how to get the absolute pathname of the current shell?

[Vincent Lefevre <vincent@vinc17.net>]

I'd like to get an absolute pathname of the current shell, for
interactive shells (i.e. scripts are not concerned), in a startup
file (so that the current working directory has not changed).
The goal is to set $SHELL to the current shell.

Currently I'm doing: ${$(whence -p ${0#-}):a}

But this doesn't work when there are symbolic links, for instance,
when one has:

  foo -> bar/subdir
  my_shell -> /bin/zsh

foo/../../my_shell executes my_shell as expected, but the ":a"
gives: /home/my_shell

Note that I have the CHASE_LINKS option set, but the solution
shouldn't depend on this setting.

I want something portable, thus I don't want to rely on a realpath
command or /proc.

-- 
Vincent Lefèvre <vincent@vinc17.net> - Web: <http://www.vinc17.net/>
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Work: CR INRIA - computer arithmetic / AriC project (LIP, ENS-Lyon)