DISCLAIMER. English language used here only for compatibility (ASCII only), so any suggestions about my bad grammar (and not only it) will be greatly appreciated.

вторник, 4 мая 2010 г.

[method][draft][part][need_check] (bash) passing arguments to function

DISCLAIMER. English language used here only for compatibility (ASCII only), so any suggestions about my bad grammar (and not only it) will be greatly appreciated.

Status: method.
State: suggestion,draft,part,need_check.
Detailed description: One more method for passing arguments to bash function.

Outline: below under 'passing argument as pointer' i mean passing argument by name:
function f() {
}

declare var='abc'
f var  # not f "$var"
This may be considered as analogy for pointers (in C), i think :-)

1. Passing argument as pointer to function sometimes may require redefinition
   it as local variable in order to preserve referred data. Though, generally
   pointer used to provide to function ability to modify caller's environment,
   in bash it may be used, to skip unnecessary copying of big argument, if we
   want to redefine all positional parameters to local variables (names
   1, 2, 3,.. is bad whenever, and also positional parameters may be used for
   some tricks, so it's better, when they do not contain useful data).

        eval "
                $(declare -p $2 | sed -e's/^[^=]\+=/local -a shortopts=/' );
                $(declare -p $3 | sed -e's/^[^=]\+=/local -a longopts=/');
        "

   Above construction is, seemingly, the only way. To correctly redefine
   pointer to local variable we should honour several contraints:
        1. we should not define any local variable before all pointers will be
           expanded, because otherwise there is a chance, that defined local
           variable name is match with some pointed to variable's name and we
           lose access to it.
        2. we should preserve all special characters inside pointed data.

   To handle first constraint, we need `eval`.

   To handle second, we should properly quote expanded pointed to variable's
   value:

        ${[@]} - split elements correctly only during first expand, but in the
        second (done in `eval`) these boundaries will be lost and bash splits
        elements by 'metacharacters', not by IFS or anything else (info bash,
        ch-2).
        ${[*]} - my be used, perhaps, with IFS=',', and then.. something
        artificial to set up quoting by brace expansion. But this rarely works,
        though (i don't know working example).
        `declare` - seems, only way to obtain correct quoting without many
        evals and artificial tricks. But its output, perhaps, should be
        processed by some program to replace variable name to desired one.


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