OpenVMS logicals, also known as logical names, are a key feature of the OpenVMS operating system that provide a flexible and convenient way to reference files, directories, devices, and other system resources using symbolic names instead of their physical locations or identifiers. Logical names can be thought of as aliases or shortcuts that simplify and abstract the way users and applications access and utilize resources within the system.
Here are some important aspects of OpenVMS logicals:
Naming: Logical names are typically composed of uppercase letters, numbers, and underscores. They can be up to 255 characters long and follow specific naming conventions. Logical names can be defined either system-wide or on a per-process basis.
Definition: Logical names are defined using the Logical Name Table (LNT) or via the SET LOGICAL command. The LNT is a system-wide database that stores logical name definitions. When a logical name is defined, it is associated with a specific value, which can be a file path, directory, device, command, or any other system resource.
Dynamic Assignment: Logical names can be dynamically assigned values at runtime. This means that their values can change during the execution of a process or depending on the system configuration. Dynamic assignment provides flexibility and adaptability to changing environments.
Scope: Logical names can have different scopes. Global logicals are visible and accessible to all processes in the system. Process logicals are specific to a particular process and are not visible or accessible by other processes.
Search Lists: OpenVMS supports search lists for logical names. A search list is an ordered collection of logical names that point to different locations. When a logical name is referenced, OpenVMS searches through the list in order until it finds a match. This allows for dynamic resolution and redirection of resources based on the current system configuration.
Logical Names as Parameters: Logical names can be used as parameters in command-line instructions or within application programs. This allows for flexible and portable code, as the logical names can be easily redefined without modifying the code itself.
System-Defined Logical Names: OpenVMS comes with a set of system-defined logical names that provide shortcuts to commonly used system resources. These logical names, such as SYS$DISK, SYS$SYSROOT, and SYS$LOGIN, allow users and applications to access important directories, devices, and files without needing to know their physical locations.
By using logical names, OpenVMS provides a level of abstraction and indirection, allowing for easier administration, portability, and maintenance of the system. Logical names simplify the way users and applications interact with resources, enhance flexibility, and provide a consistent interface across different system configurations.