#replace mkdir
function mkdir() {command mkdir "$1"  &&  cd "$1";}

#check if folder exisits
function mkdir() { 
  if [ ! -d "$1" ];
  then 
    command mkdir "$1"  &&  cd "$1";
  else 
    echo "$1 already exists.";
   fi; 
}

#or in a one liner
function mkdir() { [ ! -d "$1" ] && (command mkdir "$1"  &&  cd "$1")|| echo "$1 already exists."; }

#other options
mcd() { mkdir "$1" && cd "$1"; }