/bin
This directory contains executable programs which are needed
in single user mode and to bring the system up or repair it.
/sbin
Like /bin, this directory holds commands needed to boot the
system, but which are usually not executed by normal users.
/usr/bin
This is the primary directory for executable programs. Most
programs executed by normal users which are not needed for
booting or for repairing the system and which are not
installed locally should be placed in this directory.
系统预装的一些可执行程序，随系统升级会改变
/usr/local
This is where programs which are local to the site typically
go.
/usr/local/bin
Binaries for programs local to the site.
用户安装的可执行程序，不受系统升级影响，用户编译安装软件时，一般放到/usr/local目录下
/usr/local/sbin
Locally installed programs for system administration.
