Question: I'm new to git on OS X, and I'm using it via the command line. I come from the world of Tortoise SVN and Beyond Compare on Windows.

I want to be able to send diffs to FileMerge.

I was able to do this with TextMate simply by using:

```
git diff | mate

```

But I'm not sure how to get that set up so I can use FileMerge instead?

Answer: 
Although it's not exactly the same as piping stdin into a script, you can do this:

```
git difftool -t opendiff -y

```

That will launch FileMerge once for each file. Doing the whole project tree at once [takes a little scripting](http://kerneltrap.org/mailarchive/git/2007/11/21/435536).

See also [this question](https://stackoverflow.com/questions/255202).
First, start by telling git you want to use FileMerge (aka opendiff) as your default difftool:

```
git config --global diff.tool 

```

(This only needs to be done once)

Then it's as simple as

```bash
git difftool

```