unix, difference between path starting with '/' and '//'

Question

In unix/linux, any number of consecutive forwardslashes in a path is generally equivalent to a single forwardslash. eg.

$ cd /home/shum
$ pwd
/home/shum
$ cd /home//shum
$ pwd
/home/shum
$ cd /home///shum
$ pwd
/home/shum

Yet for some reason two forwardslashes at the beginning of an absolute path is treated specially. eg.

$ cd ////home
$ pwd
/home
$ cd ///
$ pwd
/
$ cd //
$ pwd
//
$ cd home//shum
$ pwd
//home/shum

Any other number of consecutive forwardslashes anywhere else in a patch gets truncated, but two at the beginning will remain, even if you then navigate around the filesystem relative to it.

Why is this? Is there any difference between /... and //... ?

@tchrist: This behavior is specific to Bourne shell; you are probably using zsh or the like. — u1686_grawity, Apr 29 '11 at 13:14
@grawity: No no no. It is a namei() thing, not a shell thing!! It happens on Linux, just not on the others. — tchrist, Apr 29 '11 at 13:16
@thchrist: 1) What do you mean by "namei()"? If you are referring to a syscall or library function, then I haven't yet heard of such. There's only a /usr/bin/namei, but it is not related. 2) It is a shell thing, since it only happens with sh and bash, but not with zsh/csh/tcsh, nor with the chdir() syscall. 3) It happens on "the others" - I just tested NetBSD 5.0 and FreeBSD 6.3, and cd //bin shows exactly the same behavior. — u1686_grawity, Apr 29 '11 at 13:28
I hate to nag, but it's called a ‘slash’. There's the ‘slash’ and the ‘backslash’. The ‘forward slash’ (sic) seems to be a recent construct, and I mostly hear it from people on TV. — Alexios, Feb 21 '12 at 09:05

score 58 · Accepted Answer · edited Apr 13 '17 at 12:36

58

For the most part, repeated slahes in a path are equivalent to a single slash. This behavior is mandated by POSIX and most applications follow suit. The exception is that “a pathname that begins with two successive slashes may be interpreted in an implementation-defined manner” (but ///foo is equivalent to /foo).

Most unices don't do anything special with two initial slashes. Linux, in particular, doesn't. Cygwin does: //hostname/path accesses a network drive (SMB).

What you're seeing is not, in fact, Linux doing anything special with //: it's bash's current directory tracking. Compare:

$ bash -c 'cd //; pwd'
//
$ bash -c 'cd //; /bin/pwd'
/

Bash is taking the precaution that the OS might be treating // specially and keeping it. Dash does the same. Ksh and zsh don't when they're running on Linux, I guess (I haven't checked) they have a compile-time setting.

edited Apr 13 '17 at 12:36

Community

1

answered Apr 29 '11 at 10:11

Gilles 'SO- stop being evil'

829,060

Interesting; when I do bash -c 'cd //; /bin/pwd', I get //. Cygwin (8.23-4) with bash version 4.1.17(9)-release and pwd (GNU coreutils) 8.23. Even more surprising, I still get // if I do /bin/pwd -P. – G-Man Says 'Reinstate Monica' Oct 29 '15 at 20:49
For the record, fish does not do this. If you run cd //; pwd, or fish -c 'cd //; pwd', you get /, and if you run the former with a default prompt, the prompt has just 1 slash. – trysis Sep 29 '16 at 15:02
@trysis Nor do zsh, ksh, pdksh, dash, hush, … Of the shells commonly found on Linux, bash is the only one that preserves a leading //. – Gilles 'SO- stop being evil' Sep 29 '16 at 16:15

sarnold · Answer 2 · 2011-07-10T23:58:20.787

22

From the POSIX Specification:

A pathname that begins with two successive slashes may be interpreted in an implementation-defined manner, although more than two leading slashes shall be treated as a single slash.

I assume ~~Linux~~ bash keeps this behavior in case there's a compelling future use.

(I've always heard that Al Viro kept it in place because there is a feature from Plan9 that uses it, and he'd like to have that feature in Linux, but I'm having trouble finding that feature in my Plan9 documentation. But, since it is in bash instead, Al probably doesn't have anything to do with it.)

edited Jul 10 '11 at 23:58

answered Apr 29 '11 at 02:23

sarnold

677
3
9

3

I remember that double slash to allow remote file system access in an early system V implementation (Utek V & DFS). The syntax was //servername/path. Just like Solaris automounter would use now /net/servername/path. – jlliagre Apr 29 '11 at 09:39
3

Linux doesn't do anything about //, it's bash. – Gilles 'SO- stop being evil' Apr 29 '11 at 10:11
1

@jlliagre: I remember that from AT&T RFS. (Remote File System) Was in System V. – janm Nov 08 '11 at 11:52
Python's posixpath.normpath() function also retains double leading slashes. I tried to find out why, which brought me here. It's unfortunate that it does this, because what I've read indicates that double leading slashes is no longer important. – Mr. Lance E Sloan Jul 08 '16 at 20:42

score 9 · Answer 3 · answered Oct 07 '11 at 16:47

According to the POSIX definition, paths starting with a double-slash (//) "...may be interpreted in an implementation-defined manner, although more than two leading slashes shall be treated as a single slash." If you use csh, for example, it doesn't act the same way:

% bash -c 'cd //; pwd'
//
% csh -c 'cd //; pwd'
/

Bash appears to be storing the directory, and pwd is reporting the $PWD, while csh appears to be using the getcwd() function to get the actual directory.

And zsh acts like csh, but does the trimming by itself. – Stéphane Gimenez Oct 07 '11 at 17:08 — Stéphane Gimenez, Oct 07 '11 at 17:08

unix, difference between path starting with '/' and '//'

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