The Unix File System (UFS) comes in several variations and can be found in many types of UNIX systems, including FreeBSD, HP-UX, NetBSD, OpenBSD, Apple OS X, and Sun Solaris. Many OSes have modified one or more data structures over the years to suit their needs, but they all have the same concepts. Currently, the two major variations are UFS1 and UFS2. UFS2 supports larger disks and larger time stamps. I will use the term UFS to refer to both file systems. An investigator might encounter a UFS file system when investigating a Unix system, typically a server. Ext2 and Ext3 are based on UFS, and because they were already discussed in detail, this chapter will be briefer and assume that you understand the concepts from Chapter 14, "Ext2 and Ext3 Concepts and Analysis." This chapter covers the concepts and analysis techniques of a UFS file system, and Chapter 17, "UFS1 and UFS2 Data Structures," covers the data structures. The next chapter can be read in parallel with this chapter or in series.