diff --git a/pkgs/games/keen4/builder.sh b/pkgs/games/keen4/builder.sh
index b5864b8392f5..f39edc789995 100644
--- a/pkgs/games/keen4/builder.sh
+++ b/pkgs/games/keen4/builder.sh
@@ -1,10 +1,15 @@
-source $stdenv/setup
+#!/usr/bin/env bash
 
-mkdir -p $out/share/keen4
-unzip -j $dist -d $out/share/keen4
+set -o nounset
 
-mkdir -p $out/bin
-cat > $out/bin/keen4 <<EOF
+# shellcheck source=/dev/null
+source "${stdenv}/setup"
+
+mkdir -p "${out}/share/keen4"
+unzip -j "$dist" -d "${out}/share/keen4"
+
+mkdir -p "${out}/bin"
+cat > "${out}/bin/keen4" <<EOF
 #! $SHELL -e
 if test -z "\$HOME"; then
     echo "HOME directory not set"
@@ -17,9 +22,9 @@ fi
 mkdir -p \$HOME/.keen4
 cd \$HOME/.keen4
 
-ln -sf $out/share/keen4/* .
+ln -sf "${out}/share/keen4/"* .
 
-$dosbox/bin/dosbox ./KEEN4E.EXE -fullscreen -exit || true
+"${dosbox}/bin/dosbox" ./KEEN4E.EXE -fullscreen -exit || true
 
 # Cleanup the symlinks.
 for i in *; do
@@ -28,4 +33,4 @@ for i in *; do
     fi
 done
 EOF
-chmod +x $out/bin/keen4
+chmod +x "${out}/bin/keen4"