linux判断root用户

linux 中，如何判断当前用户是否是root  呢？
判断当前用户是否是root
Shell代码 

#---------------------------- root user check ---------------------start     
if [ `id -u` -ne 0 ]; then    
    echo "Please re-run ${this_file} as root."    
    exit 1     
fi     
#---------------------------- root user check ---------------------end    
   
  
if [ x"$USER" = x"" ];then    
    if  id |grep "^uid=0(root)">/dev/null ;then    
        :     
    else    
        echo "Please rerun `basename $0` as root ."    
        exit 1     
    fi     
else    
    if [ ! x"$USER" = x"root" ];then    
        echo "Please rerun `basename $0` as root ."    
        exit 1     
    fi     
fi    
   
  
#=============== check current user start ================     
if [ ! $USER = root  ];then    
    echo "Please rerun this script as root ."    
    exit 1     
fi     
#=============== check current user end ================    
   
  
#!/bin/sh     
if  id |grep "^uid=0(root)">/dev/null ;then    
    echo is  root     
else    
    echo not  root     
fi     
   
  
check_user(){     
    if [ $UID -ne 0 ];then    
        echo "Current user is not ROOT."    
        echo "Please rerun the installer as Root user."    
        exit 11     
    fi     
}