proper bash escaping

Author: Nuwah12

wdl_tasks/peak_calling.wdl

@@ -88,7 +88,7 @@ task MACS2_CallPeaks {
       ~{if call_summits then "--call-summits" else ""} \
       ~{broad_flag} \
       ~{if defined(estimated_fragment_size) then "--nomodel" else ""} \
-      ~{if defined(estimated_fragment_size) then ("--shiftsize="+"${HALF_FRAG}") else ""}
+      ~{if defined(estimated_fragment_size) then ("--shiftsize="+$HALF_FRAG) else ""}
       ~{if defined(estimated_fragment_size) then ("--bw="+Bandwidth) else ""} \
       ~{if paired_end then "--format BAMPE" else "--format BAM"} \
       --keep-dup=1 \