how do we restrict access to a file thru unix command?

Answer 1

The command is chmod:

chmod [mode]
% chmod u+w foo
adds write permissions to the owner of file 'foo'

% chmod 750 foo
Owner (user) can do anything with the file (read, write, execute), group members can read and execute, others can do nothing with the file.

Answer 2

using the chmod command. do a "man chmod" to get the information on how to use it to set permissions