the 10

drwxr-xr-x

the first bit:

-	regular file
b	block special file
c	character special file
s	socket
d	directory
l	link
p	pipe

the other nine bits

User --- Group --- Others

chmod [parameter] [mode] [file]

// add  execute right to user 
chmod u+x a.file  //u means user, g means group, o means other , + means add, - means minus, = means assign

// add write right to group and others
chmod g+w, o+w a.file

//delete the execute right from user, delete write right from group and others
chmod u-x, g-w, o-w a.file 

use the digit in chmod

4 means r 2 means w 1 means x Ok, this is really important

chmod 744 a.file // rwxr--r--
chmod 755 a.file //rwxr-xr-x
// DON'T USE 777, That's really silly.