When is mandatory use /dev/zero over /dev/null for write/discard purpose?

Question

According with the FHS about /dev at:

• 6.1.3. /dev : Devices and special files

it contains:

The following devices must exist under /dev.
/dev/null
      All data written to this device is discarded. A read from this device will return an EOF condition.
/dev/zero
      This device is a source of zeroed out data. All data written to this device is discarded. A read from this device will return as many bytes containing the value zero as was requested.
...

Observe that both have:

All data written to this device is discarded

I read many tutorials where /dev/null is always used to discard data. But because both have the same purpose about writing (discard)

Question

• When is mandatory use /dev/zero over /dev/null for write/discard purpose?

BTW for other differences - practically mostly about read - we have available:

• Difference between /dev/null and /dev/zero

Answer 1

If you're using Linux, it's never "mandatory" to redirect to /dev/null instead of /dev/zero. As you've noticed, you'll get the same result either way.

That said, you should always redirect to /dev/null if you're discarding data. Because everyone understands that writing to /dev/null means throwing the data away; it's expressing your intention.

On the other hand, writing to /dev/zero also throws your data away, but it's not immediately obvious that that's what you're trying to do.

Besides that, I'd be concerned whether writes to /dev/zero are allowed on other Unices, like the BSDs etc. I don't think /dev/zero is even required by POSIX, while /dev/null is. So using /dev/null for its intended purpose is maximally portable; doing anything else is sacrificing portability for no gain.

Answer 2

There is a use case: if a program detects its stdout is directed into /dev/null and alters its behavior (which is not very Unix-way, in my opinion), and you don't want that.

I know only one example: tar. Imagine you want to ensure all files in some directory are actually readable from the disk (no I/O errors, etc.). One way to do it is to tar them to /dev/null:

tar -cv dir/ > /dev/null

But if you try it you may find that tar doesn't actually read the contents of the files, only the metadata (the command runs way too fast). And

tar -cv dir/ > /dev/zero

does read the files' contents (runs slower, with more disk reads).

Answer 3

There is no mandatory cases, but there are use cases:

• /dev/null: is often used when you need to provide a file name, but you do not care about the data.

  Often this is used on stdout: you do not want output of a program, so you redirect the output to /dev/null (so it is discarded). Very convenient on scripts (and cronjob), where you check the status, but text will just confuse.

  You can read /dev/null to create a empty file (but other syntaxes may be better e.g. : > empty_file_name), or when I just want the headers of a table, but I do not want to deliver data

• /dev/zero: I use it when I should *physically" erase data, e.g. with dd command: I override an hard-disk with zeros. Zeros is never ending, so it will override all the hard disk, without having a original file (or occupy space). Alternate it is to use /dev/random but it will much slower.

  I do not remember I wrote to /dev/zero (it seems /dev/null has a better name).

Note: /dev/zero will continue to feed data (and it tell program that it has data). /dev/null instead (when used as input) just tell the program that there are no more data).

It is interesting the documentation you linked. As you see /dev/null is usually used to write, and /dev/zero to read (it according with the first case for each one listed on documentation).

In general: when we are creating a script, we may find new way to use them. It depends on what I'm doing, and what parameters are required by a specific programs. We have also yes command, which deliver us y (with a new line), when we need to feed many confirmation to programs.