write it in bash

installers/common.sh

@@ -385,7 +385,7 @@ function optimize_php() {
 }
 
 function build_R1 () {
-  if ( file_exists(/boot/armbianEnv.txt)); then
+  if [-f /boot/armbianEnv.txt]; then
     sudo sed -i "1i extraargs=net.ifnames=0" /boot/armbianEnv.txt
   fi
 }