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Re: "unset var" pops var off variable stack instead of unsetting it


From: Chet Ramey
Subject: Re: "unset var" pops var off variable stack instead of unsetting it
Date: Wed, 22 Mar 2017 15:40:39 -0400

> The effect of unset on a local was what I had in mind, but really the
> manual says very little about scope. All it says right now is:
> 
> "Variables local to the function may be declared with the local builtin
> command.  Ordinarily, variables and their values are shared between the
> function and its caller."
> 
> Which doesn't exactly describe dynamic scope even for those that know
> what that means.

Here's what I have to start:

       Variables  local to the function may be declared with the local builtin
       command.  Ordinarily, variables and their values are shared between the
       function  and  its  caller.  If a variable is declared local, the vari-
       able's visible scope is restricted to that function  and  its  children
       (including the functions it calls).  Local variables "shadow" variables
       with the same name declared at previous scopes.  For instance, a  local
       variable  declared  in  a  function hides a global variable of the same
       name: references and assignments refer to the local  variable,  leaving
       the  global variable unmodified.  When the function returns, the global
       variable is once again visible.

       The shell uses dynamic  scoping  to  control  a  variable's  visibility
       within  functions.   With  dynamic scoping, visible variables and their
       values are a result of the sequence of function calls that caused  exe-
       cution  to  reach the current function.  The value of a variable that a
       function sees depends on its value within its caller, if  any,  whether
       that  caller  is the "global" scope or another shell function.  This is
       also the value that a local variable  declaration  "shadows",  and  the
       value that is restored when the function returns.

       For  example, if a variable var is declared as local in function func1,
       and func1 calls another function func2, references  to  var  made  from
       within func2 will resolve to the local variable var from func1, shadow-
       ing any global variable named var.

Chet

-- 
``The lyf so short, the craft so long to lerne.'' - Chaucer
                 ``Ars longa, vita brevis'' - Hippocrates
Chet Ramey, UTech, CWRU    address@hidden    http://cnswww.cns.cwru.edu/~chet/



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