For SL6/CentOS6 install the EPEL repo and yum install git-annex. 

I was waiting for my backups to be done hence this article, as I was using
_git-annex_ to manage my files and I decided I needed to have
git-annex on a SL5 based machine. SL5 is just an opensource
clone/recompile of RHEL5.

I haven't tried to install the newer versions of Haskell Platform and
GHC in a while on SL5 to install git-annex. But the last time I checked
when GHC7 was out, it was a pain to install GHC on SL5.

However I have discovered that someone has gone through the trouble of
packaging up GHC and Haskell Platform for RHEL based distros.

* <http://justhub.org/download> - Packaged GHC and Haskell Platform
  RPM's for RHEL based systems.

I'm primarily interested in installing _git-annex_ on SL5 based
systems. The installation process goes as such...

First install GHC and Haskell Platform (you need root for these
following steps)

    $ wget http://sherkin.justhub.org/el5/RPMS/x86_64/justhub-release-2.0-4.0.el5.x86_64.rpm
    $ rpm -ivh justhub-release-2.0-4.0.el5.x86_64.rpm
    $ yum install haskell

The RPM's don't place the files in /usr/bin, so you must add the
following to your .bashrc (from here on you don't need root if you
don't want things to be system wide)

    $ export PATH=/usr/hs/bin:$PATH

Once the packages are installed and are in your execution path, using
cabal to configure and build git-annex just makes life easier, it
should install all the needed dependancies.

    $ cabal update
    $ git clone git://git.kitenet.net/git-annex
    $ cd git-annex
    $ make git-annex.1
    $ cabal configure
    $ cabal build
    $ cabal install

Or if you want to install it globallly for everyone (otherwise it will
get installed into $HOME/.cabal/bin)

    $ cabal install --global

The above will take a while to compile and install the needed
dependancies. I would suggest any user who does should run the tests
that comes with git-annex to make sure everything is functioning as
expected.

I haven't had a chance or need to install git-annex on a SL6 based
system yet, but I would assume something similar to the above steps
would be required to do so.

The above is almost a cut and paste of <http://jcftang.github.com/2012/06/15/installing-git-annex-on-sl5/>, the above could probably be refined, it was what worked for me on SL5. Please feel free to re-edit and chop out or add useless bits of text in the above!

Note: from the minor testing, it appears the compiled binaries from SL5 will work on SL6.
