Understanding the bin, sbin, usr/bin , usr/sbin split

On Tuesday 30 November 2010 15:58:00 David Collier wrote:
> I see that busybox spreads it's links over these 4 directories.
> Is there a simple rule which decides which directory each link lives
> in.....
> For instance I see kill is in /bin and killall in /usr/bin.... I don't
> have a grip on what might be the logic for that.

You know how 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 (ooooh!).

Of course they made rules about "when the system first boots, it has to come up 
enough to be able to mount the second disk on /usr, so don't put things like 
the mount command /usr/bin or we'll have a chicken and egg problem bringing 
the system up."  Fairly straightforward.  Also fairly specific to v6 unix of 35 
years ago.

The /bin vs /usr/bin split (and all the others) is an artifact of this, a 
1970's implementation detail that got carried forward for decades by 
bureaucrats who never question _why_ they're doing things.  It stopped making 
any sense before Linux was ever invented, for multiple reasons:

1) Early system bringup is the provice of initrd and initramfs, which deals 
with the "this file is needed before that file" issues.  We've already _got_ a 
temporary system that boots the main system.

2) shared libraries (introduced by the Berkeley guys) prevent you from 
independently upgrading the /lib and /usr/bin parts.  They two partitions have 
to _match_ or they won't work.  This wasn't the case in 1974, back then they 
had a certain level of independence because everything was statically linked.

3) Cheap retail hard drives passed the 100 megabyte mark around 1990, and 
partition resizing software showed up somewhere around there (partition magic 
3.0 shipped in 1997).

Source: Understanding the bin, sbin, usr/bin , usr/sbin split