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Ya, they are binary codes and the layout is rwxrwxrwx where the 1st rwx is user, 2nd rwx is group, and the 3rd is other. Put a one in the spots you want enabled, a 0 where you want disabled. And then convert the result of each group into decimal. For instance 101 (r-x) = 5.
ok well what he is trying to get across is that you only actually need the following :
1 === execute
2 === write
4 === read
cause he first states that each group has the available permissoins rwx ..
then now he states that if you want the outcome of the results to be for example r-x, which is read access, execute access, but no write access, then to make it a little easier just use 1's and 0's ....
so a - = not enabled
and either a r,w,x in a position means = is enabled
so in our example we have r-x .. so the following is true :
r = enabled (takes a value of 1)
- = write is NOT enabled (takes a value of 0)
x = execute is enabled (takes a value of 1)
so if you put them together you get 101 because the format is rwx .. got it so far
so if we look at the chart given above we see that read = 4 and execute = 1 so if we add 4 + 1 we get the answer of 5 ...
another quick example ...
we'll use the following -> --x
so if we break it down
- = read is not enabled (takes a value of 0)
w = IS enabled (takes a value of 1)
x = IS enabled (takes a value of 1)
so you put it together and you get 011 (keep in mind the format of rwx, and also our chart above), and add them together (0 + 2 + 1 = 3) you get the answer of 3
just noticed that someone else got to it before me in explaining this little journey, but with both posts i think you'll get a better understand of what jtshaw was trying to get across ...
It's a bitmask, all possible combinations are represented by a unique number. You really only need to memorize 1, 2 and 4 (if there were more options would then go to 8, 16, 32, etc...) Just add the numbers together for the combinations. It's not that hard.
Really should use user, group and other instead of owner, group and world if you don't want to confuse people with things like chmod o+r. A newbie is more likely to think that grants the owner read permission than the world. There's no reason to make it more difficult than it really is IMHO.