Accessing Variable Values

Implicit Dereferencing

Implicit dereferencing is simply the substitution of the variable's value for the variable name. Only certain commands use implicit dereferencing. Variables used in expressions in the IF, WHILE, SETVAR, and CALC commands are dereferenced implicitly.

In these commands, any non-numeric or unquoted string in any expression is assumed to be a variable name. (Exceptions to this are the boolean values TRUE and FALSE, and other special cases, such as WARN, FATAL, and SYSTEM.) In the following example, the variable CM_NAME is set implicitly with the contents of the variable CM_FIRST_NAME, set in a previous example.

   :SETVAR CM_NAME CM_FIRST_NAME

   :SHOWVAR CM_NAME

   CM_NAME = CAROL

Explicit Dereferencing

Any variable can be dereferenced explicitly by preceding the variable name with an exclamation point. Explicit dereferencing is available with every CI command. The variable's value rather than its name is substituted in the statement or expression. Explicit dereferencing is often used to include a variable value in an expression.

   :SETVAR X "Blue"

   :SETVAR Y "!X is the best color."

   :SHOWVAR X,Y

   X = Blue

   Y = Blue is the best color.

In the preceding example, the value of variable X is substituted when the value of variable Y is set. The word "Blue" is inserted in the expression where the variable name is dereferenced.

Recursive Dereferencing

Recursive dereferencing is used to set a variable with the dynamic value of another variable. The existing variable's value is not loaded into the new variable until the new variable itself is explicitly dereferenced, guaranteeing that the current value is inserted. Recursive dereferencing is valid only when explicit dereferencing is being used. To specify recursive dereferencing, precede the variable name with two or more exclamation points.

In the following example, the ECHO command demonstrates how a variable is dereferenced. ECHO requires explicit dereferencing to display the contents of a variable. The SHOWVAR command displays the variable name and the value it contains. Using the variables X and Y set in the previous example, notice how recursive dereferencing provides the most current value.

   :SETVAR Y "!X is the best color."

   :SETVAR Z "X is best."

   :SHOWVAR Y

   Y = Blue is the best color.

   :SHOWVAR Z

   Z = !X is best.

   :ECHO !Y 

   Blue is the best color.

   :ECHO !Z

   Blue is best.

   :SETVAR X "Red"

   :SHOWVAR Y

   Y = Blue is the best color.

   :SHOWVAR Z

   Z = !X is best.

   :ECHO !Y

   Blue is the best color.

   :ECHO !Z

   Red is best.

Note that X was not dereferenced fully when the variable Z was set because recursive dereferencing of the variable was specified. The value of X was substituted only when the ECHO command was executed, requiring the dereferencing of Z itself.

For the variable Y, however, X was dereferenced when its value was loaded to Y. The value of Y always remained the same and did not reflect any change when X was reset to a new value.

The predefined variable HPPATH is the default search path used to locate UDCs and command files when file names are not fully qualified. By default, it is composed of the current group, the current account's PUB group, and the PUB.SYS group. The current group is identified as the dereferenced value of the variable, HPGROUP. The following example shows that by using recursive dereferencing, the value of HPGROUP remains current.

   :SHOWVAR HPPATH

   HPPATH = !HPGROUP,PUB,PUB.SYS

   :SETVAR HPPATH "HPGROUP,SCRIPTS.SYS,PUB.SYS"

   :SHOWVAR HPPATH

   HPPATH = !HPGROUP,SCRIPTS.SYS,PUB.SYS

If the user changes groups, the search path automatically adjusts to the current location. The variable value is not substituted until the variable is explicitly dereferenced at command execution.