grep file in the subdirectory
 

The 'grep' function can only use in the current directory, how to grep file into the subdirectory ? Thx.

Try adding -d recurse to your grep search when using a directory as a source. As an example, from /, type:

grep -d recurse DHCP /etc

-mk

You can also use find with grep to narrow down your search. For example:
find /etc -type f -exec grep DHCP {} \;

Of course, this forks a new process for each file which is grep'ped. Be nice to you cpu:

find /etc -type f -print|xargs grep DHCP