Differences database sql statement on result set limit

// mysql
select col from tbl limit 20;

// Oracle
select col from tbl where rownum<=20;

// Microsoft SQL
select top 20 col from tbl;

How to create set the hostname in Linux Redhat

% vi /etc/sysconfig/network file

Modify HOSTNAME key
HOSTNAME="your preferred hostname"

restart network service with following command

% service network restart

How to query the size of the directory and subdirectory

list usage of all files
tsongl@guiness $ du -a
4 ./test1/eee
8 ./test1
4 ./abc
4 ./cde
20 .

summarize of directory and subdirectory
tsongl@guiness $ du -s
20 .