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generate a path to go backward a given path

Assuming I have the following relative path:

MY_PATH := first/second/third

I am searching for a preferably short shell command to achieve the following:

MY_NEW_PATH := ../../..

How to do this?

Solution 
  • $ MY_PATH=first/second/third
$ sed -e 's#[^/]\+/\?#../#g' <<<"$MY_PATH"
../../../

    The easiest way to get that path without the trailing slash is just to remove it.

    $ MY_PATH=first/second/third
$ cdup=$(sed -e 's#[^/]\+/\?#../#g' <<<"$MY_PATH")
$ echo "$cdup"
../../../
$ echo "${cdup%/}"
../../..

    You could also use something like this (which generates an uglier, but still valid, path):

    $ MY_PATH=first/second/third
$ sed -e 's#[^/]\+/\?#./.#g' <<<"$MY_PATH"
./../../.

    There might be a better alternative to get a trailing-slash-less path but I'd have to think about it more.