Check if path is absolute or relative

Question

How can I check if a filesystem path is absolute or relative in a cross-platform way?

For my purposes, I consider a path absolute if points to a location that is independent of Directory[].

I only need to check paths valid on the current operating system. I never need to check Windows paths on Unix or vice versa.

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Examples of absolute paths on Unix:

• /foo/bar
• ~/baz (This is sometimes considered relative as it is user $UserName-dependent. However, it is not Directory[]-dependent.)

On Windows:

• F:\foo\bar
• F:/foo/bar

Relative paths on Unix:

• foo/bar
• ., ./foo, ..

Relative paths on Windows:

• foo\bar
• ., .\bar, .., etc.
• \foo\bar (May be either of C:\ or D:\, etc.)
• any of the above with a / separator instead of \

These are just a few examples. I may have missed several special cases. I am looking for something which always works and preferably doesn't depend on my own partial understanding of what forms an absolute or relative path may take on different OSs. I would rather not start matching strings, at least not without a reference document which gives an exhaustive list of all possible path syntaxes both for absolute and relative paths.

The ideal solution would use a feature that asks the operating system directly:

• https://msdn.microsoft.com/en-us/library/bb773660(v=vs.85).aspx

Answer 1

The only reliable way I found so far was by using Java through J/Link:

Needs["JLink`"]

Clear[absolutePathQ]
absolutePathQ[File[path_String]] := absolutePathQ[path]
absolutePathQ[path_String] :=
 Module[{file},
   InstallJava[];
   JavaBlock[
     file = JavaNew["java.io.File", path];
     file@isAbsolute[]
   ]
 ]

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My best pure Mathematica attempt was the following:

Clear[absolutePathQ]
absolutePathQ[path_] := ExpandFileName[path] === FileNameJoin@FileNameSplit[path]

As @george2079 points out, this fails on absolute paths like /var/../bin/sh.

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Neither of these two methods consider ~ absolute. That is not unreasonable and not really a problem. This case is special enough that I can always check StringStartsQ[path, "~"].