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Create a random file of a specific size

Terminal - Create a random file of a specific size
dd if=/dev/zero of=testfile.txt bs=1M count=10
2009-06-17 17:06:16
User: mstoecker
Functions: dd
1
Create a random file of a specific size

This will create a 10 MB file named testfile.txt. Change the count parameter to change the size of the file.

As one commenter pointed out, yes /dev/random can be used, but the content doesn't matter if you just need a file of a specific size for testing purposes, which is why I used /dev/zero. The file size is what matters, not the content. It's 10 MB either way. "Random" just referred to "any file - content not specific"

Alternatives

There is 1 alternative - vote for the best!

Terminal - Alternatives

Know a better way?

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What others think

This doesn't create a random file, it creates a zero'd file. What you mean is:

dd if=/dev/random of=testfile.txt bs=1M count=10
Comment by dfego 315 weeks and 1 day ago

I'm with dfego. With the modification, though, this is a great oneliner.

Comment by hooobs 315 weeks ago

Might want to consider '/dev/urandom'.

Wikipedia says:

A counterpart to /dev/random is /dev/urandom ("unlocked" random source) which reuses the internal pool to produce more pseudo-random bits. This means that the call will not block, but the output may contain less entropy than the corresponding read from /dev/random. The intent is to serve as a cryptographically secure pseudorandom number generator. This may be used for less secure applications.

Comment by mungewell 315 weeks ago

Mod'ed up as this is actually really usefull (with '/dev/zero') for creating a blank block onto which to re-image embedded files systems.

The problem is that all the old deleted files are really still there. If you 'dd' a blank page and then 'tar'/'untar' the filesystem onto it you can get your compression ratio back up.

Comment by mungewell 315 weeks ago

@mungewell I had also considered using /dev/urandom as the input source as it will also work and I have used it before. Like I said, content doesn't really matter if all you need is the size, which is what I needed for download/upload speed testing, and how I came up with this.

@dfego I don't know if it works for you, but using /dev/random never tends to generate more than 1 kb of data in the file for me. As mungewell pointed out, I believe this is do to the fact that /dev/random does not reuse bits of noise from the entropy pool it uses to create the random data and thus the reason why I didn't use it in the initial code.

Comment by mstoecker 315 weeks ago

"Random" means "random". Unfortunately, too many people have recently begun using it to mean things like "anonymous", "unknown", "miscellaneous", etc.

@mstoecker: Sometimes the content *does* matter.

Comment by dennisw 314 weeks and 1 day ago

Create an alias for bash

function mkfile {

eval dd if=/dev/urandom of=$2 bs=1M count=$1

}

mkfile

Comment by nublaii 243 weeks and 2 days ago

Your point of view

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