Computer Math Part 5: Octal

Octal, or base-8, is commonly used in Unix style operating systems. This number system, being base-8, uses digits 0-7. It’s easily translated to binary, but hex and decimal are a little harder.

Each digit in octal is 3 digits in binary, so 08 is 0002, 18 is 0012, … 78 is 1112. Because it is simple to convert octal to binary, I would suggest that you convert octal to binary and then to decimal or hex.

Unix style operating systems use octal to define file permissions. Each file has a 3 octal digit code. The first digit defines the user’s permissions. The second defines the group’s permissions. The final digit defines everyone else’s permissions.

These permissions are based in binary. The first digit is to allow reading of the file. The second digit is to allow writing to the file. The third digit is to allow execution of the file.

A file with permissions of 777 allows everyone to do everything, while 664 allows the owner and group to read and write, but everyone else only gets to read the file.

Check back Wednesday for a discussion of binary truth tables.

Home work:

Convert the following from octal to decimal: