Prince config - use function to check if env SCRATCH is set

conf/prince.config

@@ -1,6 +1,6 @@
-singularityDir = "$SCRATCH/singularity_images_nextflow"
-singularityModule = "singularity/3.2.1"
-squashfsModule = "squashfs/4.3"
+def singularityDir = set_singularity_path()
+def singularityModule = "singularity/3.2.1"
+def squashfsModule = "squashfs/4.3"
 
 params {
     config_profile_description = """
@@ -13,11 +13,21 @@ params {
 }
 
 singularity {
-      enabled = true
-      cacheDir = singularityDir
+    enabled = true
+    cacheDir = singularityDir
 }
 
 process {
         beforeScript = "module load $singularityModule $squashfsModule"
         executor = 'slurm'
 }
+
+def set_singularity_path() {
+    def scratch = System.getenv('SCRATCH')
+    if(scratch == null){
+        System.err.println("WARNING: prince.config requires the SCRATCH env var to be set.")
+        return null
+    } else {
+        return "$SCRATCH/singularity_images_nextflow"
+    }
+}