z/OS UNIX provides a hierarchical file system (HFS) for z/OS. z/OS UNIX also provides the z/OS File System (zFS). An HFS or zFS file within z/OS UNIX is called a z/OS UNIX file. HFS and zFS files are organized in a hierarchy of files and directories in a tree structure. A directory can contain files or other sub-directories. The highest level directory is called the root directory. Figure 1 shows an example of mounting an HFS or zFS directory from a UNIX client.

Figure 1. Example of mounting an HFS or zFS file from a UNIX client

A z/OS UNIX file system must be mounted by an MVS system operator using a TSO MOUNT command before that z/OS UNIX file system can be mounted by an NFS client through the z/OS NFS server. If a z/OS UNIX file system is mounted after the NFS client mount is already established, this file system's directory structure and contents will not be visible to the remote NFS Client until the NFS mount is unmounted and remounted or the z/OS NFS Server is restarted.

z/OS UNIX files are byte-oriented rather than record-oriented (unlike z/OS conventional MVS data sets). This data can be shared with TSO/E z/OS UNIX users in addition to NFS clients. All data written to a z/OS UNIX file can be read by all programs as soon as it is written. You can also copy data between z/OS UNIX files and MVS data sets using z/OS UNIX utilities like ISHELL.