Instead of the letters r, w, and x, you can use the numbers 4, 2, and 1 to set file permissions. Take a look at the following image:
Notice that the first number, 7, is basically the addition of the three numbers: 4 (r) + 2 (w) + 1 (x), which sets full permissions to the file owner. The second number, 6, is the addition of the two numbers: 4 (r) + 2(w), which sets the read and write permissions to the group owner. Finally, the third number, 4, which sets the read permission to others.
I know what you are thinking: "Why would I want to do math when I can just use the literal notation rwx?" And trust me, I feel you. A lot of people prefer the literal notation over the numeric notation, but some people just love numbers way too much!
Let's do some practice with the octal notation. There are currently zero permissions on the file mysmurf:
smurf@ubuntu-linux:~$ ls -l mysmurf
---------- 1 smurf developers 64 Oct 23 13...