DIGITAL Command Language
Encyclopedia
DCL, the DIGITAL Command Language, is the standard command language
adopted by most of the operating systems that were sold by the former Digital Equipment Corporation
(which has since been acquired by Compaq
which itself was acquired by Hewlett-Packard
). It had its roots in the IAS
, TOPS-20
, and RT-11
operating system
s and was implemented as a standard across most of Digital's operating systems, notably RSX-11
, but took its most powerful form in the OpenVMS
operating system.
Written when the Fortran
programming language
was in heavy use, DCL is a scripting language
supporting several datatypes, including strings, integers
, bit
arrays, arrays
and booleans, but not floating point
numbers. Access to OpenVMS
system services (kernel API) is through lexical functions, which perform the same as their compiled language
counterparts and allow scripts to get information on system state. DCL includes IF-THEN-ELSE
, access to all the RMS
file types including stream, indexed, and sequential, but unfortunately lacks a DO-WHILE
or other looping construct, requiring users to make do with IF and GOTO
-label statements instead.
For the OpenVMS implementation, the command line parser is a runtime library
(CLI$) that can be compiled into user applications and therefore gives a consistent command line interface for both operating system supplied commands and user written commands. The command line must start with a verb and is then followed by arguments and/or position independent qualifiers (switches in Unix
terminology), which can occur anywhere on the command line, or position dependent qualifiers which affect the parameter they appear after. Most qualifiers are position independent; that is, they can appear anywhere in the command line. Only the first most significant part of the verb and qualifier name is required. An example operating system command may look like:
set audit/alarm/enable=(authorization, breakin=all)
show device/files $1$DGA1424:
The second show command could also be typed as
sho dev $1$DGA1424:/fil
(While DCL documentation usually shows all DCL commands in uppercase, DCL commands are case-insensitive and may be typed in upper-, lower-, or mixed-case.)
Some implementations (such as OpenVMS) used a minimum uniqueness scheme in allowing commands to be shorted while others (such as RSX-11) allowed commands to be abbreviated to a minimum of three characters.
Unlike other systems which use paths
for locating commands, DCL requires commands to be defined explicitly, either via CLD (Command Language Definition) definitions or a foreign symbol. Most OpenVMS
-native commands are defined via CLD files; these are compiled by the CDU (Command Definition Utility) and added to a DCL 'table' -- SYS$LIBRARY:DCLTABLES.EXE by default, although processes are free to use their own tables -- and can then be invoked by the user. For example, defining a command FOO that accepts the option "/BAR" and is implemented by the image SYS$SYSEXE:FOO.EXE could be done with a CLD file similar to:
DEFINE VERB FOO
IMAGE "SYS$SYSEXE:FOO.EXE"
QUALIFIER BAR
The user can then type "FOO", or "FOO/BAR", and the FOO program will be invoked. The command definition language supports many types of options, for example dates and file specifications, and allows a qualifier to change the image invoked -- for example "CREATE", to create a file, vs. "CREATE/DIRECTORY" to create a directory.
The other (simpler, but less flexible) method to define commands is via foreign commands. This is more akin to the Unix method of invoking programs. By giving the command:
foo : $sys$sysexe:foo.exe
the command 'FOO' will invoke FOO.EXE, and supply any additional arguments literally to the program, for example, "foo -v". This method is generally used for programs ported from Unix and other non-native systems; for C programs using argc and argv command syntax.
Versions of OpenVMS
DCL starting with V6.2 support the DCL$PATH logical name for establishing Unix-style command paths. This mechanism is known as an Automatic Foreign Command. DCL$PATH allows a list of directories to be specified, and these directories are then searched for DCL command procedures (command.COM) and then for executable images (command.EXE) with filenames that match the command that was input by the user. Like traditional foreign commands, automatic foreign commands also allow Unix-style command input.
Scripting
DCL scripts look much like any other scripting language, with some exceptions. All DCL verbs in a script are preceded with a $ symbol; other lines are considered to be input to the previous command. For example, to use the TYPE command to print a paragraph onto the screen, one might use a script similar to:
$ TYPE SYS$INPUT:
This is an example of using the TYPE verb
in the DCL language.
$ EXIT
Indirect variable referencing
It is possible to build arrays in DCL that are referenced through translated symbols. This allows the programmer to build arbitrarily sized data structures using the data itself as an indexing function.
$ i = 1
$ variable'i' = "blue"
$ i = 2
$ variable'i' = "green"
$ j = 1
$ color = variable'j'
$ rainbow'color' = "red"
$ color = variable'i'
$ rainbow'color' = "yellow"
In this example the variable rainbowblue is assigned the value "red", and rainbowgreen is assigned the value "yellow".
Literature
External links
adopted by most of the operating systems that were sold by the former Digital Equipment Corporation
Digital Equipment Corporation
Digital Equipment Corporation was a major American company in the computer industry and a leading vendor of computer systems, software and peripherals from the 1960s to the 1990s...
(which has since been acquired by Compaq
Compaq
Compaq Computer Corporation is a personal computer company founded in 1982. Once the largest supplier of personal computing systems in the world, Compaq existed as an independent corporation until 2002, when it was acquired for US$25 billion by Hewlett-Packard....
which itself was acquired by Hewlett-Packard
Hewlett-Packard
Hewlett-Packard Company or HP is an American multinational information technology corporation headquartered in Palo Alto, California, USA that provides products, technologies, softwares, solutions and services to consumers, small- and medium-sized businesses and large enterprises, including...
). It had its roots in the IAS
RSX-11
RSX-11 is a family of real-time operating systems mainly for PDP-11 computers created by Digital Equipment Corporation , common in the late 1970s and early 1980s. RSX-11D first appeared on the PDP-11/40 in 1972...
, TOPS-20
TOPS-20
The TOPS-20 operating system by Digital Equipment Corporation was the second proprietary OS for the PDP-10 mainframe computer. TOPS-20 began in 1969 as the TENEX operating system of Bolt, Beranek and Newman...
, and RT-11
RT-11
RT-11 was a small, single-user real-time operating system for the Digital Equipment Corporation PDP-11 family of 16-bit computers...
operating system
Operating system
An operating system is a set of programs that manage computer hardware resources and provide common services for application software. The operating system is the most important type of system software in a computer system...
s and was implemented as a standard across most of Digital's operating systems, notably RSX-11
RSX-11
RSX-11 is a family of real-time operating systems mainly for PDP-11 computers created by Digital Equipment Corporation , common in the late 1970s and early 1980s. RSX-11D first appeared on the PDP-11/40 in 1972...
, but took its most powerful form in the OpenVMS
OpenVMS
OpenVMS , previously known as VAX-11/VMS, VAX/VMS or VMS, is a computer server operating system that runs on VAX, Alpha and Itanium-based families of computers. Contrary to what its name suggests, OpenVMS is not open source software; however, the source listings are available for purchase...
operating system.
Written when the Fortran
Fortran
Fortran is a general-purpose, procedural, imperative programming language that is especially suited to numeric computation and scientific computing...
programming language
Programming language
A programming language is an artificial language designed to communicate instructions to a machine, particularly a computer. Programming languages can be used to create programs that control the behavior of a machine and/or to express algorithms precisely....
was in heavy use, DCL is a scripting language
Scripting language
A scripting language, script language, or extension language is a programming language that allows control of one or more applications. "Scripts" are distinct from the core code of the application, as they are usually written in a different language and are often created or at least modified by the...
supporting several datatypes, including strings, integers
Integer (computer science)
In computer science, an integer is a datum of integral data type, a data type which represents some finite subset of the mathematical integers. Integral data types may be of different sizes and may or may not be allowed to contain negative values....
, bit
Bit
A bit is the basic unit of information in computing and telecommunications; it is the amount of information stored by a digital device or other physical system that exists in one of two possible distinct states...
arrays, arrays
Array data type
In computer science, an array type is a data type that is meant to describe a collection of elements , each selected by one or more indices that can be computed at run time by the program. Such a collection is usually called an array variable, array value, or simply array...
and booleans, but not floating point
Floating point
In computing, floating point describes a method of representing real numbers in a way that can support a wide range of values. Numbers are, in general, represented approximately to a fixed number of significant digits and scaled using an exponent. The base for the scaling is normally 2, 10 or 16...
numbers. Access to OpenVMS
OpenVMS
OpenVMS , previously known as VAX-11/VMS, VAX/VMS or VMS, is a computer server operating system that runs on VAX, Alpha and Itanium-based families of computers. Contrary to what its name suggests, OpenVMS is not open source software; however, the source listings are available for purchase...
system services (kernel API) is through lexical functions, which perform the same as their compiled language
Compiled language
A compiled language is a programming language whose implementations are typically compilers , and not interpreters ....
counterparts and allow scripts to get information on system state. DCL includes IF-THEN-ELSE
Conditional statement
In computer science, conditional statements, conditional expressions and conditional constructs are features of a programming language which perform different computations or actions depending on whether a programmer-specified boolean condition evaluates to true or false...
, access to all the RMS
Record Management Services
Record Management Services are procedures in the VMS, RSTS/E, RT-11 and high-end RSX-11 operating systems that programs may call to process files and records within files. VMS RMS is an integral part of the system software; its procedures run in executive mode...
file types including stream, indexed, and sequential, but unfortunately lacks a DO-WHILE
While loop
In most computer programming languages, a while loop is a control flow statement that allows code to be executed repeatedly based on a given boolean condition. The while loop can be thought of as a repeating if statement....
or other looping construct, requiring users to make do with IF and GOTO
Goto
goto is a statement found in many computer programming languages. It is a combination of the English words go and to. It performs a one-way transfer of control to another line of code; in contrast a function call normally returns control...
-label statements instead.
For the OpenVMS implementation, the command line parser is a runtime library
Library (computer science)
In computer science, a library is a collection of resources used to develop software. These may include pre-written code and subroutines, classes, values or type specifications....
(CLI$) that can be compiled into user applications and therefore gives a consistent command line interface for both operating system supplied commands and user written commands. The command line must start with a verb and is then followed by arguments and/or position independent qualifiers (switches in Unix
Unix
Unix is a multitasking, multi-user computer operating system originally developed in 1969 by a group of AT&T employees at Bell Labs, including Ken Thompson, Dennis Ritchie, Brian Kernighan, Douglas McIlroy, and Joe Ossanna...
terminology), which can occur anywhere on the command line, or position dependent qualifiers which affect the parameter they appear after. Most qualifiers are position independent; that is, they can appear anywhere in the command line. Only the first most significant part of the verb and qualifier name is required. An example operating system command may look like:
set audit/alarm/enable=(authorization, breakin=all)
show device/files $1$DGA1424:
The second show command could also be typed as
sho dev $1$DGA1424:/fil
(While DCL documentation usually shows all DCL commands in uppercase, DCL commands are case-insensitive and may be typed in upper-, lower-, or mixed-case.)
Some implementations (such as OpenVMS) used a minimum uniqueness scheme in allowing commands to be shorted while others (such as RSX-11) allowed commands to be abbreviated to a minimum of three characters.
Unlike other systems which use paths
Path (computing)
A path, the general form of a filename or of a directory name, specifies a unique location in a file system. A path points to a file system location by following the directory tree hierarchy expressed in a string of characters in which path components, separated by a delimiting character, represent...
for locating commands, DCL requires commands to be defined explicitly, either via CLD (Command Language Definition) definitions or a foreign symbol. Most OpenVMS
OpenVMS
OpenVMS , previously known as VAX-11/VMS, VAX/VMS or VMS, is a computer server operating system that runs on VAX, Alpha and Itanium-based families of computers. Contrary to what its name suggests, OpenVMS is not open source software; however, the source listings are available for purchase...
-native commands are defined via CLD files; these are compiled by the CDU (Command Definition Utility) and added to a DCL 'table' -- SYS$LIBRARY:DCLTABLES.EXE by default, although processes are free to use their own tables -- and can then be invoked by the user. For example, defining a command FOO that accepts the option "/BAR" and is implemented by the image SYS$SYSEXE:FOO.EXE could be done with a CLD file similar to:
DEFINE VERB FOO
IMAGE "SYS$SYSEXE:FOO.EXE"
QUALIFIER BAR
The user can then type "FOO", or "FOO/BAR", and the FOO program will be invoked. The command definition language supports many types of options, for example dates and file specifications, and allows a qualifier to change the image invoked -- for example "CREATE", to create a file, vs. "CREATE/DIRECTORY" to create a directory.
The other (simpler, but less flexible) method to define commands is via foreign commands. This is more akin to the Unix method of invoking programs. By giving the command:
foo : $sys$sysexe:foo.exe
the command 'FOO' will invoke FOO.EXE, and supply any additional arguments literally to the program, for example, "foo -v". This method is generally used for programs ported from Unix and other non-native systems; for C programs using argc and argv command syntax.
Versions of OpenVMS
OpenVMS
OpenVMS , previously known as VAX-11/VMS, VAX/VMS or VMS, is a computer server operating system that runs on VAX, Alpha and Itanium-based families of computers. Contrary to what its name suggests, OpenVMS is not open source software; however, the source listings are available for purchase...
DCL starting with V6.2 support the DCL$PATH logical name for establishing Unix-style command paths. This mechanism is known as an Automatic Foreign Command. DCL$PATH allows a list of directories to be specified, and these directories are then searched for DCL command procedures (command.COM) and then for executable images (command.EXE) with filenames that match the command that was input by the user. Like traditional foreign commands, automatic foreign commands also allow Unix-style command input.
Scripting
DCL scripts look much like any other scripting language, with some exceptions. All DCL verbs in a script are preceded with a $ symbol; other lines are considered to be input to the previous command. For example, to use the TYPE command to print a paragraph onto the screen, one might use a script similar to:
$ TYPE SYS$INPUT:
This is an example of using the TYPE verb
in the DCL language.
$ EXIT
Indirect variable referencing
It is possible to build arrays in DCL that are referenced through translated symbols. This allows the programmer to build arbitrarily sized data structures using the data itself as an indexing function.
$ i = 1
$ variable'i' = "blue"
$ i = 2
$ variable'i' = "green"
$ j = 1
$ color = variable'j'
$ rainbow'color' = "red"
$ color = variable'i'
$ rainbow'color' = "yellow"
In this example the variable rainbowblue is assigned the value "red", and rainbowgreen is assigned the value "yellow".
Literature