OPS145 Lab 4 Newversion

Binary review

Go back and review the binary stuff we looked at in the course introduction.

This lab is about POSIX permissions, and you need to be completely comfortable with binary-to-decimal and decimal-to-binary conversion from 000 to 111 (decimal 0 to 7).

Here's a summary of the absolute minimum you need to remember from that:

Binary	Decimal
000	0+0+0=0
001	0+0+1=1
010	0+2+0=2
011	0+2+1=3
100	4+0+0=4
101	4+0+1=5
110	4+2+0=6
111	4+2+1=7

Read, Write, Execute (rwx) permissions

On a Linux filesystem every file and directory has 9 bits of information allocated for reording basic permissions. Those 9 bits are split into three groups of 3 bits.

Each group of 3 bits records whether the following permissions are granted:

• read permission (most significant bit, on the left, decimal 4)
• write permission (second bit, in the middle, decimal 2)
• execute permission (least significant bit, on the right, decimal 1)

These are usually called "octal" rather than decimal, but I suspect you won't find any value in that extra complication, so you can just think of them as decimal numbers.

Permissions for files

The read and write permissions for files are pretty self-explanatory.

Also:

• binary review
• read, write, execute permissions
  • for files
  • for directories
• owner user, owner group, others
• chmod with octal
• quick adduser with id 1145, passwd
• try examples with two users