Unix Paths

August 13, 2013

[ Today’s exercise was written by guest author Robert Fisher. Having worked on shrinkwrapped consumer software, e-commerce web apps, embedded systems, and insurance OLTP systems, Robert currently writes C/C++ and Javascript for HP Tippingpoint and writes Scheme whenever he can. Suggestions for exercises are always welcome, or you may wish to contribute your own exercise; feel free to contact me if you are interested. ]

Under Unix, files are identified by paths. These are strings of directory names separated by slash characters followed by the name of the file. Paths starting with a slash are absolute, but paths that don’t start with a slash are relative to the current directory. Relative paths may include “..” elements. These represent a parent directory. Likewise, “../..” would represent a grandparent directory. For example, if the current directory is “/home/bob” then “praxis/prelude.scm” represents the file “/home/bob/praxis/prelude.scm” and “../tom/bin/scheme” represents “/home/tom/bin/scheme”.

Your task is to write a function that takes the current directory path and a target path and returns an absolute and minimal path equivalent to the target path. When you are finished, you are welcome to read or run a suggested solution, or to post your own solution or discuss the exercise in the comments below.


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One Response to “Unix Paths”

  1. Josh said

    This works for paths starting with ‘.’, as well for the convoluted (‘/home/josh’, ‘./../josh/file.txt’) -> ‘/home/josh/file.txt’

    def abs_path(cwd, fp):
        if fp.startswith('/'):
            return fp
        if fp.startswith('./'):
            return abs_path(cwd, fp[2:])
        if fp.startswith('../'):
            return abs_path(cwd.rsplit('/', 1)[0], fp[3:])
        return '%s/%s' % (cwd, fp)

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