chmod octal numbers

fakie_flip · 03-19-2010, 11:05 AM

Is there an easy way to remember what the each of the 8 numbers, 0-7 represent for chmod and umask? That way I would not have to keep guessing until I get it right.

centosboy · 03-19-2010, 11:27 AM

Quote:

Originally Posted by fakie_flip

Is there an easy way to remember what the each of the 8 numbers, 0-7 represent for chmod and umask? That way I would not have to keep guessing until I get it right.

in short -
wrt chmod, simplest way to remember apart from doing all the binary stuff is

r=4
w=2
x=1

the stat command will give a helping hand here..... stat <file>
with umask an example is

000 = creation of folders with 777 perms (rwxrwxrwx)
creation of files 666 (rw-rw-rw)

777 = creation of folders 000 perms (---------)
and same with files

colucix · 03-19-2010, 11:31 AM

Yes. Think at permissions:

Code:

-rwxrwxrwx

as numbers

Code:

-421421421

each set of permissions (user, group, other) is given by the sum of the three numbers. So that (just some examples):

Code:

-rw-------  is 4+2 0 0 = 600
-rwxr-xr-x  is 4+2+1 4+1 4+1 = 755
-rw-r--r--  is 4+2 4 4 = 644

The umask is the reverse and you have to subtract permissions from 777 one digit at a time, for example:

Code:

               777 -
permissions:   755 =
---------------------
umask:         022

Edit: too late... sorry for redundancy!

fakie_flip · 03-24-2010, 01:26 PM

Thanks.

Now I know I only have to remember that r=4, w=2, x=1, and I can figure out the rest with simple addition.

chrism01 · 03-24-2010, 07:28 PM

In fact you can use the symbolic notation ie rwx for chmod and umask; I always do. There is no difference for chmod, but for umask, you quote the protections you want instead of the reverse ie no subtraction involved.
http://linux.die.net/man/1/umask

fakie_flip · 03-27-2010, 03:03 AM

Quote:

Originally Posted by chrism01

In fact you can use the symbolic notation ie rwx for chmod and umask; I always do. There is no difference for chmod, but for umask, you quote the protections you want instead of the reverse ie no subtraction involved.
http://linux.die.net/man/1/umask

I am aware, but I like using octal numbers, maybe because I'm a computer nerd

plus it's shorter to write.

One thing I have to remember when using rwx for chmod is that 'o' does not stand for owner. It stands for other. I've often made that mistake. It's easy to do. Even the man page for chmod says this.

"A combination of the letters ugoa controls which users' access to the
file will be changed: the user who owns it (u)"

Code:

chris@ubuntu910:/tmp$ touch test{1,2}
chris@ubuntu910:/tmp$ chmod u+r,u-wx,g+w,g-rx,o+x,o-rw test1
chris@ubuntu910:/tmp$ ls -l test1 
-r---w---x 1 chris chris 0 2010-03-27 02:50 test1
chris@ubuntu910:/tmp$ chmod 421 test2 
chris@ubuntu910:/tmp$ ls -l test2 
-r---w---x 1 chris chris 0 2010-03-27 02:50 test2
chris@ubuntu910:/tmp$

See how much shorter and simpler to write the command using octal numbers is? Now that I know that figuring out octal numbers for permissions can be found using simple addition rather than memorizing, I find it much easier.

chrism01 · 03-29-2010, 01:24 AM

You don't write it that way:

chmod u=r,g=w,o=x filename

some people find letters (user, group, other), (read, write, exe) easier than nums. Its just personal preference.
I can do either...

fakie_flip · 03-31-2010, 10:54 AM

Quote:

Originally Posted by chrism01

You don't write it that way:

chmod u=r,g=w,o=x filename

some people find letters (user, group, other), (read, write, exe) easier than nums. Its just personal preference.
I can do either...

Shows how little I use it. This thread is about the octal numbers.

MTK358 · 03-31-2010, 11:00 AM

I prefer octal.

Anyway, reading about the binary system could help you understand it better.