change dir to //

[Michał Piotrowski]

Hi,

I wonder why such a thing is possible
[michal@ozzy ~]$ cd /
[michal@ozzy /]$ pwd
/
[michal@ozzy /]$ cd //
[michal@ozzy //]$ pwd
//
[michal@ozzy //]$ cd ///
[michal@ozzy /]$

Why can I change dir to // ?

[Andre Robatino]

Michał Piotrowski <mkkp4x4 <at> gmail.com> writes:
> Hi,
>
> I wonder why such a thing is possible
> [michal <at> ozzy ~]$ cd /
> [michal <at> ozzy /]$ pwd
> /
> [michal <at> ozzy /]$ cd //
> [michal <at> ozzy //]$ pwd
> //
> [michal <at> ozzy //]$ cd ///
> [michal <at> ozzy /]$
>
> Why can I change dir to // ?

Good question - a directory listing shows it's in /, as expected, but $PWD is
wrong, and only when there are exactly 2 /'s. The same thing happens in F13,
F14, and Rawhide. It only happens in sh/bash, and not in csh/tcsh, ksh, or zsh.

[Andre Robatino]

Andre Robatino <robatino <at> fedoraproject.org> writes:
> Good question - a directory listing shows it's in /, as expected, but $PWD is
> wrong, and only when there are exactly 2 /'s. The same thing happens in F13,
> F14, and Rawhide. It only happens in sh/bash, and not in csh/tcsh, ksh, or zsh.

More generally, it happens for anything like "//usr/bin" as well - you end up in
the expected directory, but "echo $PWD" shows the double slash.

[Michał Piotrowski]

2011/1/17 Andre Robatino :
> Andre Robatino <robatino <at> fedoraproject.org> writes:
> > Good question - a directory listing shows it's in /, as expected, but $PWD is
> > wrong, and only when there are exactly 2 /'s. The same thing happens in F13,
> > F14, and Rawhide. It only happens in sh/bash, and not in csh/tcsh, ksh, or zsh.
>
> More generally, it happens for anything like "//usr/bin" as well - you end up in
> the expected directory, but "echo $PWD" shows the double slash.

Actually I'm testing things like

while [ true ]; do
  ls ///////////////////////////////////////////////////(1400 /)
done

it works, but when I try

while [ true ]; do
  ls ///////////////////////////////////////////////////(5400 /)
done

I get "too long file name" warning.

Also works for ~/// and ///dir1///dir2
> --
> test mailing list
> [e-mail]
> To unsubscribe:
> https://admin.fedoraproject.org/mailman/listinfo/test

[Andre Robatino]

Andre Robatino <robatino <at> fedoraproject.org> writes:
> More generally, it happens for anything like "//usr/bin" as well - you end up
> in the expected directory, but "echo $PWD" shows the double slash.

It also persists if you then cd using a relative path. The double slash in $PWD
won't go away unless you cd using an absolute path.

[Andre Robatino]

Well, apparently it's deliberate behavior (not a bug). See this from 2005:

http://linuxfromscratch.org/pipermail/blfs-support/2005-December/057882.html

[Andre Robatino]

Andre Robatino <robatino <at> fedoraproject.org> writes:
> Well, apparently it's deliberate behavior (not a bug). See this from 2005:
>
> http://linuxfromscratch.org/pipermail/blfs-support/2005-December/057882.html

Which then raises the question - if bash is doing the right thing, are all the
other shells doing the wrong thing (meaning should it be reported as a bug
against them)?

[Michał Piotrowski]

2011/1/17 Andre Robatino :
> Well, apparently it's deliberate behavior (not a bug). See this from 2005:
>
> http://linuxfromscratch.org/pipermail/blfs-support/2005-December/057882.html

Fair enough

at least
ls /././/././..//../
works and AFAICS doesn't crash anything.

[John Wendel]

On 01/16/2011 04:13 PM, Michał Piotrowski wrote:
> 2011/1/17 Andre Robatino:
> > Well, apparently it's deliberate behavior (not a bug). See this from 2005:
> >
> > http://linuxfromscratch.org/pipermail/blfs-support/2005-December/057882.html
>
> Fair enough
>
> at least
> ls /././/././..//../
> works and AFAICS doesn't crash anything.

In my job as a programmer, I maintain a very old app that actually 
depends on "//" in pathnames behaving the same as "/". The "//" triggers 
some special app behavior while still being a valid pathanme when passed 
to the kernel filesystem. It works this way on Linux, Solaris, AIX, and 
SGI boxes.

Regards,

JOhn

[Eric Blake]

On 01/16/2011 03:54 PM, Michał Piotrowski wrote:
> Hi,
>
> I wonder why such a thing is possible
> [michal@ozzy ~]$ cd /
> [michal@ozzy /]$ pwd
> /
> [michal@ozzy /]$ cd //
> [michal@ozzy //]$ pwd
> //

Because POSIX says that leading // is allowed (but not required) to have
a separate interpretation from / (no other spelling gets this special
treatment; leading /// must be identical to leading /).  Some systems,
like Cygwin, use this POSIX requirement to implement an alternate access
space, where //server/share is used, in perfect compliance with POSIX,
to represent windows remote share drives.  Since bash is ported to
cygwin, the bash maintainer chose to preserve // as special for all
systems, whether or not it actually is special.  On Linux, it happens to
not be special.  Other shells, like zsh, only treat // as special on
platforms where it really is special, and collapse it to / otherwise.

However, you have not pointed out any bugs - since POSIX is explicit
that // is special, you are better off avoiding // and not worrying
about whether a program collapses // into /.