diff --git a/contrib/tools/hooks/purge-varnish.sh b/contrib/tools/hooks/purge-varnish.sh
index 871bc1c..ca0a022 100755
--- a/contrib/tools/hooks/purge-varnish.sh
+++ b/contrib/tools/hooks/purge-varnish.sh
@@ -9,6 +9,6 @@ ip="127.0.1.1"
 varnish="127.0.0.1:8000"
 
 # purge thread page
-curl --interface "$ip" -X PURGE http://$varnish/thread-$(sha1sum <<< "$3" | cut -d' ' -f1).html &> /dev/null
+curl --interface "$ip" -X PURGE http://$varnish/thread-$(echo -n "$3" | sha1sum | cut -d' ' -f1).html &> /dev/null
 # purge board page
 curl --interface "$ip" -X PURGE http://$varnish/$1-0.html &> /dev/null