linux 判断当前用户是否是root

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 

#---------------------------- 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 
}