this post was submitted on 24 May 2025
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cat /proc/sys/kernel/random/uuid [I ❤️ Linux] (feddit.org)
submitted 2 weeks ago* (last edited 1 week ago) by A_norny_mousse@feddit.org to c/linux@programming.dev
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That's all. I just found this in a random script. Generates a random UUID every time it's called. I didn't know.

Of course I can also use uuidgen or pipe /dev/(u)random into something to get a random alphanumeric string - but this is built right into the kernel!

In /proc/sys/kernel/random/, there's also boot_id which ~~seems to do the same~~ is static, and some tweakable parameters.

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[–] timhh@programming.dev 11 points 2 weeks ago (2 children)

Yeah but please don't actually use this. Use a proper UUID library that works cross-platform and lets you choose the UUID type and can be seeded etc.

[–] A_norny_mousse@feddit.org 3 points 1 week ago* (last edited 1 week ago) (1 children)

Can you explain?
Use for what?
Also it is being seeded, according to the file urandom_min_reseed_secs which is also writeable. Here are the other files:

-r--r--r-- 1 root root 0 25. 5. 11:13 boot_id
-r--r--r-- 1 root root 0 25. 5. 11:19 entropy_avail
-r--r--r-- 1 root root 0 25. 5. 11:19 poolsize
-rw-r--r-- 1 root root 0 25. 5. 11:19 urandom_min_reseed_secs
-r--r--r-- 1 root root 0 25. 5. 11:19 uuid
-rw-r--r-- 1 root root 0 25. 5. 11:19 write_wakeup_threshold

edit: the type is always DCE/random

[–] timhh@programming.dev 1 points 4 days ago

There are a few reasons you shouldn't use this in proper programs. If you're the sort of person that thinks hacky Bash scripts are acceptable then sure, use it there.

  1. It isn't cross-platform. Not available on Mac/Windows.
  2. There are several types of UUID with different properties. This doesn't let you choose which one to use.
  3. To make programs deterministic (really useful for testing!) you want to be able to seed all their randomness with a specific seed so that it generates the same UUID each time you run it. (Obviously in normal use you would use a random seed.)