When you want to use a pre-existing .gitmodules file to set up submodules on another repository.

git-submodule-add-from-gitmodules-file

#!/bin/sh
# http://stackoverflow.com/a/11258810/1154693
set -e

git config -f .gitmodules --get-regexp '^submodule\..*\.path$' |
    while read path_key path
    do
        url_key=$(echo $path_key | sed 's/\.path/.url/')
        url=$(git config -f .gitmodules --get "$url_key")
        git submodule add $url $path
    done