Appendix I. Important System Directories

Sysadmins and anyone else writing administrative scripts should be intimately familiar with the following system directories.

[148] Some early UNIX systems had a fast, small-capacity fixed disk (containing /, the root partition), and a second drive which was larger, but slower (containing /usr and other partitions). The most frequently used programs and utilities therefore resided on the small-but-fast drive, in /bin, and the others on the slower drive, in /usr/bin.

This likewise accounts for the split between /sbin and /usr/sbin, /lib and /usr/lib, etc.