[TUHS] Understanding the /bin, /sbin, /usr/bin, /usr/sbin Split

[Hendrik Jan Thomassen]

If I recall well the /sbin directory first appeared in HP/UX,
and the name stood for 'Static binaries'. Its reason for existence
was that with the introduction of shared libraries the file with
the C-library had become a single point of failure. Therefore this
sbin was introduced to hold Statically linked executables for the
most important commands needed to fix a broken system (sh, ls, mv,
cp, find, tar, fsck etcetera). With this directory earlier than
/bin in his PATH the administrator could at least restore a working
libc.so file. And, while we are at it, the name 'executable' was
not commonly used: they were called 'binaries' (except in IBM
mainframe terminology were they were called 'load modules').
Only much later the habit of /sbin = System binaries emerged.

An important reason to split /bin vs. /usr/bin and /lib vs. /usr/lib
etc. was that the root file system had to be kept small. The fsck
program, while at work, builds tables. If possible, they stay in
memory, but if memory runs out fsck writes them to disk. However,
you don't want them to be written to an untrusted/unchecked file
system, and certainly not to the file system currently under repair.
Since the root file system has to be checked before the others, it
had to be small enough so ensure that fsck could run memory-based
only. Therefore: the /bin, /lib and other root-fs based directories
held the minimal stuff needed for booting and for repairs/restores,
while all the rest had to go into the "overflow" directories /usr/bin,
/usr/lib etc (and, obviously, /usr was a separately mounted partition).
Remember: those were the days that every reboot included a full
fsck on all your partitions. 
-- 
Hendrik-Jan Thomassen     <hjt at ATComputing.nl>
AT Computing
6546 BE  Nijmegen NL      Fax +31 24 352 72 92
info at atcomputing.nl       www.atcomputing.nl
'If you think education is expensive, then try ignorance.'