/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.
/usr/sbin This directory contains program binaries for system
administration which are not essential for the boot process, for mounting
/usr, or for system repair.