A shell script for setting the CFBundleVersion of an Xcode build based on the current git revision

git-version.sh

# A script for generating pseudo-version numbers for a git repository
# and updating the BundleVersion of an Xcode app build with this version

alias git=$(which git)

git_version()
{
    # The cornerstone of this hack is "git describe", which
    # relies on there being at least one "real" tag (i.e. not 
    # a lightweight tag) somewhere in the current code's commit
    # parentage. So I normally start by tagging my initial commit 
    # as "0.1". Then, N commits down the line git describe returns
    # 0.1-[N]-[commit-sha], e.g. 0.1-24-g0bf6847
    #
    # The cut -d- -f1-2 splits the input on hyphens and returns
    # the first two tokens, joined back together with tokens. So
    # now I've got 0.1-24
    #
    # The tr - . replaces hyphens with dots, so now I've got 0.1.24
    GITVERSION=`git describe | cut -d- -f1-2 | tr - .`

    # If there are modifications in the repository, append "-M" to
    # the "revision number"
    GITMODIFIED=`(git status | grep "modified:\|added:\|deleted:" -q) && echo "-M"`
    echo $GITVERSION$GITMODIFIED
}

# I use the git-flow framework and build App Store builds from master, Ad Hoc
# builds from develop. This if clause ensures that I don't mess with the
# version number of an App Store build.
if [[ $(git symbolic-ref -q HEAD) != 'refs/heads/master' ]]; then
    # Get location of parsed Info.plist
    GS_INFO_PLIST_PATH="$BUILT_PRODUCTS_DIR/$FULL_PRODUCT_NAME/Info"

    # Use git version as version number
    export GS_NEW_VERSION=`git_version`
    echo Version is $GS_NEW_VERSION

    # Write new version number to parsed Info.plist
    defaults write "$GS_INFO_PLIST_PATH" CFBundleVersion $GS_NEW_VERSION
    defaults write "$GS_INFO_PLIST_PATH" CFBundleShortVersionString $GS_NEW_VERSION
fi

You might try using git rev-parse. Look at how the Linux kernel does it: http://git.kernel.org/?p=linux/kernel/git/torvalds/linux.git;a=blob;f=scripts/setlocalversion;hb=HEAD