Editing OpenVMS (section)

===Command Language Interpreter===

An OpenVMS Command Language Interpreter (CLI) implements a [[command-line interface]] for OpenVMS, responsible for executing individual commands and ''command procedures'' (equivalent to [[shell script]]s or [[batch file]]s).<ref name="user-guide">{{cite web|url=https://vmssoftware.com/docs/VSI_USERS_MANUAL.pdf|title=OpenVMS User's Manual|at=Chapter 14, Advanced Programming with DCL|website=VSI|access-date=April 9, 2021|date=July 2020}}</ref> The standard CLI for OpenVMS is the [[DIGITAL Command Language]], although other options are available.

Unlike [[Unix shell]]s, which typically run in their own isolated process and behave like any other user-mode program, OpenVMS CLIs are an optional component of a process, which exist alongside any executable image which that process may run.<ref name="dcl-supervisor-thread">{{cite newsgroup|url=https://comp.os.vms.narkive.com/Wc1uHZU3/how-dangerous-is-it-to-be-able-to-get-into-dcl-supervisor-mode|title=How dangerous is it to be able to get into DCL supervisor mode?|author=Simon Clubley|date=July 3, 2017|newsgroup=comp.os.vms|access-date=February 1, 2021}}</ref> Whereas a Unix shell will typically run executables by creating a separate process using [[fork-exec]], an OpenVMS CLI will typically load the executable image into the same process, transfer control to the image, and ensure that control is transferred back to CLI once the image has exited and that the process is returned to its original state.<ref name="idsm-5.2" />

Because the CLI is loaded into the same address space as user code, and the CLI is responsible for invoking image activation and image rundown, the CLI is mapped into the process address space at supervisor access mode, a higher level of privilege than most user code. This is in order to prevent accidental or malicious manipulation of the CLI's code and data structures by user-mode code.<ref name="idsm-5.2"/><ref name="dcl-supervisor-thread" />