Changing file extension associations on a Mac

One of the problems with making the switch from Windows to Mac is that you lose a lot of little tricks and tips that you have picked up over the years. Despite claims that Macs are much more intuitive than Windows machines (something that I have never found, personally), I quite often find myself frustrated at the lack of an obvious way to do something that I know how to do in Windows.

Yesterday, it was file extensions that briefly gave me angst. For historical (i.e. forgotten!) reasons, I have a lot of programs generating *.tdt files, which are tab delimited text. (The correct extension for such things actually seems to be *.tsv, or tab separated values.) Being a biologist, I generally like to open my delimited files to look at in Microsoft Excel - the number one bioinformatics tool in world! (By use, that is!) Naive Macs (and naive Windows machines) do not know what a *.tdt file is, though, so they will generally offer a text editor instead. Similarly, I have a lot of draft notes in Markdown, saved as *.md files, but Finder wants to open them in TextWrangler rather than Mou, my Markdown Editor of choice.

Happily, the internet has once again come to the rescue, in the form of a 2009 post on OSXDaily, Change File Associations in Mac OS X. Simply right-click (or ctrl+click) in Finder and select Get Info.

You can then change the program to open that file with and choose Change All to change the default action for all similar files (e.g. those with the same extension). This was so useful that I thought I would re-share here.