Re: bug in pwd on Mandrake Linux 9.2

[Seán McGarraghy]

Bob, 

Thanks very much for your comprehensive reply, and the explanation: I am now 
clear on how the builtin works.

Re your question: I got the version number from the man page for pwd.  It 
said it was GNU pwd and gave address@hidden as the contact email.

Apologies for raising as a bug something that is actually reasonable 
behaviour!

Thanks again,

Sean

On Fri, 4 Mar 2005, Bob Proulx wrote:

> Seán McGarraghy wrote:
> > I am running Mandrake Linux 9.2, the version of pwd is 
> > 
> > pwd 5.0 March 2003
> 
> How did you obtain that version number from pwd?  (I think it is very
> strange.)
> 
> > I found the following behaviour (running as root at the command line) 
> > 
> > address@hidden linux32]# pwd
> > /usr/local/lindoapi/bin/linux32
> > address@hidden linux32]# mv ../../../lindoapi ../../../lindoapi3.0
> > address@hidden linux32]# pwd
> > /usr/local/lindoapi/bin/linux32
> > address@hidden linux32]# ls -al /usr/local/lin*
> > drwxr-xr-x   10 root     root          368 Mar  4 17:20 lindoapi3.0/
> > 
> > That is, I renamed a directory further up the path of the current working 
> > directory, but pwd didn't catch this (although ls shows it happened).
> 
> Generally that is true.  The default behavior of most modern shells is
> to report the logical path and not the physical path.
> 
> > I don't know if this bug is still in later versions, but I thought
> > you should know.  Its severity is not too big.
> 
> Thank you for reporting this.  As has been said, many eyes make all
> bugs known.  However you should know that 'pwd' is a shell builtin in
> most shells.  In bash, for example:
> 
>   type pwd
>   pwd is a shell builtin
> 
> Therefore when you type in 'pwd' you are not getting an external pwd
> such as the GNU pwd usually installed at /bin/pwd but are instead
> getting your shell's built in pwd.  Therefore this is not a GNU
> coreutils thing but a shell thing.
> 
> In the case of the shell it usually tracks the PWD environment
> variable by logical path and not by physical path.  The behavior you
> are seeing is actually a feature of the shell.  It does this so that
> the user does not notice symbolic links in the path.  Here is an
> example.
> 
>   $ mkdir /tmp/foo
>   $ ln -s foo /tmp/bar
>   $ cd /tmp/bar
>   $ pwd
>   /tmp/bar
>   $ /bin/pwd
>   /tmp/foo
> 
> Even though the shell reported the present working directry as
> /tmp/bar it only knew that because it tracked how we got there.  That
> is the logical path.  We got there through the symlink and so it
> reported that value.  But we were really in /tmp/bar because of the
> symlink.  The /bin/pwd program did not know about the tracking of the
> logical path.  It called the systems getcwd() routine and reported the
> real physical path.
> 
> In GNU bash the 'set -o physical' option turns off this logical path
> tracking of the shell.  Setting that option with your example would
> report the new name of the physical present working directory.
> 
> In bash:
> 
>   $ set -o physical
>   $ mkdir /tmp/foo
>   $ cd /tmp/foo
>   $ mv /tmp/foo /tmp/bar
>   $ pwd
>   $ /tmp/bar
> 
> Hope that helps to explain this.
> 
> Bob
> 

-- 
Dr Seán McGarraghy
Dept of Management Information Systems
University College Dublin
Belfield, Dublin 4, Ireland.
email: address@hidden
tel +353 1 716 4734, fax +353 1 716 4783