diff --git a/agent/agent.sh b/agent/agent.sh
index bf0c392..5fa02fc 100644
--- a/agent/agent.sh
+++ b/agent/agent.sh
@@ -38,13 +38,13 @@ function downloadUpdate {
   URL="https://raw.githubusercontent.com/kroese/virtual-dsm/master/agent/agent.sh"
   remote_size=$(curl -sIk -m 4 "${URL}" | grep -i "content-length:" | tr -d " \t" | cut -d ':' -f 2)
 
-  [ "$remote_size" == "" ] && return
-  [ "$remote_size" == "0" ] && return
+  [[ "$remote_size" == "" ]] && return
+  [[ "$remote_size" == "0" ]] && return
 
   SCRIPT=$(readlink -f ${BASH_SOURCE[0]})
   local_size=$(stat -c%s "$SCRIPT")
 
-  [ "$remote_size" == "$local_size" ] && return
+  [[ "$remote_size" == "$local_size" ]] && return
 
   if ! curl -sfk -m 10 -o "${TMP}" "${URL}"; then
     echo "$HEADER: curl error" && return
@@ -56,7 +56,7 @@ function downloadUpdate {
 
   line=$(head -1 "${TMP}")
 
-  if [ "$line" != "#!/usr/bin/env bash" ]; then
+  if [[ "$line" != "#!/usr/bin/env bash" ]]; then
     echo "$HEADER: update error, invalid header: $line" && return
   fi