diff --git a/shady.sh b/shady.sh
index d06d500..d852221 100755
--- a/shady.sh
+++ b/shady.sh
@@ -1,6 +1,14 @@
 #!/bin/bash
 
-shaderfile="$(shuf -n1 -e shaders/*)"
+if [[ -n "$4" ]]; then
+    shaderfile="shaders/${4}.frag"
+    if [[ ! -f "$shaderfile" ]]; then
+        echo "Error: shader file '$shaderfile' does not exist." >&2
+        exit 1
+    fi
+else
+    shaderfile="$(shuf -n1 -e shaders/*)"
+fi
 
 export EGL_PLATFORM=surfaceless
 export MESA_GL_VERSION_OVERRIDE=3.3