docs: document relative paths

Author: dafi

unixshell.md

@@ -47,3 +47,24 @@ $ sudo ln -s /Applications/VisualDiffer.app/Contents/Helpers/visdiff /usr/bin/vi
 
 VisualDiffer was sandboxed starting from version 1.4.2 and many users found using it very annoying because any comparison prompts to pick folders/files.  
 This problem was fixed in version 1.4.3 by introducing the so-called "Trusted Paths"; please refer to [Trusted Paths](trustedPaths.html) for further details.
+
+[How visdiff resolves relative paths](#relative_paths)
+========================================================
+
+`visdiff` is a sandboxed application. Because of that, even when you launch it from a regular terminal, relative paths are resolved from the home directory of the sandbox container, not from the current working directory of the shell.
+
+This can make VisualDiffer report that a file does not exist because it looks for it in the wrong location.
+
+The most reliable solution is to expand the relative path with `$PWD` so the final path is built from the current working directory before `visdiff` receives it.
+
+Instead of:
+
+{% highlight bash %}
+$ visdiff ../myfile.txt
+{% endhighlight %}
+
+Use:
+
+{% highlight bash %}
+$ visdiff $PWD/../myfile.txt
+{% endhighlight %}