Add git-submodules-add

Run `git submodule add` on submodules in `.gitmodules`
if said `.gitmodules` has been simply copied into the project.

diff --git a/git-submodules-add b/git-submodules-add
new file mode 100755 (executable)
index 0000000..d60f9b4
--- /dev/null
+++ b/git-submodules-add
@@ -0,0 +1,30 @@
+#! /bin/sh
+# Run `git submodule add` on submodules in `.gitmodules`
+# if said `.gitmodules` has been simply copied into the project.
+
+# Adapted from https://stackoverflow.com/a/53899440/7976758
+
+git config -f .gitmodules --get-regexp '^submodule\..*\.path$' |
+    while read -r KEY MODULE_PATH
+    do
+        # If the module's path exists, remove it.
+        # This is done b/c the module's path is currently 
+        # not a valid git repo and adding the submodule will cause an error.
+        [ -d "${MODULE_PATH}" ] && rm -rf "${MODULE_PATH}"
+
+        NAME="$(echo "${KEY}" | sed 's/^submodule\.\(.*\)\.path$/\1/')"
+
+        url_key="$(echo "${KEY}" | sed 's/\.path$/.url/')"
+        branch_key="$(echo "${KEY}" | sed 's/\.path$/.branch/')"
+
+        URL="$(git config -f .gitmodules --get "${url_key}")"
+        BRANCH="$(git config -f .gitmodules --get "${branch_key}")"
+
+        if [ -n "$BRANCH" ]; then
+           git submodule add --force -b "${BRANCH}" --name "${NAME}" "${URL}" "${MODULE_PATH}"
+        else
+           git submodule add --force --name "${NAME}" "${URL}" "${MODULE_PATH}"
+        fi
+    done
+
+exec git submodule update --init --recursive