Setting access permissions numerically
There is a shorthand way of setting permissions by using octal numbers. 
Read permission is given the value  4 , write permission the value  2  and 
execute permission  1 .

    r  w  x
     4    2    1 

These values are added together for any one user category:

     1    =   execute only
     2    =   write only
     3    =   write and execute ( 1 + 2 )
     4    =   read only
     5    =   read and execute ( 4 + 1 )
     6    =   read and write ( 4 + 2 )
     7    =   read and write and execute ( 4 + 2 + 1 )

So access permissions can be expressed as three digits. For example:

                          user    group   others

    chmod  640  file1       rw-     r --     ---
 
    chmod  754  file1       rwx     r-x     r --
 
    chmod  664  file1       rw-     rw-     r--