HP OpenVMS Systems Documentation

OpenVMS User's Manual

Note the following:

• The logical name DCL$PATH can be a search-list type logical.
• Only the node, device, and directory portions of each translation of the logical name are used.
• Normal logical precedence takes place. Users can override a system definition of DCL$PATH by defining their own. If a system definition exists and the user does not want the feature, it can be turned off by overriding the logical with a definition of " ".
• The set of valid characters for DCL verbs and symbol names differs from the set of valid characters for file names. For example, DCL symbols cannot contain a hyphen (-) or start with a dollar sign ($). If the image or procedure you wish to execute is not valid as a DCL symbol name, it cannot be directly invoked by this new feature.
• DCL has not parsed the command. It is up to the image being invoked to perform its own command parsing. For C programs, use the "argc" and "argv" parameters to the main() routine. For programs written in other languages, call LIB$GET_FOREIGN to obtain the entire command line, which must then be parsed by the program.
• If a directory contains both a command procedure and an executable image, whichever file is found first will be invoked. On OpenVMS systems, directories are in alphabetical order, so a ".COM" file will be found before a ".EXE" file. A network file specification in the DCL$PATH logical pointing to a node running some other operating systems could result in a ".EXE" file being found before a ".COM" file.
  Because DCL performs the search with the invalid verb as the file specification and "DCL$PATH:.*" as the default file specification, it is possible to define a logical in such a way that a specific file is found. For example, if you define the logical FOO to be "FOO.EXE", and type "FOO" at the DCL prompt, you will never invoke FOO.COM, only FOO.EXE.

Note the following restrictions:

• You cannot use automatic foreign commands on any versions of the OpenVMS operating system prior to Version 6.2.
• Because new verbs can be added to the DCL command table at any time, a command that works with automatic foreign commands one day may not work at a later date.
• The automatic foreign commands feature does not work in all cases. In the following example, DCL (which looks only at the first four characters of any verb) finds a match with the SHOW verb (the first four letters of SHOWME) and executes the SHOW USERS command instead of the SHOWME.COM procedure. If you defined SHOWME as a DCL symbol, then the SHOWME command would invoke SHOWME.COM.

  $ DEFINE DCL$PATH SYS$SYSTEM,SYS$DISK:[]FOO $ TYPE SHOWME.COM $ SHOW SYMBOL P1 $ EXIT $ SHOWME USERS OpenVMS User Processes at MARCH 2, 1999 01:40 PM Total number of users = 1, number of processes = 11 Username Interactive Subprocess Batch RSMITH 9 2

A command procedure is a file that contains DCL commands and data lines used by DCL commands. Some simple command procedures might contain only one or two DCL commands; complex command procedures can function as sophisticated computer programs. When a command procedure runs, the DCL interpreter reads the file and executes the commands it contains.

If your system manager has set up a system login command procedure, it is executed whenever you log in. A system login command procedure lets your system manager ensure that certain commands are always executed when you and other users on the system log in.

After running the system login command procedure, the system runs your personal login command procedure, if one exists. Your personal login command procedure lets you customize your computing environment. The commands contained in it are executed every time you log in. When you log in, the system automatically executes up to two login command procedures (the systemwide login command procedure and your own login command procedure, if it exists).

The person who sets up your account might have placed a login command procedure in your top-level directory. If a login command procedure is not in your top-level directory, you can create one yourself. Name it LOGIN.COM and place it in your top-level directory. Unless your system manager tells you otherwise, the LOGIN.COM file that you create will run whenever you log in.

This chapter is divided into major sections that include the following:

• Basic information for writing command procedures
• Step-by-step procedure for writing command procedures
• Executing command procedures
• Exiting, interrupting, and error handling command procedures
• Login command procedures

There are two types of DCL command procedures:

• Simple
  Execute a series of DCL commands in the order in which they are written
• Complex
  Perform program-like functions

There are two ways to create command procedures:

• Use a text editor such as EVE to create a new file
• Use the DCL command CREATE to create a new file

The file that you create can contain command lines, labels, comments, conditional statements, and variables.

13.1.1 Default File Type

The default file type for command procedures is .COM. If you specify the .COM file type when you name a command procedure, you can execute the procedure by specifying the file name only. The SUBMIT and execute procedure (@) commands assume the file type is .COM unless you specify otherwise.

13.1.2 Writing Commands

The following are suggestions for including commands in command procedures:

• Use complete names for commands and qualifiers. This will help to ensure that your command procedure is upwardly compatible to future releases of OpenVMS.
• Use continuation lines to make a procedure easier to read. Note that continuation lines do not begin with dollar signs. For example:

  $ PRINT LAB.DAT - /AFTER=17:00 - /COPIES=20 - /NAME="COMGUIDE"

When writing command lines:

• You must use a dollar sign ($) to begin each line containing a command, comment, or label.
• If you want to include a line containing data, omit the dollar sign ($) on that line.
• If you need to include a data line that begins with a dollar sign ($), use the DCL commands DECK and EOD. For example:

  $ ! Everything between the commands DECK and EOD $ ! is written to the file WEATHER.COM $ ! $ CREATE WEATHER.COM $ DECK $ FORTRAN SUMMER $ LINK SUMMER $ RUN SUMMER $ EOD $ ! $ ! Now execute WEATHER.COM $ @WEATHER $ EXIT

Note that command lines that do not begin with a dollar sign might be correctly interpreted by DCL, but Compaq strongly recommends that any DCL command line start with a dollar sign.

13.2 Using Labels in Command Lines

Labels are used in DCL command procedures to mark the beginning of loops, sections of code, or subroutines. Note the following rules when using labels:

• Put labels on separate lines to make loops, subroutines, and conditional code more visible.
• Use label names that contain fewer than 255 characters and no blank spaces.
• Differentiate labels from commands by placing labels immediately after the dollar sign ($) and by preceding commands with spaces.
• End each label with a colon.
• You cannot delete labels.

As the command interpreter encounters labels, it enters them in a special section of the local symbol table. The amount of space available for labels is limited. If a command procedure uses many symbols and contains many labels, the command interpreter might run out of symbol table space and issue an error message. If this occurs, include the DELETE/SYMBOL command in your procedure to delete symbols as they are no longer needed. (Note, however, that you cannot delete labels.)

13.2.2 Duplicate Labels

If a command procedure uses the same label more than once, the new definition replaces the existing one in the local symbol table.

When duplicate labels exist, the GOTO command transfers control to the label that DCL has processed most recently. The GOTO command also uses the following rules when processing duplicate labels:

• If all duplicate labels precede the GOTO command, control transfers to the label nearest the GOTO command.
• If duplicate labels precede and follow the GOTO command, control transfers to the preceding label nearest the GOTO command.
• If all duplicate labels follow the GOTO command, control transfers to the label nearest the GOTO command.

It is good programming practice to include comments in command procedures. Comments can be helpful when updating or troubleshooting the command procedure. Comments can be used as follows:

• At the beginning of a procedure to describe the procedure and the parameters passed to it.
• At the beginning of each block of commands to describe that section of the procedure.
• To separate command sequences with lines containing both a dollar sign and an exclamation point ($!). This makes it easier to see the outline of the command procedure. If you insert blank lines, the command interpreter interprets them as data lines and produces a message warning you that the data lines were ignored.

The following rules apply when writing comments in command procedures:

• Use an exclamation point (!) to indicate the beginning of a comment; the command interpreter ignores all text to the right of an exclamation point when the command procedure executes.
• To include a literal exclamation point in a command line, enclose the exclamation point in quotation marks (" ").

Before you begin writing a command procedure, perform the tasks interactively that the command procedure will execute. As you type the necessary commands, note any variables and conditionals that are used, and any iterations that occur.

The following sections contain the steps to write a simple command procedure. The example used throughout these sections is a command procedure called CLEANUP.COM. This procedure can be used to clean up a directory.

Definitions

• Variable
  Data that changes each time you perform a task.
• Conditional
  Any command or set of commands that can vary and therefore must be tested each time you perform the task.
• Iteration
  Any command or set of commands that are performed repetitively until a condition is met.

Follow these steps to write a command procedure:

13.5.1 Step 1: Design the Command Procedure

There are certain commands that are usually executed during clean up operations. The following table lists those commands and the tasks that they perform:

Variables

Any data that changes when you perform a task is a variable. If you create or delete files in your directory, the file names will be different each time you clean your directory; therefore, the file names in CLEANUP.COM are variables.

Conditionals

Any command that must be tested each time you execute a command procedure is considered conditional. Because any or all of the commands in CLEANUP.COM might be executed, depending on the operation you need to perform, each command is conditional.

Design Decisions

After you have determined what variables and conditionals you will use in the CLEANUP.COM command procedure, you must decide how to load the variables, test the conditionals, and exit from the command procedure. For the CLEANUP.COM command procedure, the following decisions have been made:

To make command procedures easier to understand and maintain, write statements so the procedures execute from the first command to the last command.

13.5.2 Step 2: Assign Variables and Test Conditionals

There are many ways to assign values to variables. In this section, we will discuss using the INQUIRE command. For additional methods, see Chapter 14.

Follow these steps to assign values to variables and test conditionals:

The INQUIRE command prompts for a value, reads the value from the terminal, and assigns the value to a symbol.

By default, the INQUIRE command:

• Converts responses to uppercase
• Replaces multiple blanks and tabs with a single space
• Removes leading and trailing spaces
• Performs apostrophe substitutions if the response includes symbols or lexical functions

The following command line is used in CLEANUP.COM to prompt the user for a command name. The INQUIRE command equates the value entered to the symbol COMMAND.

$ INQUIRE COMMAND-
  "Enter command (DELETE, DIRECTORY, PRINT, PURGE, TYPE)"

To preserve lowercase characters, multiple spaces and tabs when using the INQUIRE command, enclose your response in quotation marks (" "). To include quotation marks in your response, enclose the quoted text in quotation marks (""text"").

13.5.2.3 Testing Conditionals Using IF and THEN

After the INQUIRE command prompts for a variable, the command procedure must include a statement that determines what action is to be taken. For example, to determine which command to execute, you must include statements in the command procedure that check the command entered by the user against each possible command.

To test whether a condition is true, use the IF and THEN commands. The following table shows the possibilities that you must check for in CLEANUP.COM:

A program stub is a temporary section of code that you use in your procedure while you test the design. Usually, a program stub outputs a message stating the function that it is replacing. After the overall design works correctly, replace each stub with the correct coding.

Example: Assigning Variables and Testing Conditionals

The following example shows how to assign variables and test conditionals:

$ INQUIRE COMMAND-
  "Enter command (DELETE, DIRECTORY, EXIT, PRINT, PURGE, TYPE)"
$ IF COMMAND .EQS. "EXIT" THEN EXIT
$!
$! Execute if user entered DELETE
$ DELETE:
$    IF COMMAND .NES "DELETE" THEN GOTO DIRECTORY         (1) (2)
$    WRITE SYS$OUTPUT "This is the DELETE section."       (3)
$! Execute if user entered DIRECTORY
$ DIRECTORY:                                              (4)
$    IF COMMAND .NES "DIRECTORY" THEN GOTO PRINT
$    WRITE SYS$OUTPUT "This is the DIRECTORY section."
   .
   .
   .
$! Execute if user entered TYPE
$ TYPE:
$    IF COMMAND .NES "TYPE" THEN GOTO ERROR               (5)
$    WRITE SYS$OUTPUT "This is the TYPE section."
$!
$ ERROR:
$    WRITE SYS$OUTPUT "You have entered an invalid command." (6)
$!
$ EXIT

As you examine the example, note the following:

1. This IF statement tests to see if the command that the user entered (COMMAND) is equal to "DELETE". If COMMAND is equal to DELETE, then the command procedure executes the next command.
2. This statement also includes a GOTO command. A GOTO command is used to change the flow of execution to a label in the procedure. In this case, the procedure will go to the DIRECTORY label if COMMAND is not equal to DELETE.
3. This statement is a program stub. After the logic of the command procedure is tested, this line will be replaced with the actual commands required for a DELETE operation.
4. This is the label for the DIRECTORY subroutine. Note that the labels that identify each command block are the same as the commands on the option list. This allows you to use the symbol COMMAND (which is equated to the user's request) in the GOTO statement.
5. This IF statement tests to see if the "TYPE" command was entered. If "TYPE" was entered, the procedure will output "This is the TYPE section." However, because this is the last command you will be testing for, if the command entered is not "TYPE," the program will display an error message.
6. If all commands have been tested and no valid command name is found, then the program will output, "You have entered an invalid command."