Linux+ numerical file permissions

rima-wilson

Hi!

I am in the process of learning file permissions for the Linux+ Certification. Specifically, I am looking at the numerical notation for file permissions. A thought occurred to me as to why the numbers 1-7 represent what they do.

I'll bet you have already explained this concept somewhere in your videos but here it is:

0 - None
1 - Execute
2 - Write
3 - Write and Execute
4 - Read
5 - Read and Execute
6 - Read and Write
7 - Read, Write and Execute

Here are the values that you mentioned for numerical notation. Are these numbers in 3 binary sets?

ie.
6 - read and write
110 ( which is 6 in Binary)

4 - read
100

Does it make sense to think of the numeric file permissions in the form of binary numbers in which each value represents (read, write, execute) --> (111)?

Thank you!

Ronnie Wong

@rima-wilson ,
Each number you mention correlates to 4 binary bits (0000 - 1111) technically, even so the highest order bit is rarely used (0000).

As far as permissions go, the single digits are normally combined to give the proper permissions for a file: so instead of just seeing a single file listed with a 7, you'll see something more along the lines of 3 digits like 755 or 555 etc.

if you access a terminal and type ld -l

ronnie@SiteA-01 ~ $ ls -l
total 32
drwxr-xr-x 2 ronnie ronnie 4096 Jun  3 14:57 Desktop
drwxr-xr-x 2 ronnie ronnie 4096 Jun  8 20:12 Documents
drwxr-xr-x 2 ronnie ronnie 4096 Jun  8 21:29 Downloads
drwxr-xr-x 2 ronnie ronnie 4096 Jun  2 17:17 Music
drwxr-xr-x 2 ronnie ronnie 4096 Jun  2 17:17 Pictures
drwxr-xr-x 2 ronnie ronnie 4096 Jun  2 17:17 Public
drwxr-xr-x 2 ronnie ronnie 4096 Jun  2 17:17 Templates
drwxr-xr-x 2 ronnie ronnie 4096 Jun  2 17:17 Videos
ronnie@SiteA-01 ~ $

The string you see at the beginning: drwxr-xr-x is a symbolic notation of the octal notation you're referring.
the d would be for the directory the 1st set of 'rwx' = 7. This is what the OWNER can do, the 2nd tells us the permissions of the group member r-x = 5 and 3rd tells us what everyone else can do : r-x = 5 = overall giving an octal code of 755 for the directory.

So yes it makes sense to find that correlation. I'm not sure if you'll find any real practical matter, even if you're changing permissions, you're changing the octal and not necessarily needing to change it in binary.

Cordially,
Ronnie Wong
Host, ITProTV

rima-wilson

@Ronnie-Wong ,

I see, Thank you Ronnie! And thanks for the amazingly helpful content you all put out!