how to get the absolute pathname of the current shell?

how to get the absolute pathname of the current shell?

[Vincent Lefevre]

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/>
100% accessible validated (X)HTML - Blog: <http://www.vinc17.net/blog/>
Work: CR INRIA - computer arithmetic / AriC project (LIP, ENS-Lyon)

Re: how to get the absolute pathname of the current shell?

[Bart Schaefer]

On Dec 6,  2:27pm, Vincent Lefevre wrote:
}
} 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).

As far as I can tell there is no portable way to do this.  Even C
programs have to use different code depending on the OS.

} Currently I'm doing: ${$(whence -p ${0#-}):a}
} 
} But this doesn't work when there are symbolic links

If the above is otherwise good enough, why not ${$(whence -p ${0#-}):A}
which uses realpath() when it is available?  [See above about different
code depending on OS ...]

Re: how to get the absolute pathname of the current shell?

[Vincent Lefevre]

On 2013-12-06 08:30:20 -0800, Bart Schaefer wrote:
> On Dec 6,  2:27pm, Vincent Lefevre wrote:
> } Currently I'm doing: ${$(whence -p ${0#-}):a}
> } 
> } But this doesn't work when there are symbolic links
> 
> If the above is otherwise good enough, why not ${$(whence -p ${0#-}):A}
> which uses realpath() when it is available?  [See above about different
> code depending on OS ...]

Using :A instead of :a doesn't solve the problem.

-- 
Vincent Lefèvre <vincent@vinc17.net> - Web: <http://www.vinc17.net/>
100% accessible validated (X)HTML - Blog: <http://www.vinc17.net/blog/>
Work: CR INRIA - computer arithmetic / AriC project (LIP, ENS-Lyon)

Re: how to get the absolute pathname of the current shell?

[Vincent Lefevre]

On 2013-12-09 09:53:30 +0100, Vincent Lefevre wrote:
> On 2013-12-06 08:30:20 -0800, Bart Schaefer wrote:
> > On Dec 6,  2:27pm, Vincent Lefevre wrote:
> > } Currently I'm doing: ${$(whence -p ${0#-}):a}
> > } 
> > } But this doesn't work when there are symbolic links
> > 
> > If the above is otherwise good enough, why not ${$(whence -p ${0#-}):A}
> > which uses realpath() when it is available?  [See above about different
> > code depending on OS ...]
> 
> Using :A instead of :a doesn't solve the problem.

The following code seems to work:

  tmp=$(whence -p ${0#-})
  SHELL=${${tmp##/*}:+$PWD/}$tmp

It avoids buggy symbolic link resolution.

-- 
Vincent Lefèvre <vincent@vinc17.net> - Web: <http://www.vinc17.net/>
100% accessible validated (X)HTML - Blog: <http://www.vinc17.net/blog/>
Work: CR INRIA - computer arithmetic / AriC project (LIP, ENS-Lyon)

Re: how to get the absolute pathname of the current shell?

[Vincent Lefevre]

On 2013-12-09 09:53:30 +0100, Vincent Lefevre wrote:
> On 2013-12-06 08:30:20 -0800, Bart Schaefer wrote:
> > On Dec 6,  2:27pm, Vincent Lefevre wrote:
> > } Currently I'm doing: ${$(whence -p ${0#-}):a}
> > } 
> > } But this doesn't work when there are symbolic links
> > 
> > If the above is otherwise good enough, why not ${$(whence -p ${0#-}):A}
> > which uses realpath() when it is available?  [See above about different
> > code depending on OS ...]
> 
> Using :A instead of :a doesn't solve the problem.

After looking more closely, this is because "resolution of `..' occurs
before resolution of symbolic links" (see zshexpn(1) man page). Since
zsh's resolution of `..' is broken by design, i.e. incompatible with
real resolution, as done by realpath (and the Linux kernel), :A and :a
mustn't be used here.

-- 
Vincent Lefèvre <vincent@vinc17.net> - Web: <http://www.vinc17.net/>
100% accessible validated (X)HTML - Blog: <http://www.vinc17.net/blog/>
Work: CR INRIA - computer arithmetic / AriC project (LIP, ENS-Lyon)