Re: Passing variables by reference conflicts with local

[Dennis Williamson]

As Chet said, use internal variables that are unlikely to conflict.

# Param: $1  variable name to return value to
   blackbox() {
       local __bb_a    # internal: __, blackbox: bb, a: _a
       eval $1=bar
   }

f() {
       local a
       blackbox a
       echo $a
   }
   f    # no conflict

I prefer to avoid using eval by using declare, but declare inside a
function makes the variable local. Wouldn't it be nice to have a
global flag (declare -g) like zsh's typeset -g.

d='ls /;true'   # aren't you glad it's not "rm -rf" instead of "ls"
f "$d"  # oops

So in the mean time, you should sanitize those evals.

On Sat, May 1, 2010 at 3:18 AM, Freddy Vulto <fvulto@gmail.com> wrote:
> On 100430 08:19, Greg Wooledge wrote:
>> On Fri, Apr 30, 2010 at 12:02:58AM +0200, Freddy Vulto wrote:
>> > Passing variables by reference however, has a caveat in that
>> > local variables override the passing by reference, e.g.:
>> >
>> >     t() {
>> >         local a
>> >         eval $1=b
>> >     }
>> >     unset a; t a; echo $a  # Outputs nothing, expected "b"
>>
>> Why did you declare 'a' to be local if that's not what you wanted?
>> Why did you expect 'b' to be output, if you used a local variable?
>
> t() is an example library function which - as a black box - can contain
> many local variables.  Local 'a' is just an example.
>
> I would like to call t(), and let it return me a filled variable by
> reference, that is without polluting the global environment.
>
> Problem is, all variables work except 'a' because t() happens to declare
> this local - which I'm not supposed to know because t() is a black box.
>
> Maybe the example is not clear because 'a' becomes global after all,
> and is this a better example:
>
>    # Param: $1  variable name to return value to
>    blackbox() {
>        local a
>        eval $1=bar
>    }
>
> This goes all right:
>
>    f() {
>        local b
>        blackbox b
>        echo $b
>    }
>    f  # Echos "bar" all right
>
> But if I change 'b' to 'a', this conflicts with blackbox() local 'a':
>
>    f() {
>        local a
>        blackbox a
>        echo $a
>    }
>    f  # Outputs nothing unexpected
>
>
> Freddy Vulto
> http://fvue.nl/wiki/Bash:_passing_variables_by_reference
>
>
>