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.
You used to be able to sudo cat /proc/kcore > /dev/dsp and listen to your ram
The RAM whisperer.
Julia Evans has a bunch of these really handy cheatsheets. I have several saved that I reference semi-regularly.
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!
/procwas 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
I’m surprised it doesn’t mention that
/proc/selfautomagically points to the directory of the current process. So if you’re writing a program, you can just look there for information about itself
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!
Wait until you see the Plan 9 API
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.
Thanks for explaining, I was wondering about that.
Wait, is that why renaming or moving files on android takes forever?
No, that’s because Android. It’s also overlaying all filesystems to enforce stupid rules, like no files named “CON” and no files named the same in a different case (like in DOS).
Julia Evans has such great and accessible content, they’re awesome
they are a youtuber?
Better, they make zines! https://jvns.ca/
Maybe a dumb question, but is… a zine just a blog entry?
thanks, it’s got really hard nowdays to find quality content.
I didn’t know any of this. Amazing. I usually just look at
/proc/net/for routes and bonding config etc.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
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
Anyone else getting turned on rn?
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.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.Remember the old days before /dev was its own virtual filesystem?
It still is! If you try and boot a kernel with nothing in /dev, you won’t get most of the kernel boot messages since /dev/console doesn’t exist. You need a basic set of device nodes in /dev before udev is started.
If you’re going to rsync a system to new hardware and exclude /dev /proc /sys, it’s better to setup a new mount of your / to a different directory and sync off that, so you get the underlying stuff without the cruft.
I’m aware of /proc but usually use lsof to find open fd’s for a process.
Is one better than the other?
lsofjust reads from/procand gives you formatted outputhttps://github.com/lsof-org/lsof/blob/master/lib/dialects/linux/dproc.c#L297
/proc is quite useful when it comes to hiding root on Android.
Do you know how to build portable executables?
configure --prefix=/proc/self/pwdIt even works in .so files and libtool.
May I have the explanation?
The executable will search for .so files not inside predefined paths like /usr/lib but inside it’s current directory. You may theoretically put
.as an argument toconfigure, but it converts that into an absolute path, snd libtool and linker fail when encountering relative paths inside shared library dependencies.Thank you
I would generally just set the RPATH to
$ORIGINin the ELF fileHow would this differ from
--prefix=$(pwd)?--prefix=$(pwd)is resolved at compile time. If you move your compiled .so files to a different directory, they stop working.I see what you mean, very clever solution if a program is using the configure prefix by compiling it into the binary.
That can’t be very common though is it? Usually the configure prefix is just used by
make installto install the binaries into the prefix. If the compiled program needs to know where it is installed it can readargv[0].Have you seen this used somewhere?
I am using this myself in Android app, where you can write files only inside
/data/data/your.app.id/So naturally, if you want to publish a different app,
your.app.idwill be different, so you need to recompile all your Linux tools that you bundle inside your app.If you are not running your Linux tools on Android, you’d probably be better off using Docker or a chroot.
I love *nix’s commitment to the bit on the core abstractions.
I am saving this, both for the post AND for the comments.
