/bin: Its critical directory used to bring the system online in single-user mode to repair it. This contains executable programs(can say scripts) for this. /sbin: This directory holds commands needed to boot the system in normal condition but not executed or used by normal users. /usr: This is one of the largest directories in the Linux system, mostly mounted from a separate partition. Birnies and files all programs are installed reside here. /usr/bin: This directory contains programs, executables and scripts not used for boot process but used by users to execute. Most of the programs or executable executed by users rather than root /usr/sbin: Program binaries or executables required for a system administrator is kept under this directory. This program binaries or executables are not required for boot process or normal users.