see http://mogproject.blogspot.jp/2015/03/working-with-multiple-github-accounts.html

git-wrapper

#!/bin/bash

GIT=/usr/local/bin/git
SCRIPT_DIR=$( cd "$( dirname "$0" )" && pwd -P )

if [ "$1" = "clone" ]; then
    origin_url="$2"
else
    # check if current directory is under git repository
    origin_url=$("$GIT" config --get remote.origin.url) || {
        "${GIT}" "$@"
        exit $?
    }
fi

# definitions
case ${origin_url} in
    git@github.com:your-company/*)
        readonly user_name='Your name 1'
        readonly user_email='youremail1@example.com'
        readonly git_ssh='ssh-yourname-1'
        ;;
    *)
        readonly user_name='Your name 2'
        readonly user_email='youremail2@example.com'
        readonly git_ssh='ssh-yourname-2'
        ;;
esac

# check name and email
if [ "$1" != "clone" ]; then
    current_name=$("$GIT" config --get user.name) || {
        echo "Failed to get user name. Aborted."
        exit 1
    }

    current_email=$("$GIT" config --get user.email) || {
        echo "Failed to get user email. Aborted."
        exit 1
    }

    if [ "${current_name}" != "${user_name}" ]; then 
        echo "Setting user name: \"${GIT}\" config user.name \"${user_name}\""
        "${GIT}" config user.name "${user_name}"
    fi

    if [ "${current_email}" != "${user_email}" ]; then 
        echo "Setting user email: \"${GIT}\" config user.email \"${user_email}\""
        "${GIT}" config user.email "${user_email}"
    fi
fi

# launch git command
GIT_SSH="${SCRIPT_DIR}/../ssh/${git_ssh}" "${GIT}" "$@"