Skip to content

DoubleSlash

Jump to bottom
Simon Fowler edited this page Jun 30, 2017 · 2 revisions
  1. summary How to deal with the double-slash problem.
  2. labels Phase-Implementation

Introduction

Little known fact - POSIX requires that the cd command allow you to change directory into // in addition to /.

This is mentioned in section E10 here:

E10) Why does `cd //' leave $PWD as `//'?

POSIX.2, in its description of `cd', says that *three* or more leading
slashes may be replaced with a single slash when canonicalizing the
current working directory.

This is, I presume, for historical compatibility.  Certain versions of
Unix, and early network file systems, used paths of the form
//hostname/path to access `path' on server `hostname'.

Rationale

So I was left with a dilemma - either the command logger strips away the leading extra slash when recording the ${PWD}, or we offload some complexity to the query writer.

My original thinking was that when troubleshooting strange issues on a system, knowing what the system believes the ${PWD} is can actually be important. For example, if a script checks the current working directory and compares with a hard-coded value such as /home/me- if ${PWD} reports //home/me and the script fails - this indicates a bug in the script.

I chose to keep the actual value of ${PWD} for this reason.

Queries

Query authors need to deal with the fact that if a user is in a double-slash rooted directory and wishes to view history based on the value of ${PWD}, in most cases they also want to see history for the single-slash rooted equivalent as well.

For example:

me@my_host:/home/me $ ash_query -q CWD  # I want to see my homedir history
me@my_host:/home/me $ cd //home/me  # Enter the double-slash-rooted homedir
me@my_host://home/me $ ash_query -q CWD  # I ALSO want to see my homedir history

If the CWD query did not have all three tests of the current working directory, the first and second invocations of ash_query in the example above would have totally different results.

Also, if you really want to make this distinction, you can still write your own query to do just that!

Clone this wiki locally