Unix/Linux file permisions
Permissions are usually represented as letters or numbers depending on what you are doing they have the same meaning.
The below are the basic permissions. Notice how the number 3 is missing well it will make sense shortly.
| Meaning | Number | Letter |
| No Permissions | 0 | – |
| Execute | 1 | x |
| Write | 2 | w |
| Read | 4 | r |
(Note: there are actually more permissions such as t, T, s, S and another x but this is just general information. I will add advanced topics latter)
Unix/Linux has 3 category of users that can access files they are the owner/user, group, and others. you will see this when you use the command:
$ ls -l // this will list non-hidden files in a directory
$ ls -al // This will list all files in a directory including hidden.
$ ls -l total 760
drwxr-xr-x 2 kirk www-data 4096 May 31 12:15 test
-rw-r--r-x 1 kirk kirk 6633 Jun 2 15:23 test_1.php
-rw-r--r-- 1 kirk kirk 413 Jun 2 08:18 test.js
drwxr-xr-x 2 kirk kirk 4096 Jun 2 15:32 test_xlsThere the first section has 10 places the first one usually has a ‘-‘ for a regular file ‘d’ for a directory and ‘l’ for a symbolic link. There are others but they are for advance uses.
The 2nd through 4th position show the user/owners permission it will display one of the following in each of the 3 positions for the user/owner ‘-,x,w,r’.
This will continue for the groups permission the next 3 positions and the others permissions in the last 3 positions.
| Below is an example of a directory | |||||||||
| Type | USER/OWNER | GROUP | OTHERS | ||||||
| 1st | 2nd | 3rd | 4th | 5th | 6th | 7th | 8th | 9th | 10th |
| d | r | w | x | r | – | x | r | – | x |
| directory | Owner Yes read |
Owner Yes write |
Owner Yes execute |
Group Yes read |
Group No write |
Group Yes execute |
Other yes read |
Others No write |
Others Yes execute |
| Below is an example of a regular file. The user/owner has full control, the group can only write to the file, others can do nothing | |||||||||
| – | r | w | x | – | w | – | – | – | – |
| Regular File | Owner Yes read |
Owner Yes write |
Owner Yes execute |
Group No read |
Group Yes write |
Group No execute |
Other No read |
Others No write |
Others No execute |
Now we know what each letter and number represent and that there are 3 permission categories (User, Group, Others) that we use to allow access to our files.
Now back to the missing number 3. Computers do math using binary(0 and 1s) and we understand it better if we use octal (0 through 7) and you will see this below.
Remember 0 means no permission, 1 means you can EXECUTE a file, 2 means you can WRITE to a file, and 4 means you can read a file. If you are paying attention you noticed that the order is opposite from what is displayed by the ls -l command (rwx) or is it….
Binary like decimal have the smallest number is on the right and they get bigger as you move to the left
| Binary | 00000001 | 00000010 | 00000100 |
| octal | 1 | 2 | 4 |
| symbol | x | w | r |
Now I will Reverse that table and expand it to include the 3,5,6, and 7
| Binary | 00000111 | 00000110 | 00000101 | 00000100 | 00000011 | 00000010 | 00000001 | 00000000 |
| Octal | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
| Symbol | xwr | wr | xr | r | wx | w | x | – |
| The Math | 1+2+4 | 2+4 | 1+4 | 1+2 |
As you can hopefully see that the numbers combine for more or permissions. Another way to look at it is to ignore the first zeros in the binary and think of it as three light switches. The switches are off when one a O (zero) and on when 1 (one). Here is my switchboard image maybe it helps maybe not
