|Document revision date: 15 July 2002|
A loop is a group of statements that execute repeatedly until a condition is met. A loop works as follows:
To write a loop, follow this procedure:
|1||Begin the loop with a label.|
|2||Test a variable to determine whether you need to execute the commands in the loop.|
|3||If you do not need to execute the loop, go to the end of the loop.|
|4||If you need to execute the loop, perform the commands in the body of the loop, then return to the beginning of the loop.|
|5||End the loop.|
The following example shows the usage of loops in the CLEANUP.COM command procedure:
$ GET_COM_LOOP: $ INQUIRE COMMAND- $ "Enter command (DELETE, DIRECTORY. EXIT, PRINT, PURGE, TYPE)" $ IF COMMAND .EQS. "EXIT" THEN GOTO END_LOOP $! $! Execute if user entered DELETE $ DELETE: $ IF COMMAND .NES. "DELETE" THEN GOTO DIRECTORY $ WRITE SYS$OUTPUT "This is the DELETE section." $ GOTO GET_COM_LOOP . . . $ END_LOOP: $ WRITE SYS$OUTPUT "Directory ''F$DIRECTORY()' has been cleaned" $ EXIT
Once a command executes, control is passed back to the GET_COM_LOOP
label until a user enters the EXIT command. When an EXIT command is
entered, the procedure outputs a message stating that the directory has
13.5.4 Step 4: End the Command Procedure
To end a command procedure, follow this procedure:
|1||Decide where you might need to exit or quit from the command procedure.|
|2||Place EXIT or STOP commands as appropriate.|
You can put an EXIT command in your command procedure to:
The following is an example of using an EXIT command to avoid executing an error handling routine that is located at the end of a procedure:
. . . $ EXIT ! End of normal execution path $ ERROR_ROUTINE . . .
The following is an example of using the EXIT command to end a procedure that has more than one execution path:
$ START: $ IF P1 .EQS. "TAPE" .OR. P1 .EQS. "DISK" THEN GOTO 'P1' $ INQUIRE P1 "Enter device (TAPE or DISK)" $ GOTO START $ TAPE: !Process tape files . . . $ EXIT $ DISK: ! Process disk files . . . $ EXIT
The commands following each of the labels (TAPE and DISK) provide different paths though the procedure. The EXIT command before the DISK label ensures that the commands after the DISK label do not execute unless the procedure branches explicitly to the label.
The EXIT command is not required at the end of procedures because the
end-of-file of the procedure causes an implicit EXIT command. However,
Compaq recommends use of the EXIT command.
184.108.40.206 Using the STOP Command
You can use the STOP command in a command procedure to ensure that the procedure terminates if a severe error occurs. If the STOP command is in a command procedure that is executed interactively, control is returned to the DCL level. If a command procedure is being executed in batch mode, the batch job terminates.
This command line tells the procedure to stop if a severe error occurs:
$ ON SEVERE_ERROR THEN STOP
Once you have written the code using program stubs, you should test the overall logic of the command procedure. You should test all possible paths of execution.
Follow this procedure to test and debug command procedures:
|1||Test the program logic by entering each valid command in the command procedure.|
|2||Continue testing the program logic by entering an invalid command.|
|3||Finish testing the program logic by exiting from the command procedure using the EXIT command.|
|4||If necessary, debug the program using the SET VERIFY, SET PREFIX, or SHOW SYMBOL commands.|
The following example shows how to test the command procedure by entering and executing every possible command, an invalid command, and then exiting:
$ @CLEANUP Enter command (DELETE, DIRECTORY, EXIT, PRINT, PURGE, TYPE): DELETE This is the DELETE section. Enter command (DELETE, DIRECTORY, EXIT, PRINT, PURGE, TYPE): DIRECTORY This is the DIRECTORY section. . . . Enter command (DELETE, DIRECTORY, EXIT, PRINT, PURGE, TYPE): PRINF You have entered an invalid command. Enter command (DELETE, DIRECTORY, EXIT, PRINT, PURGE, TYPE): EXIT $
You can use the following commands to help debug command procedures:
Example: Debugging Using the SET VERIFY Command
In the following example, the label END_LOP is spelled incorrectly. You can see exactly where the error is because verification is turned on:
$ SET VERIFY $ @CLEAN $ GET_COM_LOOP: $ INQUIRE COMMAND - "Enter command (DELETE, DIRECTORY, EXIT, PRINT, PURGE, TYPE)" Enter command (DELETE, DIRECTORY, EXIT, PRINT, PURGE, TYPE): EXIT $ IF COMMAND .EQS. "EXIT" THEN GOTO END_LOP %DCL-W-USGOTO, target of GOTO not found - check spelling and presence of label
To correct the error, change the label to END_LOOP.
Example: Debugging Using the SET PREFIX Command
The following example illustrates the use of time-stamping:
$ SET VERIFY $ @TEST $ SET DEFAULT SYS$LOGIN $ SHOW DEFAULT USER$:[SMYTHE] $ SET PREFIX "(!5%T) " $ @TEST (17:52) $ SET DEFAULT SYS$LOGIN (17:52) $ SHOW DEFAULT USER$:[SMYTHE]
Example: Debugging Using the SHOW SYMBOL Command
The following example shows how the SHOW SYMBOL command is used to determine how the symbol COMMAND is defined:
$ SET VERIFY $ @CLEAN $ GET_COM_LOOP: $ INQUIRE COMMAND - "ENTER COMMAND (DELETE, DIRECTORY, EXIT, PRINT, PURGE, TYPE)" ENTER COMMAND (DELETE, DIRECTORY, EXIT, PRINT, PURGE, TYPE): EXIT $ SHOW SYMBOL COMMAND COMMAND = "EXIT" $ IF COMMAND .EQS. "exit" THEN GOTO END_LOOP . . .
The SHOW SYMBOL command reveals that the symbol COMMAND has the value
"EXIT". Because the INQUIRE command automatically converts input to
uppercase and the IF statement that tests the command uses lowercase
characters in the string "exit", DCL determines that the strings are
not equal. To correct the error, make sure that the quoted string in
the IF statement is written in capital letters. The rest of the string
can use either uppercase or lowercase letters.
220.127.116.11 Enabling Verification During Execution
You can also interrupt a command procedure while it is executing to enable verification. As long as the command procedure does not contain the SET VERIFY command or a Ctrl/Y key sequence, you can enable verification by following these steps:
|1||Press Ctrl/Y to interrupt execution.|
|2||Enter the SET VERIFY command.|
|3||Enter the CONTINUE command to continue execution of the command procedure (with verification enabled).|
In general, execution of a command procedure should not change the user's process state. Therefore, a command procedure should include a set of commands that return the process to its original state. This set of commands is usually part of a subroutine that is labeled "CLEAN_UP". Common cleanup operations include closing files and resetting the default device and directory.
Follow this procedure to add cleanup tasks to your command procedure:
|1||Begin the cleanup subroutine with a label, such as CLEAN_UP.|
|2||Test for any open files using the F$GETJPI lexical function.|
|3||Delete any temporary or extraneous files using the DELETE or PURGE command.|
|4||If you have changed any defaults (such as the device or directory), restore them to their original state using the SET DEFAULT command.|
|5||Include an ON CONTROL_Y statement to ensure that the cleanup operations are performed.|
If you have any open files, make sure that they are closed before the procedure exits. You can use the lexical function F$GETJPI to examine the remaining open file quota (FILCNT) for the process. If FILCNT is the same at the beginning and end of the command procedure, you know that no files have been left open.
These are the commands that you would use to warn a user that a file has been left open:
$ FIL_COUNT = F$GETJPI ("","FILCNT") . . . $ IF FILCNT .NE. F$GETJPI ("", "FILCNT") THEN- WRITE SYS$OUTPUT "WARNING -- file left open)
If you have created temporary files, delete them. In general, if you have updated any files, you should purge them to delete the previous copies. Before you delete files you have not created, make sure you want to delete them. For example, if you have updated a file that contains crucial data, you might want to make the purging operation optional.
If you change the default device, the directory, or both, reset the original defaults before the command procedure exits. To save the name of the original default directory, use the DEFAULT keyword of the F$ENVIRONMENT lexical function. At the end of the command procedure, include a SET DEFAULT command that restores the saved device and directory.
The command lines shown in this example save and restore the device and directory defaults:
$ SAV_DEFAULT = F$ENVIRONMENT ("DEFAULT") . . . $ SET DEFAULT 'SAV_DEFAULT'
The following table lists other commonly changed process characteristics, the lexical functions used to save them, and the lexical function or command used to restore them:
Used to Save
Used to Restore
|DCL prompt||F$ENVIRONMENT||SET PROMPT|
|Default protection||F$ENVIRONMENT||SET PROTECTION/DEFAULT|
|Privileges||F$SETPRV||F$SETPRV or SET PROCESS/PRIVILEGES|
|Control characters||F$ENVIRONMENT||SET CONTROL|
|Message format||F$ENVIRONMENT||SET MESSAGE|
|Key state||F$ENVIRONMENT||SET KEY|
For complete descriptions of these lexical functions, refer to the
OpenVMS DCL Dictionary.
18.104.22.168 Ensuring Cleanup Operations Are Performed
To ensure that cleanup operations are performed even if the command procedure is aborted, begin each command level in the command procedure with the following statement:
$ ON CONTROL_Y THEN GOTO CLEANUP
For additional information on using the ON CONTROL_Y command, see
13.5.7 Step 7: Complete the Command Procedure
When your general design works correctly, follow these steps to complete your command procedure:
|1||Substitute commands for the first program stub in the command procedure.|
|2||Test the command procedure to make sure that the new commands work properly.|
|3||Debug the command procedure, if necessary.|
|4||When the first program stub works, move to the next one, and so on, until all program stubs have been replaced.|
Example: Replacing a Program Stub with Commands
The following example shows the code for the TYPE section of CLEANUP.COM:
$! Execute if user entered TYPE $! TYPE: $ IF COMMAND .NES. "TYPE THEN GOTO ERROR $ INQUIRE FILE "File to type" $ TYPE 'FILE' $ GOTO GET_COM_LOOP
This would replace the existing code:
$ WRITE SYS$OUTPUT "This is the TYPE section."
Example: CLEANUP.COM Command Procedure
Following is an example of the completed CLEANUP.COM command procedure:
$ GET_COM_LOOP: $ INQUIRE COMMAND - "Enter command (DELETE, DIRECTORY, EXIT, PRINT, PURGE, TYPE)" $ IF COMMAND .EQS. "EXIT" THEN GOTO END_LOOP $! $!Execute if user entered DELETE $ DELETE: $ IF COMMAND .NES. "DELETE" THEN GOTO DIRECTORY $ INQUIRE FILE "File to delete? " $ DELETE 'FILE' $ GOTO GET_COM_LOOP $! $!Execute if user entered DIRECTORY $ DIRECTORY: $ IF COMMAND .NES. "DIRECTORY" THEN GOTO PRINT $ DIRECTORY $ GOTO GET_COM_LOOP $! $!Execute if user entered PRINT $ PRINT: $ IF COMMAND .NES. "PRINT" THEN GOTO PURGE $ INQUIRE FILE "File to print? " $ PRINT SYS$OUTPUT 'FILE' $ GOTO GET_COM_LOOP $! $!Execute if user entered PURGE $ PURGE: $ IF COMMAND .NES. "PURGE" THEN GOTO TYPE $ PURGE $ GOTO GET_COM_LOOP $! $!Execute if user entered TYPE $ TYPE: $ IF COMMAND .NES. "TYPE" THEN GOTO ERROR $ INQUIRE FILE "File to type" $ TYPE 'FILE' $ GOTO GET_COM_LOOP $! $ ERROR: $ WRITE SYS$OUTPUT "You entered an invalid command." $ GOTO GET_COM_LOOP $! $ END_LOOP: $ WRITE SYS$OUTPUT "Directory ''F$DIRECTORY()' has been cleaned." $ $ EXIT
To make a command procedure run, you must execute it. You can execute command procedures:
The following sections describe each of these methods.
13.6.1 Executing Command Procedures from Within Other Command Procedures
You can execute another command procedure from within a command procedure by including an execute procedure (@) command .
The following command procedure, WRITEDATE.COM, invokes the command procedure GETDATE.COM:
$! WRITEDATE.COM $! $ INQUIRE TIME "What is the current time in hh:mm format?" $ @GETDATE [JONES.COM]GETDATE.COM
You can use the TYPE command to execute command procedures in the top-level directory of another account on a remote node. You can execute command procedures that:
Enter the TYPE command followed by an access control string. Use the following format:
$ TYPE nodename"username password"::"TASK=command_procedure"
The variables username and password are the user name and password for the account on the remote node.
This command procedure displays the users logged in to the remote node on which the command procedure resides:
$!SHOWUSERS.COM $! $ IF F$MODE() .EQS. "NETWORK" THEN DEFINE/USER SYS$OUTPUT SYS$NET $ SHOW USERS
In the following example, SHOWUSERS.COM is located in the top-level directory of BIRD's account on node ORIOLE, and the password is BOULDER. SHOWUSERS.COM executes the DCL command SHOW USERS on the remote node ORIOLE. The TYPE command displays the output from SHOWUSERS.COM on the local node; that is, on the terminal from which you enter the type command:
$ TYPE ORIOLE"BIRD BOULDER"::"TASK=SHOWUSERS" OpenVMS User Processes at 11-DEC-1999 17:20:13.30 Total number of users = 4, number of processes = 4 Username Node Interactive Subprocess Batch FLICKER AUTOMA 2 1 ROBIN FABLES 1 2 1 DOVE MURMUR 1 DUCK FABLES 1 1
Your password will be visible on your terminal when you use the TYPE
command with an access control string. Take the appropriate security
precautions as described in Chapter 18.
13.6.3 Executing Command Procedures with DCL Qualifiers or Parameters
You can create a command procedure that specifies DCL command parameters or qualifiers. This type of command procedure is useful when there is a set of parameters or qualifiers that you use frequently with one or more commands.
Enter the execute procedure command (@) in a command line where you would normally specify qualifiers or parameters.
This command procedure can be used to enter a set of qualifiers to the LINK command:
$! This command procedure contains command $! qualifiers for the LINK command. $! /DEBUG/SYMBOL_TABLE/MAP/FULL/CROSS_REFERENCE
This command line links an object named SYNAPSE.OBJ, using the qualifiers specified in DEFLINK.COM:
$ LINK SYNAPSE@DEFLINK
This command procedure can be used to enter the parameters CHAP1.TXT, CHAP2.TXT, and CHAP3.TXT with a DCL command:
$! PARAM.COM $! This command procedure contains a list of $! parameters that can be used with commands. $! CHAP1, CHAP2, CHAP3
This command line specifies the command procedure PARAM in place of a list of parameters. In the following example, the parameters are the file names listed in PARAM.COM:
$ DIRECTORY/SIZE @PARAM
When using the execute procedure command (@), the entire specified file is treated as command input by DCL.
|privacy and legal statement|