diff --git a/Makefile b/Makefile
new file mode 100644
index 0000000..d6346e0
--- /dev/null
+++ b/Makefile
@@ -0,0 +1,5 @@
+.PHONY: all
+all: test
+
+test:
+	shellcheck --check-sourced --external-sources de-p1st-installer.sh
diff --git a/de-p1st-installer.sh b/de-p1st-installer.sh
index 6d211da..f8936f1 100755
--- a/de-p1st-installer.sh
+++ b/de-p1st-installer.sh
@@ -1,14 +1,33 @@
 #!/bin/bash
+# Exit on error.
 set -e
+# Exit on undefined variable reference.
 set -u
 
-# load config
-source /etc/de-p1st-installer/installer.cfg
+# Check if run from installation or locally (development setup).
+# -> Print commands before they get executed (set -v).
+# -> Determine library directory.
+script_dir="$(dirname "${BASH_SOURCE[0]}")" || exit 1
+if [ "${script_dir}" = '/usr/bin' ]; then
+  # This script has been installed.
+  lib_dir='/usr/lib/de-p1st-installer'
+else
+  # This script is run locally (development setup).
+  lib_dir='./lib'
+  set -v
+fi
 
-# load functions
-source /usr/lib/de-p1st-installer/util.sh
-source /usr/lib/de-p1st-installer/user-input.sh
-source /usr/lib/de-p1st-installer/block-device.sh
+# Source library files.
+# shellcheck source=lib/util.sh
+source "${lib_dir}"/util.sh
+# shellcheck source=lib/user-input.sh
+source "${lib_dir}"/user-input.sh
+# shellcheck source=lib/block-device.sh
+source "${lib_dir}"/block-device.sh
+
+# Source the configuration file.
+# shellcheck source=installer.cfg
+source /etc/de-p1st-installer/installer.cfg
 
 function main() {
   # @pre
@@ -91,7 +110,7 @@ function increase_cow_space() {
   # May be useful when running 'pacman -Syu' on the live medium.
   # Usually not necessary!
 
-  # make sure that we are on a live medium:
+  # Make sure that we are on a live medium:
   findmnt /run/archiso/cowspace >/dev/null || {
     echo 'Not on live medium, did not increase cowspace!'
     return 1
@@ -326,7 +345,7 @@ function run_pacstrap() {
 
   local args=()
   if [ "${PACSTRAP_INTERACTIVE}" = 'true' ]; then
-    args+=('-i')  # run interactively
+    args+=('-i')  # Run interactively
   fi
   args+=('/mnt')
 
@@ -409,7 +428,7 @@ function user_and_pwd() {
   #   ROOT_PWD (optional)
 
   echo 'Adding user and changing shell to /bin/zsh ...'
-  # -m: create home
+  # -m: Create home
   # -U: Create a group with the same name as the user, and add the user to this group.
   sudo arch-chroot /mnt useradd -m -s /usr/bin/zsh -g wheel "${USERNAME}"
   sudo arch-chroot /mnt chsh       -s /usr/bin/zsh
@@ -430,7 +449,7 @@ function bootloader() {
   echo 'Installing grub ...'
   case "${BOOT_FIRMWARE}" in
   uefi)
-    # portable fallback efi name for grub:
+    # Portable fallback efi name for grub:
     # * https://www.rodsbooks.com/efi-bootloaders/installation.html#alternative-naming
     # * arch-chroot /mnt cp /boot/EFI/GRUB/grubx64.efi /boot/EFI/BOOT/bootx64.efi
     sudo arch-chroot /mnt grub-install --target=x86_64-efi --bootloader-id=GRUB --efi-directory=/boot --removable