Red Hat LINUX 7.2 Reference Manual page 23

Section 1.2:Overview of Filesystem Hierarchy Standard (FHS)
The /opt Directory
The /opt directory provides an area for usually large, static application software packages to be
stored.
For packages that wish to avoid putting their files throughout the filesystem, /opt provides a logical
and predictable organizational system under that package's directory. This gives the system adminis-
trator an easy way to determine the role of each file within a particular package.
For example, if sample is the name of a particular software package located within /opt, then all
of its files could be placed within directories inside /opt/sample, such as /opt/sample/bin
for binaries and /opt/sample/man for manual pages.
Large packages that encompass many different sub-packages, each of which accomplish a particular
task, also go within /opt, giving that large package a standardized way to organize itself. In this
way, our sample package may have different tools that each go in their own sub-directories, such as
/opt/sample/tool1 and /opt/sample/tool2, each of which can have their own bin, man,
and other similar directories.
The /proc Directory
The /proc directory contains special "files" that either extract information from or send information
to the kernel.
Due to the great variety of data available within /proc and the many ways this directory can be
used to communicate with the kernel, an entire chapter has been devoted to the subject. For more
information, please see Chapter 4, The /proc Filesystem.
The /sbin Directory
The /sbin directory is for executables used only by the root user. The executables in /sbin are
only used to boot and mount /usr and perform system recovery operations. The FHS says:
"/sbin typically contains files essential for booting the system in addition to the binaries
in /bin. Anything executed after /usr is known to be mounted (when there are no
problems) should be placed in /usr/sbin. Local-only system administration binaries
should be placed into /usr/local/sbin."
At a minimum, the following programs should be in /sbin:
arp, clock,
getty, halt,
init, fdisk,
fsck.*, grub,
ifconfig, lilo,
mkfs.*, mkswap,
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