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--

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