, the information is simply gone.

[swoll1980]

Please don't do either... your going to make some poor ubuntuforum user cry when you ask them to fix your system without reinstalling.

resolv.conf is easily repairable. No worries.

[Penguin Guy]

Is there a set number of characters in the string it produces, or will it go on forever.
It goes on forever.

[swoll1980]

It goes on forever.

So other than cleaning a hard drive what would this be good for?

[sisco311]

You can redirect /dev/null INTO a file, which is an effective way of emptying a file without deleting it.
e.g.


cat /dev/null > file_I_dont_care_about.really


I use that to empty log file sometimes, since deleting them and recreating them causes some programs to quit logging.

If you don't like to type too much, then:

> /path/to/file
:evil:


so if I accidentally did it backwards;

cat /dev/null > /etc/resolv.conf
It would make /etc/resolv.conf a blank document?

Only if you are logged in as root, otherwise you have to run something like:


cat /dev/null | sudo tee /path/to/fileor

echo | sudo tee /path/to/file
(some config files need a newline at the end ;) )

[Simian Man]

So other than cleaning a hard drive what would this be good for?

I use it for testing that my programs don't fail hard no matter what retarded things the user types in.

[whiskeylover]

What happens in /dev/null stays in /dev/null