The /usr directory consists of several subdirectories that contain additional UNIX commands and data files. It is also the default location of user home directories.
The /usr/bin directory contains more UNIX commands. These commands are used less frequently or are considered nonessential to UNIX system operation.
The /usr/include directory contains header files for compiling C programs.
The /usr/lib directory contains more libraries and data files used by various UNIX commands.
The /usr/spool directory contains various directories for storing files to be printed, mailed, or passed through networks.
The /usr/tmp directory contains more temporary files.
The /usr/adm directory contains data files associated with system administration and accounting. In particular, the /usr/adm/messages and /usr/adm/syslog files contain a record of system error messages, including those sent to the system console. These files are especially useful for locating system problems, including hardware problems. For example, an unusual number of disk errors on a drive indicates a defective or misaligned drive. Because messages in the file can accumulate rapidly, these files must be deleted periodically. See ``Checking and clearing system log files'' for more information.