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There is a nice option in MacOS X that you can right-click on a particular file, select "open with", then select the app that you want to use to open the file type and check the mark "always open with".

So far so good, this works very well. However, this association resets itself after time (or after a system restart for example). How can I achieve it that a particular file type stays associated with my selected "always open with" app?

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Actually these associations should stick and remain active after a reboot. Maybe something else (e.g. another application) is reseting them. Does it happen for all custom-set ones or only for specific types? – patrix Apr 6 '13 at 14:56
I don't have an exact answer with the details provided, but do search for launch services database. If the database were corrupt on your Mac - that would explain it not behaving in a stable manner. As matrix mentioned - these mappings only change when a new app gets installed (or arrives mounted on a connected drive) and feeds new apps into the hierarchy of who can and should open a specific document type. Basically, the system is quite complicated - so it could be several things that have broken. – bmike Apr 6 '13 at 15:02
That's weird, now the connection stays after reboot. But there is another thing: when I associate file.txt with my application for example, all other files of this type are still opened by the default program. So I have to apply the "always open with" procedure to each individual file. Is there a way to do this globally? – bluewoodtree Apr 6 '13 at 15:28
Okay got it! Thanks for your comments, I will keep an eye on the database. – bluewoodtree Apr 6 '13 at 15:31
up vote 6 down vote accepted

Press ⌘I, select an application, and press the Change All button:

Another option is to use duti. Download the installer and run /usr/local/bin/duti ~/.duti after saving a file like this as ~/.duti:

org.videolan.vlc .mkv all
net.sourceforge.skim-app.skim .pdf all public.plain-text all
com.macromates.TextMate.preview public.unix-executable all
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Apple used to provide this capability, but there's an invaluable, free Preference Pane available from RubiCode called RCDefaultApp that sets default open-with, file types, extensions and more.

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