chmod by the Numbers

Up to this point, we’ve been setting the mode with letters. It turns out that you can also set the mode numerically. Here’s how it works:

  1. Write the permissions you want the file to have. To make your life easier, write the permissions grouped into sets of three letters. For example, let’s say you want file to have these permissions

    - rwx r-x r--
  2. Under each letter, write a digit 1; under each dash write a digit zero. Ignore the dash at the very beginning that tells you whether it’s a file or directory. This gives you three binary numbers.

    - rwx r-x r--
      111 101 100
  3. Now convert each set of three digits to a single digit using this table:

    BinaryBecomes 0000 0011 0102 0113 BinaryBecomes 1004 1015 1106 1117

    From our example, the 111 101 100 translates to the number 754.

  4. Now use that number in a chmod command to set your desired permissions on the file:

    chmod 754