z/OS® UNIX® System Services (z/OS UNIX) allows z/OS users to create UNIX file systems and file system directory trees on z/OS, and to access UNIX files on z/OS and other systems.

Think of a UNIX file system as a container that holds part of the entire UNIX directory tree. Unlike a traditional z/OS library, a UNIX file system is hierarchical and byte-oriented. To find a file in a UNIX file system, you search one or more directories (see Figure 1). There is no concept of a z/OS catalog that points directly to a file.

Figure 1. A hierarchical file system structure

In z/OS, a UNIX file system is mounted over an empty directory by the system programmer (or a user with mount authority).

You can use the following file system types with z/OS UNIX:

• zSeries® File System (zFS), which is a file system that stores files in VSAM linear data sets.
• Hierarchical file system (HFS), a mountable file system, which is being phased out by zFS.
• z/OS Network File System (z/OS NFS), which allows a z/OS system to access a remote UNIX (z/OS or non-z/OS) file system over TCP/IP, as if it were part of the local z/OS directory tree.
• Temporary file system (TFS), which is a temporary, in-memory physical file system that supports in-storage mountable file systems.

As with other UNIX file systems, a path name identifies a file and consists of directory names and a file name. A fully qualified file name, which consists of the name of each directory in the path to a file plus the file name itself, can be up to 1023 bytes long.

/dir1/dir2/dir3/MyFile

Like UNIX, z/OS UNIX is case-sensitive for file and directory names. For example, in the same directory, the file MYFILE is a different file than MyFile.

The files in a hierarchical file system are sequential files, and are accessed as byte streams. A record concept does not exist with these files other than the structure defined by an application.

The zFS data set that contains the UNIX file system is a z/OS data set type (a VSAM linear data set). zFS data sets and z/OS data sets can reside on the same DASD volume. z/OS provides commands for managing zFS space utilization.

The integration of the zFS file system with existing z/OS file system management services provides automated file system management capabilities that might not be available on other UNIX platforms. This integration allows file owners to spend less time on tasks such as backup and restore of entire file systems.