diff --git a/config/functions b/config/functions
index a7c846789e..2c533c9e0b 100644
--- a/config/functions
+++ b/config/functions
@@ -365,7 +365,7 @@ add_user() {
   # Usage: add_user "username" "password" "userid" "groupid" "description" "home" "shell"
   mkdir -p ${INSTALL}/etc
   touch ${INSTALL}/etc/passwd
-  if [ -z "`grep "$1:" ${INSTALL}/etc/passwd`" ]; then
+  if ! grep -q "^$1:" ${INSTALL}/etc/passwd; then
     echo "$1:x:$3:$4:$5:$6:$7" >> ${INSTALL}/etc/passwd
   fi
 
@@ -376,7 +376,7 @@ add_user() {
   if [ "$PASSWORD" = "x" ]; then
     PASSWORD="*"
   fi
-  if [ -z "`grep "$1:" ${INSTALL}/usr/config/shadow`" ]; then
+  if ! grep -q "^$1:" ${INSTALL}/usr/config/shadow; then
     echo "$1:$PASSWORD:::::::" >> ${INSTALL}/usr/config/shadow
   fi
 }