fix bug

tasks/MD5.wdl

@@ -1,12 +1,13 @@
 task MD5 {
 
 	File file
+	String name = basename(file)
 	String docker
 	String cluster_config
 	String disk_size
 
 	command <<<
-		md5sum ${file} > $(basename ${file}.md5)
+		md5sum ${file} > ${name}.md5
 	>>>
 
 	runtime {
@@ -17,6 +18,6 @@ task MD5 {
 	}
 
 	output {
-		File MD5 = "$(basename ${file}.md5)"
+		File MD5 = "${name}.md5"
 	}
 }