Re: Silly shell question

[Dan LaBell <dan4l-nospam%verizon.net@localhost>]

On Mar 22, 2016, at 4:39 PM, Swift Griggs wrote:

> In ksh, when you use the 'export' keyword, what is actually going  
> on? Does it create a copy of the variable in memory? I doubt it  
> since I tried a test and I could see the exported version changing  
> even if I just change the original variable:
>
> # FOO=abc
> # export FOO
> # ksh
> [...]
Try printenv before and after.  Man printenv and environ, and getenv.
Basically, exported variables become part an array of character  
strings in
the memory space of child process (as a copy). In a way, it's similar to
to command line arguments. It's actually part of C , not just unix.
In C you can optionally pass a 3rd parameter to main
--
#include <stdio.h>
#include <stdlib.h>

int main( int argc, char * argv[] , char **environ)
{
  printf("main %p\n", main);
  printf("argv  %p\n", argv);
  printf("environ %p\n", environ);
  printf("malloc'ed byte %p\n",malloc(1));
}
Compile with symbols, and launch gdb, set a break point and
you can poke around. (print environ[4], print strlen(environ[4]), etc ).

> Sorry if this seems like a dumb question. I'm just curious about  
> the intrinsics of how
> it works. I looked at the source code for ast-ksh, but it's pretty  
> huge and hard to
Might want to look at the source for something simpler, like  
printenv, and env.
/usr/src/usr.bin/env/env.c
/usr/src/usr.bin/printenv/printenv.c