如何判断linux指定用户对指定目录具有的权限

#!/bin/sh 
username3=$1 
dir_name2=$2 
# get existing directory 
file_path=$dir_name2 
while true 
do 
if [ -d $file_path ];then 
break; 
fi 
file_path=${file_path%/*} 
done 
dir_name2=$file_path 
# Judge whether the user exists 
grep "^$username3:" /etc/passwd >/dev/null 
if [ $? -ne 0 ];then 
echo "This user \"$username3\" does not exist." 
exit 4 
fi 
#echo "username : $username3" 
group4=` grep "^$username3:" /etc/passwd |awk -F : {'print $4'}|xargs -i grep {} /etc/group|cut -d":" -f1` 
#echo "group : $group4" 
su -l $username3 -c "test -r $dir_name2" 
is_read=$? 
su -l $username3 -c "test -x $dir_name2" 
is_exe=$? 
su -l $username3 -c "test -w $dir_name2" 
is_write=$? 
$is_read_str 
$is_exe_str 
$is_write_str 
if [ $is_read -eq 0 ];then 
is_read_str="r" 
else 
is_read_str="-" 
fi 
if [ $is_exe -eq 0 ];then 
is_exe_str="x" 
else 
is_exe_str="-" 
fi 
if [ $is_write -eq 0 ];then 
is_write_str="w" 
else 
is_write_str="-" 
fi 
echo "${is_read_str}${is_write_str}${is_exe_str}"