My guy you've just given me the same sense of wonder you got yourself. Fuck man I love linux
this post was submitted on 24 May 2025
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When writing shell scripts that run in a loop I always look for files in /sys or /proc before using standard utilities. There's a lot
- /proc/mounts instead of the mount command
- /sys/class/..../backlight to influence screen brightness
- /sys/class/net to get info and stats about interfaces
- ....
Someone should make a comprehensive list I guess.
Interesting,
non-scientific speed test:
kernel 6.1.0-37-amd64:
$ time for i in $(seq 1 100000); do cat /proc/sys/kernel/random/uuid > /dev/null; done
real 3m53,388s
user 1m37,366s
sys 2m13,847s
$ for i in $(seq 1 100000); do cat /proc/sys/kernel/random/uuid ; done | wc -l
100000
vs. uuid 1.6.2-1.5+b11:
$ time for i in $(seq 1 100000); do uuid -v4 > /dev/null; done
real 4m44,854s
user 1m37,867s
sys 3m4,414s
$ for i in $(seq 1 100000); do uuid -v4 ; done | wc -l
100000
EDIT: I'm blind (wrong result).
boot_id seems to be static, probably set at boot.
So don't use it for a random uuid.
it's fine as long as you reboot before each call
Oops you’re correct. Editing OP.
That’s what you get when you define a file system as “a system that names things”.
Yeah but please don't actually use this. Use a proper UUID library that works cross-platform and lets you choose the UUID type and can be seeded etc.
Can you explain?
Use for what?
Also it is being seeded, according to the file urandom_min_reseed_secs which is also writeable. Here are the other files:
-r--r--r-- 1 root root 0 25. 5. 11:13 boot_id
-r--r--r-- 1 root root 0 25. 5. 11:19 entropy_avail
-r--r--r-- 1 root root 0 25. 5. 11:19 poolsize
-rw-r--r-- 1 root root 0 25. 5. 11:19 urandom_min_reseed_secs
-r--r--r-- 1 root root 0 25. 5. 11:19 uuid
-rw-r--r-- 1 root root 0 25. 5. 11:19 write_wakeup_threshold
edit: the type is always DCE/random
I’ve used this in some bash scripts, very useful!
That's what I mean. No more searching for ways to create shell-friendly randomness ($RANDOM does not always cut it).
Thank you so much for sharing this!
This is awesome. Thank you
Works in Termux on Android
See also: /dev/null
It’s basically a black hole where you can throw anything.
Yes, but what if it were a subscription? May I present: /dev/null-as-a-Service.
The most secure Bitcoin endpoint yet!
lol.
Okay, let’s be honest, it’s basically a crappy Pentium 4 box with Debian and nginx installed.
cat /proc/sys/kernel/random/uuid > /dev/nullWould have to be cat /proc/sys/kernel/random/uuid > /dev/null
You can't pipe to a file, only to programs, and since /dev/null isn't an executable your command will simply give an error.
To make it more clear, consider using dd, which lets you explicitly specify an input and output file. For example: dd if=/proc/sys/kernel/random/uuid of=/dev/sda1 wait shit that wasn't the right output oh god oh fu
dd is just cp but more confusing here.
The only thing dd can do that cp can't is stop ahead of time, which only really matters for infinite files like /dev/random
cp /proc/sys/kernel/random/uuid /dev/sda
dd if=/proc/sys/kernel/random/uuid of=/dev/sda1
Peanuts. Real men do dd if=/proc/sys/kernel/random/uuid of=/dev/sda
i saw this and came to do THE THING but you beat me too it. GOOD ANYA
Thanks, I'm overworked lately.
lol, the last part
while :; do cat /proc/sys/kernel/random/uuid > /dev/null; done
edit: on all cores for maximum "efficiency"
The information will be evenly distributed upon its surface and some believe one day it will be be radiated back out into the rest of the system.
That's a horrifying concept. Better not think about it.
no.
That reminds me of the CPU stress test I ran many years ago.
dd if=/dev/random of=/dev/null
If you have 8 cores, just open 8 terminals, and run that code in each of them.
/dev/urandom should stress the CPU more. /dev/random can be entropy limited
Can you guarantee that each process will run on its own core?
Absolutely not, quite the opposite actually. However, the end result is close to 100% CPU load, which is good enough for some purposes. Let’s say you want to test the performance of your CPU cooler, or overclock stability, this should good enough. There are also dedicated tools for people with more advanced needs.
for i in {1..n} # where n == number of cores
do
dd if=/dev/urandom of=/dev/null &
done
# to stop:
jobs -p | xargs kill
Oh yeah. This looks like a much better way to do it. My solution is pretty bare bones by comparison.
the advantage of yours is that you can actually see the performance number afterwards.