Discipline Functions
Each variable can have zero or more discipline functions associated with it. The shell initially understands the discipline names get, set, and unset but on most systems others can be added at run time via the C programming interface extension provided by the builtin built-in utility. If the get discipline is defined for a variable, it is invoked whenever the given variable is referenced. If the variable .sh.value is assigned a value inside the discipline function, the referenced variable will evaluate to this value instead. If the set discipline is defined for a variable, it is invoked whenever the given variable is assigned a value. The variable .sh.value is given the value of the variable before invoking the discipline, ...
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