diff --git a/aptpkg b/aptpkg
index 70624d1..a58e30f 100755
--- a/aptpkg
+++ b/aptpkg
@@ -28,7 +28,7 @@ error_usage_die(){
 
 # Verify sha512sums
 verify(){
-	echo "$sha512sums" | sha512sum -c
+	echo "${sha512sums:?}" | sha512sum -c
 }
 
 # Copy/ install wrapper for files
@@ -50,31 +50,31 @@ check(){
 # Generate the debian control file
 gen_control_file(){
 	mkdir -p "$builddir"/DEBIAN
-	echo "Package: $name
-Version: $version-$rev
+	echo "Package: ${name:?}
+Version: ${version:?}-${rev:?}
 Architecture: amd64
 Maintainer: $(grep -am 1 -hr "Maintainer: " build | sed 's/# Maintainer: //')
 Section:
 Priority:
-Homepage: $url
-Description: $description
-$customcontrol
+Homepage: ${url:?}
+Description: ${description:?}
+${customcontrol:?}
 " > "$builddir"/DEBIAN/control
 }
 
 # Download a debian package
 build_deb(){
-	echo "Getting $source"
+	echo "Getting ${source:?}"
 	wget "$source" -qP dist/
 
-	( cd dist ; verify )
+	( cd dist || error_usage_die "Problem with the dist/ directory" && verify )
 }
 
 # Download and build using files
 build_file(){
 	builddir=${name}_${version}-${rev}_amd64
 
-	cd "$1"
+	cd "$1" || error_usage_die "Problem with changing directory"
 	mkdir -p "$builddir"
 
 	for url in $source; do
@@ -93,7 +93,7 @@ build_file(){
 		[ -f $maintfile ] && cin-bin $maintfile "$builddir"/DEBIAN/$maintfile
 	done
 
-	cd -
+	cd - || error_usage_die "Problem with changing directory"
 
 	dpkg-deb --build --root-owner-group "$1"/"$builddir" dist/
 }
@@ -103,9 +103,9 @@ load_build(){
 	mkdir -p dist
 
 	# shellcheck disable=SC1090
-	source "$1/build"
+	source "${1/build:?}"
 
-	case "$type" in
+	case "${type:?}" in
 		deb)
 			build_deb ;;
 		file)