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If you ran the following, what would happen?

# Do not run.
# cat /dev/random > ~/randomFile

Would it be written until the drive runs out of space, or would the system see a problem with this and stop it (like with an infinite symlink loop)?

DaveTheMinion
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tkbx
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    I just wanted to add that concatenating random into devices was worth a few hours of fun in my youth. The framebuffer and sound card resulted in noise and the disk drive required me to do a low level reformat... good times. – Bob Roberts Feb 05 '13 at 16:40
  • OT: 'executable /dev/dsp? sounds like fun' – sendmoreinfo Feb 05 '13 at 21:04
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    @BobRoberts I has similar adventures, with similar outcomes. I also used to ssh into colleagues' linux workstations and cat DTMF tones to the internal speaker, then play a busy tone, so it sounded like their PC was trying to FAX something out. Ah, good times. – Tim Kennedy Feb 06 '13 at 15:59
  • @TimKennedy how do you do that? Is there a /dev/ice for the 30mm audio jack? – tkbx Feb 06 '13 at 16:41
  • @tkbx well, i'm dating myself, but back in the day you could cat a file to /dev/audio, and if it was a sound file (.wav) it would get played. – Tim Kennedy May 01 '13 at 15:24
  • If you are putting dangerous commands up, I would recommend preceding it with an octothorpe as an additional safeguard. – msouth Feb 22 '15 at 03:25

1 Answers1

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It writes until the disk is full (usually there is still some space reserved for the root user). But as the pool of random data is limited, this could take a while.

If you need a certain amount of random data, use dd. For 1MB:

dd if=/dev/random iflag=fullblock of=$HOME/randomFile bs=1M count=1

Other possibilities are mentioned in answers to a related question.

However, in almost all cases it is better to use /dev/urandom instead. It does not block if the kernel thinks that it get out of entropy. For better understanding, you can also read myths about /dev/urandom.

Installing haveged speeds up /dev/random and also provides more entropy to /dev/urandom.

EDIT: dd needs the fullblock option as /dev/random (in opposite of /dev/urandom) can return incomplete blocks if the entropy pool is empty.

If your dd does not support units, write them out:

dd if=/dev/random iflag=fullblock of=$HOME/randomFile bs=1048576 count=1
Community
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jofel
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    100 MiB of high-quality random data is a bit much... you should get a few bytes to seed some high-quality pseudo random number generator, like the Mersenne twister. If the application is critical (in a cryptographic sense) you _must_ go and read up on the matter, and perhaps hire an expert. – vonbrand Feb 05 '13 at 14:52
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    @vonbrand No, reading 100MB from `/dev/urandom` is fine, there's no reason not to do it. And do not use a Mersenne twister to do crypto. And [don't use `/dev/random` on Linux](http://security.stackexchange.com/questions/3936/is-a-rand-from-dev-urandom-secure-for-a-login-key/3939#3939). – Gilles 'SO- stop being evil' Feb 05 '13 at 21:46
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    @jofel: No, the output of `/dev/urandom` _is_ fine for cryptographic use. Do not use `/dev/random`. – Thomas Pornin Feb 05 '13 at 22:12
  • @Gilles, maybe I'm loosing it... I agree with everything you say, ans can't see where I said anything different. – vonbrand Feb 05 '13 at 22:13
  • @Gilles & vonbrand: Thanks for you comments, I improved my answer (now only 1MB in the example, /dev/urandom recommended). – jofel Feb 06 '13 at 10:02
  • @jofel `dd: bs: illegal numeric value`, 1000000 works – tkbx Feb 06 '13 at 12:30
  • @tkbx With dd from e.g. GNU coreutils, you can use units like "M". I found, that `iflag=fullblock` is needed. Answer is updated. – jofel Feb 06 '13 at 13:42
  • I tend to write `head -c 123 /dev/urandom` to read a given amount of data from a device. I'd expect this to be less affected by the incomplete block scenario. Might be less performant, though. – MvG Feb 15 '13 at 20:32