Linux Directory Structure

Ken Thompson and Dennis Ritchie created Unix on a PDP-7 in 1969. Well around 1971 they upgraded to a PDP-11 with a pair of RK05 disk packs (1.5 megabytes each) for storage.

When the operating system grew too big to fit on the first RK05 disk pack (their root filesystem) they let it leak into the second one, which is where all the user home directories lived (which is why the mount was called /usr). They replicated all the OS directories under there (/bin, /sbin, /lib, /tmp...) and wrote files to those new directories because their original disk was out of space. When they got a third disk, they mounted it on /home and relocated all the user directories to there so the OS could consume all the space on both disks and grow to THREE WHOLE MEGABYTES. And thereafter /usr is used to store user programs while /home is used to store user data.

source: http://lists.busybox.net/pipermail/busybox/2010-December/074114.html

THREE WHOLE MEGABYTES
Me in 2024 holding a 4TB NVMe stick: Still not enough (it's never enough)
- Same, but with a 22TB drive for /data loooool
This thread is 3 MB

You're not the only one 😅 🙋

I thought it was United System Resources.
And I still don't know what's the point in separating /bin, /sbin, /usr/bin and /usr/sbin.
Also /mnt and /media
Or why it's /root and not /home/root

Mostly historical reasons, /home was often a network mounted directory, but /root must be local.
And only regular users have their home in /home
- Idk why I feel compelled to add this info, but / doesn't have to be local as long as the necessary kernel modules for mounting it are available in the initrd or built into the kernel.
  
  Yes, that is true. I was speaking in the context of very early Unix/Linux before initrd was a thing.
/home is often on a separate volume. You’d want root to be available in a maintenance situation where /home may not be mounted.
I don't recall the reasons for the addition but /media is newer than /mnt.
- I don't recall the reasons for the addition but /media is newer than /mnt.
  Something to do with hard-coded mounts in /etc/fstab vs. dynamically-mounted removable media (USB drives etc.), I think.
  
  I’ve also seen autofs network automounts go in /net
And I still don't know what's the point in separating /bin, /sbin, /usr/bin and /usr/sbin.
This goes back to the olden days when disk space was measured in kilo and megabytes. /sbin/ and /usr/sbin have the files needed to start a bare bone Unix/Linux system, so that you could boot from a 800kb floppy and mount all other directories via network or other storage devices as needed.
- Is there a reason to keep this structure other than „we’ve always been doing it like that“/backwards compatibility?
  
  The structure is changing, many distributions already are merging more and more of the duplicated subdirectories in /usr/ with the counterparts in / but it takes time to complete that and at the moment those subdirectories are often still there but as symlinks to be compatible with older software (and sysadmins).
They hold "system binaries" meant for root user. It's not a hard distinction but many if not most Linux fundamentals have their roots in very early computing, mainframes, Bell and Xerox, and this good idea has been carried into the here&now. Not sure about the provenance of this one, but it makes sense. isn't /mnt /media different between distros? These aren't hard and fast rules - some distros choose to keep files elsewhere from the "standard".
/bin and /usr/bin, one is typically a symbolic link to another - they used to be stored on disks of different size, cost, and speed.
https://refspecs.linuxfoundation.org/FHS_3.0/fhs/ch03s16.html
https://unix.stackexchange.com/questions/5915/difference-between-bin-and-usr-bin
I think /mnt is where you manually mount a hard drive or other device if you’re just doing it temporarily, and /media has sub folders for stuff like cdrom drives or thumb drives?
- Yeah, but why?
  You can mount a hard drive anywhere, and why not put all the cdrom and thumbdrive folders in /mnt, too?
  
  /media is for removable drives. If you mount something there, file managers like Gnome will show you the "eject" or "disconnect" button.
  /mnt drives show up as regular network drives without that "eject" functionality.
  
  I don't know if I'm doing something wrong, but I have a secondary SSD in my laptop that I mount on /mnt/elyssa and in every DE and distro I tried it appeared as a removable drive with the "eject" button. Right now I use Fedora with Gnome and if I install this extension or enable the removable drives option in Dash to Dock, it shows me that drive. Maybe some mount option in Gnome Disks, but since it's not that big of a problem, I haven't looked too much into it.
  
  It gets even more complicated nowadays because most DE will mount removable drives somewhere in folders like /run/$USER/
  
  /mnt is meant for volumes that you manually mount temporarily. This used to be basically the only way to use removable media back in the day.
  /media came to be when the automatic mounting of removable media became a fashionable thing.
  And it's kind of the same to this day. /media is understood to be managed by automounters and /mnt is what you're supposed to mess with as a user.
/sbin are system binaries, eg root only stuff, dunno the rest but I would guess there are some historical reasons for the bin usr/bin separation
- I know the distinction between /bin and /sbin, I just don't know what purpose it serves.
  Historically, /bin contained binaries that were needed before /usr was mounted during the boot process (/usr was usually on a networked drive).
  Nowadays that's obsolete, and most distros go ahead and merge the directories.
  
  It's easier to manage security that way.
  Instead of having one binary folder full of stuff that's intended to be run with privilege access and non-privilege access, all the privileged stuff goes in sbin and you don't even see it in your path as a regular user. It also means that access rights can be controlled at the folder level instead of the individual file level.

I think it originally did under old Unix, it was what /home is nowadays; "Unix System Resources" is a backronym.

You are correct and this can be seen in some of the old AT&T demos from the '80s floating around on YouTube. There is even a chart that specifically labels a directory like /usr/bwk as the user's home.
Plan 9 also uses this old convention; users live under /usr and there is no /home.

It's always been for USeR binaries. It's the first time I've seen this bizarre backronym (40 years of Unix here).

I was just about to post the same thing. I've been using Linux for almost 10 years. I never really understood the folder layout anyway into this detail. My reasoning always was that /lib was more system-wide and /usr/lib was for stuff installed for me only. That never made sense though, since there is only one /usr and not one for every user. But I never really thought further, I just let it be.

Yup same. I always wondered why there was a user folder when we already have home.

Same. I actually feel like I remember the professor of my only unix class saying that. Hoping I'm wrong.

Likewise.

It's also only just now dawning on me /bin is short for /binaries. I always thought it was like... A bin. like a junk drawer hidden in a cupboard

@FQQD @sag U So Rong 🙂