== Using a non-standard shell as your login shell

Suppose that you want to use a non-standard shell as your login shell
(for the purposes of this, 'non-standard' means that it's not installed
system-wide; you have to compile your own version). Further suppose,
for the sake of argument, that the shell you want to use doesn't have
a command-line option to tell it to be a login shell.

If your shell did have such a command line option, using it as your
login shell would be simple. Set your system shell to _/bin/sh_ and
then have a _.profile_ that just said something like:

>  SHELL=$HOME/bin/shell; export SHELL
>  exec $SHELL --login

(This doesn't work on some systems in some circumstances, but that's
another entry entirely.)

Without such an option, you need some way to make your non-standard
shell think that it is being run as a login shell by _login_ (and things
that imitate it, like your SSH daemon). As you might guess, there is a
long-standing convention for how _login_ communicates this to the shell:
~~a login shell has a '_-_' as the first character of its program name~~.

(This works because the entire argument list, including the program name
(aka 'argument zero'), is entirely up to the program that runs you; it
need not have anything to do with your actual file name.)

This really means *the first character*, which means that you need some
way to make the first character be a '_-_'. As it happens, most or all
shells just use the bare program name if you run something that is found
in your _$PATH_. So what you need is two-fold: first, you need a version
of your shell that's called '_-shell_' (easily gotten with a symlink or
a hardlink), and second, you need to add _$HOME/bin_ to your _$PATH_ so
you can run your shell without an absolute or relative path.

This gives you the magic incantation:

>  SHELL=$HOME/bin/shell
>  PATH=$HOME/bin:$PATH
>  export SHELL PATH
>  exec -shell

(Then you run into [[a bash irritation BashExecAnnoyance]].)