diff --git a/modules/pre-commit.nix b/modules/pre-commit.nix
index 85d68206..019bf9f5 100644
--- a/modules/pre-commit.nix
+++ b/modules/pre-commit.nix
@@ -281,12 +281,22 @@ in
           defaultText = "<derivation>";
         };
 
+      environmentSetupScript =
+        mkOption {
+          type = types.str;
+          description = lib.mdDoc
+            ''
+              A bash snippet that provides functions for updating the config file and installing the hook scripts.
+            '';
+          readOnly = true;
+        };
+
       installationScript =
         mkOption {
           type = types.str;
           description = lib.mdDoc
             ''
-              A bash snippet that installs nix-pre-commit-hooks in the current directory
+              A bash snippet that installs nix-pre-commit-hooks in the current directory.
             '';
           readOnly = true;
         };
@@ -372,70 +382,21 @@ in
           default_stages = cfg.default_stages;
         };
 
-      installationScript =
+      environmentSetupScript =
         ''
           export PATH=${cfg.package}/bin:$PATH
-          if ! type -t git >/dev/null; then
-            # This happens in pure shells, including lorri
-            echo 1>&2 "WARNING: pre-commit-hooks.nix: git command not found; skipping installation."
-          elif ! ${git}/bin/git rev-parse --git-dir &> /dev/null; then
-            echo 1>&2 "WARNING: pre-commit-hooks.nix: .git not found; skipping installation."
-          else
-            GIT_WC=`${git}/bin/git rev-parse --show-toplevel`
-
-            # These update procedures compare before they write, to avoid
-            # filesystem churn. This improves performance with watch tools like lorri
-            # and prevents installation loops by via lorri.
-
-            if readlink "''${GIT_WC}/.pre-commit-config.yaml" >/dev/null \
-              && [[ $(readlink "''${GIT_WC}/.pre-commit-config.yaml") == ${configFile} ]]; then
-              echo 1>&2 "pre-commit-hooks.nix: hooks up to date"
-            else
-              echo 1>&2 "pre-commit-hooks.nix: updating $PWD repo"
-
-              [ -L .pre-commit-config.yaml ] && unlink .pre-commit-config.yaml
-
-              if [ -e "''${GIT_WC}/.pre-commit-config.yaml" ]; then
-                echo 1>&2 "pre-commit-hooks.nix: WARNING: Refusing to install because of pre-existing .pre-commit-config.yaml"
-                echo 1>&2 "    1. Translate .pre-commit-config.yaml contents to the new syntax in your Nix file"
-                echo 1>&2 "        see https://github.com/cachix/pre-commit-hooks.nix#getting-started"
-                echo 1>&2 "    2. remove .pre-commit-config.yaml"
-                echo 1>&2 "    3. add .pre-commit-config.yaml to .gitignore"
-              else
-                ln -fs ${configFile} "''${GIT_WC}/.pre-commit-config.yaml"
-                # Remove any previously installed hooks (since pre-commit itself has no convergent design)
-                hooks="pre-commit pre-merge-commit pre-push prepare-commit-msg commit-msg post-checkout post-commit"
-                for hook in $hooks; do
-                  pre-commit uninstall -t $hook
-                done
-                # Add hooks for configured stages (only) ...
-                if [ ! -z "${concatStringsSep " " install_stages}" ]; then
-                  for stage in ${concatStringsSep " " install_stages}; do
-                    if [[ "$stage" == "manual" ]]; then
-                      continue
-                    fi
-                    case $stage in
-                      # if you amend these switches please also review $hooks above
-                      commit | merge-commit | push)
-                        stage="pre-"$stage
-                        pre-commit install -t $stage
-                        ;;
-                      prepare-commit-msg | commit-msg | post-checkout | post-commit)
-                        pre-commit install -t $stage
-                        ;;
-                      *)
-                        echo 1>&2 "ERROR: pre-commit-hooks.nix: either $stage is not a valid stage or pre-commit-hooks.nix doesn't yet support it."
-                        exit 1
-                        ;;
-                    esac
-                  done
-                # ... or default 'pre-commit' hook
-                else
-                  pre-commit install
-                fi
-              fi
-            fi
-          fi
+          _pre_commit_hooks_nix_git=${git}/bin/git
+          _pre_commit_hooks_nix_config=${configFile}
+          _pre_commit_hooks_nix_install_stages='${concatStringsSep " " install_stages}'
+
+          source ${../src/pre-commit-install.sh}
+        '';
+
+      installationScript =
+        ''
+          ${cfg.environmentSetupScript}
+
+          _pre_commit_hooks_nix_install_main
         '';
     };
 }
diff --git a/nix/default.nix b/nix/default.nix
index 2ec99f78..349562d3 100644
--- a/nix/default.nix
+++ b/nix/default.nix
@@ -22,6 +22,9 @@ let
           src = ../.;
           hooks = {
             shellcheck.enable = true;
+            # `types_or = [ ... something ... ]` doesn't pick up our .sh file
+            shellcheck.types_or = nixpkgs.lib.mkForce [ ];
+
             nixpkgs-fmt.enable = true;
           };
         };
diff --git a/src/pre-commit-install.sh b/src/pre-commit-install.sh
new file mode 100644
index 00000000..549b6b33
--- /dev/null
+++ b/src/pre-commit-install.sh
@@ -0,0 +1,123 @@
+# shellcheck shell=bash
+
+# This file provides utilities that are used by the `shellHook` among other things.
+# They are meant to be loaded into interactive shells, so use an underscore prefix
+# to avoid polluting the command tab completion.
+
+# PUBLIC API
+#
+# We make a reasonable effort to keep the functions marked (public) stable.
+# Functions marked (private) are not guaranteed to be stable for reasons including:
+#  - dependency on global variables that are not part of the public API
+#  - likely to be changed in the future to improve behavior or implementation details
+# The public API uses the following global variables, which are checked here:
+(
+  : "${_pre_commit_hooks_nix_git:?}"
+  : "${_pre_commit_hooks_nix_config:?}"
+  : "${_pre_commit_hooks_nix_install_stages:?}"
+) || {
+  echo >&2 "ERROR: pre-commit-hooks.nix: missing global variables. Please initialize them in your shellHook."
+}
+
+# (public)
+#
+# Install the hooks and the config file.
+_pre_commit_hooks_nix_install_main() {
+  if _pre_commit_hooks_nix_local_config_file="$(_pre_commit_hooks_nix_find_git_toplevel "skipping installation.")/.pre-commit-config.yaml"; then
+    _pre_commit_hooks_nix_ensure_config_file_up_to_date && _pre_commit_hooks_nix_install_stages
+  fi
+}
+
+# (public)
+#
+# Only update the config file.
+_pre_commit_hooks_nix_install_config_file_only() {
+  if _pre_commit_hooks_nix_local_config_file="$(_pre_commit_hooks_nix_find_git_toplevel "not updating local config file link.")/.pre-commit-config.yaml"; then
+    _pre_commit_hooks_nix_ensure_config_file_up_to_date
+  fi
+}
+
+# (private)
+#
+# Find the root of the worktree.
+_pre_commit_hooks_nix_find_git_toplevel() {
+  local msg="$1"
+  if ! type -t git >/dev/null; then
+    # This happens in pure shells, including lorri
+    echo 1>&2 "WARNING: pre-commit-hooks.nix: git command not found; $msg"
+    return 1
+  elif ! $_pre_commit_hooks_nix_git rev-parse --git-dir &> /dev/null; then
+    echo 1>&2 "WARNING: pre-commit-hooks.nix: .git not found; $msg"
+    return 1
+  else
+    $_pre_commit_hooks_nix_git rev-parse --show-toplevel
+  fi
+}
+
+# (private)
+_pre_commit_hooks_nix_is_config_up_to_date() {
+  readlink "${_pre_commit_hooks_nix_local_config_file}" >/dev/null \
+      && [[ "$(readlink "${_pre_commit_hooks_nix_local_config_file}")" == "$_pre_commit_hooks_nix_config" ]]
+}
+
+# (private)
+_pre_commit_hooks_nix_ensure_config_file_up_to_date() {
+  # These update procedures compare before they write, to avoid
+  # filesystem churn. This improves performance with watch tools like lorri
+  # and prevents installation loops by via lorri.
+  if _pre_commit_hooks_nix_is_config_up_to_date; then
+    echo 1>&2 "pre-commit-hooks.nix: hooks up to date"
+    return 0;
+  fi
+
+  echo 1>&2 "pre-commit-hooks.nix: updating $PWD repo"
+
+  [ -L .pre-commit-config.yaml ] && unlink .pre-commit-config.yaml
+
+  if [ -e "${_pre_commit_hooks_nix_local_config_file}" ]; then
+    echo 1>&2 "pre-commit-hooks.nix: WARNING: Refusing to install because of pre-existing .pre-commit-config.yaml"
+    echo 1>&2 "    1. Translate .pre-commit-config.yaml contents to the new syntax in your Nix file"
+    echo 1>&2 "        see https://github.com/cachix/pre-commit-hooks.nix#getting-started"
+    echo 1>&2 "    2. remove .pre-commit-config.yaml"
+    echo 1>&2 "    3. add .pre-commit-config.yaml to .gitignore"
+    return 1;
+  fi
+
+  ln -fs "$_pre_commit_hooks_nix_config" "${_pre_commit_hooks_nix_local_config_file}"
+}
+
+# (private)
+#
+# Perform the installation of the hooks. Relies on some global variables.
+_pre_commit_hooks_nix_install_stages() {
+  # Remove any previously installed hooks (since pre-commit itself has no convergent design)
+  hooks="pre-commit pre-merge-commit pre-push prepare-commit-msg commit-msg post-checkout post-commit"
+  for hook in $hooks; do
+    pre-commit uninstall -t "$hook"
+  done
+  # Add hooks for configured stages (only) ...
+  if [ -n "$_pre_commit_hooks_nix_install_stages" ]; then
+    for stage in $_pre_commit_hooks_nix_install_stages; do
+      if [[ "$stage" == "manual" ]]; then
+        continue
+      fi
+      case $stage in
+        # if you amend these switches please also review $hooks above
+        commit | merge-commit | push)
+          stage="pre-$stage"
+          pre-commit install -t "$stage"
+          ;;
+        prepare-commit-msg | commit-msg | post-checkout | post-commit)
+          pre-commit install -t "$stage"
+          ;;
+        *)
+          echo 1>&2 "ERROR: pre-commit-hooks.nix: either $stage is not a valid stage or pre-commit-hooks.nix doesn't yet support it."
+          exit 1
+          ;;
+      esac
+    done
+  # ... or default 'pre-commit' hook
+  else
+    pre-commit install
+  fi
+}