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An amazing directory: /proc
(lemmy.nz)
Julia Evans has such great and accessible content, they're awesome
they are a youtuber?
Better, they make zines! https://jvns.ca/
thanks, it's got really hard nowdays to find quality content.
Maybe a dumb question, but is.. a zine just a blog entry?
Zine is short for magazine, I think. Offline zines are often self-published paper booklets, produced by lone wolves or communities, with a focus on one subject. Digital zines are often PDFs. Sometimes a printable booklet version is provided so they can be printed and distributed. Here's an example of a digital zine: https://digitalselfdefence.net/pdf/dsdc-zine.pdf
Okay, this is legitimately cool as heck. This finally helps me wrap my head around the “everything is a file” concept in Linux. Of course it can just copy the executable to memory for later reference. That’s how the system would not completely have a meltdown when you do live updates.
Excellent share!
It's a bit more interesting than that...
Linux, unlike Windows, will let you delete a file that is actively in use. It removes the reference on the file system but the contents of the file won't be deleted until all file pointers to it close. In fact it will still be seen as taking up disk space until it's garbage collected (deleting a large file that's in use can be frustrating).
So the file is actually still available to that running process. If you replace it with a new executable and run that then you get the new version.
I bet this is done with inodes. Deleted files aren't truely deleted until nothing has it open and its inode gets dropped. Like ARC for files.
You can also do interesting things like overwrite a "file" (as in a specific filesystem path) with new contents while keeping anything that already has the file open on the old contents by unlinking the old inode to the path and writing the new contents under a new inode. I believe mv does this. The kind of lesser known feature that probably strikes a good balance between preventing super annoying silent errors/corruption and causing them.
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You used to be able to sudo cat /proc/kcore > /dev/dsp and listen to your ram
The RAM whisperer.
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I'm surprised it doesn't mention that /proc/self automagically points to the directory of the current process. So if you're writing a program, you can just look there for information about itself
Got to love UNIX's everything is a file.
ProcFS is from Plan9. https://en.wikipedia.org/wiki/Plan_9_from_Bell_Labs#/proc
Plan9 is worth a deep dive, but maybe not worth running. It is more UNIX than UNIX. (Also where UTF8 came from.)
I did not know that about plan9! /proc was one of the things I missed most when I switched from daily driving linux to daily driving osx.
Well that is retrograde of it.
Another cool thing from Plan9:
https://github.com/torvalds/linux/blob/master/Documentation/filesystems/9p.rst
This is awesome, and I really no no idea about this!
I no no idea you liked this, nice
d'oh! That's what I get for trying to type quickly.
That's ok, sometimes I to to type quickly too
The hand written effect make these so much easier to digest.
I know what a symlink is, but what is a magic symlink?
A symlink that has been blessed by Richard Stallman.
"Now that's a name I haven't heard in a long time" - Saint IGNUcius
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If you delete a file that's still open by the app, the link in proc will still exist and you can copy the file back out of proc.
Does that not make them a hard link (pointing to a filesystem node) instead of a symlink (pointing to another filename)?
No, you can't have a hard link cross filesystems and proc is its own fs type.
Ah - it has its own special magical file system type, that’s the secret sauce. Neat!
/dev and /sys are the other two special ones
These days there can be a dozen virtual filesystems doing various stuff. If you ever want to get quite baffled, install a terminal emulator on an Android phone (not Termux) and run mount.
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Also known as zombie files. If you are unlucky they can take up all space on a drive, or even tmpfs (so your ram) and you can't easily find them. At least until you end the right process or restart.
Had that happen once and had to write my own script to trace it to the log of an open terminal emulator tab that had billions of lines.
Julia Evans has a bunch of these really handy cheatsheets. I have several saved that I reference semi-regularly.
I have some of her paper zines on my desk, warmly recommended.
Might look esoteric at first but if you think we will be using Linux at least on desktop, servers, consoles and more for decades then totally worth learning about.
I didn't know any of this. Amazing. I usually just look at /proc/net/ for routes and bonding config etc.
Do you know how to build portable executables?
configure --prefix=/proc/self/pwd
It even works in .so files and libtool.
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Great tips! Although if i may 🤓 just a little for the top right, it works because what you deleted isn't the binary, it's the pointer that points to the binary's location. The data is still exactly as it was before "deletion"; the symlink is simply a copy of the original pointer's info; and I'm speculating that the existence of any pointer prevents the system from recycling those addressed bits.
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I never really considered using proc to solve my problems. I jump to logs and such but honestly, how have I never played with proc?
Subscribe.
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I’m aware of /proc but usually use lsof to find open fd’s for a process.
Is one better than the other?
lsof just reads from /proc and gives you formatted output
https://github.com/lsof-org/lsof/blob/master/lib/dialects/linux/dproc.c#L297
I am saving this, both for the post AND for the comments.
We need the whole set of these in one post
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I love *nix’s commitment to the bit on the core abstractions.
Did you know openBSD does not have this folder? I think the BSD's do not use /proc. I do not know why though.
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this post was submitted on 20 Aug 2026
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