usr 是 UNIX Software Resource 的缩写，也就是 Unix操作系统软件资源 所放置的目录。
一 /bin:Essential user command binaries(for use by all users)
1 Purpose
/bin contains commands that may be used by both the system administrator and by users,but which are required when no other filesystems are mounted(e.g. in single user mode).It may also
contain commands which are used indirectly by scripts.
2 Requirements
There must be no subdirectories in /bin,
The following commands,or symbolic links to commands,are required in /bin.
cat
chgrp
chmod
chown
cp
date
dd
df
dmesg
echo
false
hostname
kill
ln
login
ls
mkdir
mknod
more
mount
mv
ps
pwd
rm
rmdir
sed
sh
stty
su
sync
true
umount
uname
二 /sbin:System binaries
1 Purpose
Utilities used for system administration (and other root-only commands) are stored in /sbin, /usr/sbin, and /usr/local/sbin. /sbin contains binaries essential for booting, restoring, recovering, and/or repairing the system in addition to the binaries in /bin. Programs executed after /usr is known to be mounted (when there are no problems) are generally placed into /usr/sbin. Locally-installed system administration programs should be placed into /usr/local/sbin.
2 Requirements
shutdown
三 /usr/sbin:Non-essential standard system binaries
1 Purpose
This directory contains any non-essential binaries used exclusively by the system administrator. System administration programs that are required for system repair, system recovery, mounting /usr, or other essential functions must be placed in /sbin instead.
四 /usr/local/sbin
五 /usr/bin 绝大部分用户可使用命令都放在这里。