diff --git a/xCAT-server/lib/xcat/plugins/blade.pm b/xCAT-server/lib/xcat/plugins/blade.pm
index 52cc45b77..52c63906a 100644
--- a/xCAT-server/lib/xcat/plugins/blade.pm
+++ b/xCAT-server/lib/xcat/plugins/blade.pm
@@ -2178,7 +2178,7 @@ sub process_request {
 
 
   my $children = 0;
-  $SIG{CHLD} = sub { my $cpid; while ($cpid = waitpid(-1, WNOHANG) > 0) { if ($mm_comm_pids{$cpid}) { delete $mm_comm_pids{$cpid}; $children--; } } };
+  $SIG{CHLD} = sub { my $cpid; while (($cpid = waitpid(-1, WNOHANG)) > 0) { if ($mm_comm_pids{$cpid}) { delete $mm_comm_pids{$cpid}; $children--; } } };
   my $inputs = new IO::Select;;
   foreach my $info (@$moreinfo) {
     $info=~/^\[(.*)\]\[(.*)\]\[(.*)\]/;
diff --git a/xCAT-server/lib/xcat/plugins/kvm.pm b/xCAT-server/lib/xcat/plugins/kvm.pm
index 97980b7af..13ebcb51a 100644
--- a/xCAT-server/lib/xcat/plugins/kvm.pm
+++ b/xCAT-server/lib/xcat/plugins/kvm.pm
@@ -849,7 +849,7 @@ sub process_request {
   }
 
   my $children = 0;
-  $SIG{CHLD} = sub { my $cpid; while ($cpid = waitpid(-1, WNOHANG) > 0) { if ($vm_comm_pids{$cpid}) { delete $vm_comm_pids{$cpid}; $children--; } } };
+  $SIG{CHLD} = sub { my $cpid; while (($cpid = waitpid(-1, WNOHANG)) > 0) { if ($vm_comm_pids{$cpid}) { delete $vm_comm_pids{$cpid}; $children--; } } };
   my $inputs = new IO::Select;;
   my $sub_fds = new IO::Select;
   %hyphash=();
diff --git a/xCAT-server/lib/xcat/plugins/xen.pm b/xCAT-server/lib/xcat/plugins/xen.pm
index daaf3fdb3..3556197e0 100644
--- a/xCAT-server/lib/xcat/plugins/xen.pm
+++ b/xCAT-server/lib/xcat/plugins/xen.pm
@@ -714,7 +714,7 @@ sub process_request {
   }
 
   my $children = 0;
-  $SIG{CHLD} = sub { my $cpid; while ($cpid = waitpid(-1, WNOHANG) > 0) { if ($vm_comm_pids{$cpid}) { delete $vm_comm_pids{$cpid}; $children--; } } };
+  $SIG{CHLD} = sub { my $cpid; while (($cpid = waitpid(-1, WNOHANG)) > 0) { if ($vm_comm_pids{$cpid}) { delete $vm_comm_pids{$cpid}; $children--; } } };
   my $inputs = new IO::Select;;
   my $sub_fds = new IO::Select;
   %hyphash=();