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Control structures are fundamental components in any programming language, including DCL (Digital Command Language) scripting on OpenVMS. They allow you to control the flow of execution in your scripts, making them more dynamic and responsive to different conditions. In this section, we will cover the following control structures:

Conditional Statements
Loops
Branching

Conditional Statements

Conditional statements allow you to execute certain parts of your script based on specific conditions. The primary conditional statement in DCL is the IF statement.

IF Statement

The IF statement evaluates a condition and executes a block of code if the condition is true.

Syntax:

$ IF condition THEN
$     command
$ ELSE
$     command

Example:

$ SET VERIFY
$ number = 10
$ IF number .GT. 5 THEN
$     WRITE SYS$OUTPUT "Number is greater than 5"
$ ELSE
$     WRITE SYS$OUTPUT "Number is 5 or less"

Explanation:

SET VERIFY enables command verification, which displays each command before it is executed.
number = 10 assigns the value 10 to the variable number.
The IF statement checks if number is greater than 5 (.GT. stands for "greater than").
If the condition is true, it prints "Number is greater than 5"; otherwise, it prints "Number is 5 or less".

Loops

Loops allow you to execute a block of code multiple times. DCL supports several types of loops, including FOR, WHILE, and GOTO loops.

FOR Loop

The FOR loop iterates over a range of values.

Syntax:

$ FOR variable IN start TO end DO
$     command

Example:

$ SET VERIFY
$ FOR i IN 1 TO 5 DO
$     WRITE SYS$OUTPUT "Iteration: ", i

Explanation:

The FOR loop iterates from 1 to 5.
In each iteration, it prints the current value of i.

WHILE Loop

The WHILE loop continues to execute as long as a specified condition is true.

Syntax:

$ WHILE condition DO
$     command

Example:

$ SET VERIFY
$ count = 1
$ WHILE count .LE. 5 DO
$ BEGIN
$     WRITE SYS$OUTPUT "Count: ", count
$     count = count + 1
$ END

Explanation:

The WHILE loop checks if count is less than or equal to 5 (.LE. stands for "less than or equal").
If true, it prints the current value of count and increments count by 1.
The loop continues until count exceeds 5.

Branching

Branching allows you to jump to different parts of your script based on conditions or labels.

GOTO Statement

The GOTO statement transfers control to a specified label within the script.

Syntax:

$ GOTO label

Example:

$ SET VERIFY
$ number = 3
$ IF number .EQ. 3 THEN GOTO label1
$ WRITE SYS$OUTPUT "This will not be printed"
$label1:
$ WRITE SYS$OUTPUT "Jumped to label1"

Explanation:

The IF statement checks if number is equal to 3 (.EQ. stands for "equal").
If true, it jumps to label1.
The script prints "Jumped to label1" and skips the line "This will not be printed".

Practical Exercise

Exercise 1: Conditional Statements

Task: Write a DCL script that checks if a number is positive, negative, or zero and prints an appropriate message.

Solution:

$ SET VERIFY
$ number = -5
$ IF number .GT. 0 THEN
$     WRITE SYS$OUTPUT "Number is positive"
$ ELSE IF number .LT. 0 THEN
$     WRITE SYS$OUTPUT "Number is negative"
$ ELSE
$     WRITE SYS$OUTPUT "Number is zero"

Exercise 2: Loops

Task: Write a DCL script that prints the numbers from 1 to 10 using a WHILE loop.

Solution:

$ SET VERIFY
$ count = 1
$ WHILE count .LE. 10 DO
$ BEGIN
$     WRITE SYS$OUTPUT "Number: ", count
$     count = count + 1
$ END

Exercise 3: Branching

Task: Write a DCL script that uses a GOTO statement to jump to a label if a variable is equal to a specific value.

Solution:

$ SET VERIFY
$ value = 7
$ IF value .EQ. 7 THEN GOTO found
$ WRITE SYS$OUTPUT "Value not found"
$found:
$ WRITE SYS$OUTPUT "Value found"

Conclusion

In this section, we covered the essential control structures in DCL scripting: conditional statements, loops, and branching. These constructs allow you to control the flow of your scripts, making them more flexible and powerful. Practice the provided exercises to reinforce your understanding, and you'll be well-prepared to handle more complex scripting tasks in OpenVMS.

Control Structures

Conditional Statements

IF Statement

Loops

FOR Loop

WHILE Loop

Branching

GOTO Statement

Practical Exercise

Exercise 1: Conditional Statements

Exercise 2: Loops

Exercise 3: Branching

Conclusion

OpenVMS Programming Course

Module 1: Introduction to OpenVMS

Module 2: Basic OpenVMS Commands

Module 3: OpenVMS File System

Module 4: Scripting with DCL

Module 5: OpenVMS System Management

Module 6: Networking on OpenVMS

Module 7: Advanced OpenVMS Programming

Module 8: OpenVMS Clustering

Module 9: OpenVMS Security

Module 10: Troubleshooting and Optimization