Changing permissions with chmod (numbers)

leeondet · 10-28-2016, 03:24 AM

Hi,

I am unsure how the following command #chmod 755 file, results in the permission: rwxr-xr-x

My understanding is that you have a 9 bit permission and you add up numbers (r=4, w=2, x=1)) to set each of the 3 parts of the permission(owner, group, and others)

I would have thought that the permission 755 would read rwx-xr-xr not rwxr-xr-x

I am new to Linux and this is an example in the Linux bible 9th edition, page 111.

Thanks,

Lee

kakistocrat · 10-28-2016, 04:07 AM

The grouping is a little different: rwx r-x r-x

The first triplet is for the first octal number.
The middle triplet is for the middle octal number.
The last triplet is for the last octal number.

So the letters have to be grouped that way.

Jjanel · 10-28-2016, 05:48 AM

Welcome! Great book! (I just got it from library today!)

For example, in your last 3 characters (perm. mode for 'others'; same for 'group')
where you guessed "-xr",
you have the right idea: no w=no2, an x=1, and an r=4 (your order, but not Linux's!)
but `ls -l` specifically 'reports' them in a particular order:
first: an r or - if no4, then second: a w or - if no2, then third: an x or - if no1,
so, to put it in a [strange] Y/N way: 5=binary101=YNY is reported as "r-x".

By the way, you'll see the owner called "user" and referred to as 'u',
to leave the 'o' for "other", on the next page in that book ['ugo']
(owner=User vs. Other/world can get confusing too)

Enjoy reading! Best wishes! (p.s. What 'distro' do you use? PC/boot?)

linux4evr5581 · 10-28-2016, 10:24 AM

[7=rwx|6=rw-|5=r-x|4=r--|3=-wx|2=-w-|1=--x] Just so you know the numerical version of chmod is for setting permissions absolutely, and should used when creating new files. The letter version adds or subtracts permissions as opposed to setting absolute values, for example:

chmod ugo+rwx or chmod u-rw or chmod u=rwx,g=rwx,o=rwx or a=rwx (a means all)

chmod -R ugo-rwx * (revokes all permissions for the current directory aswell as its sub directories), chmod ugo= * (revokes all permissions for all files in the current directory, but not the current directory its sub directories)

kakistocrat · 10-28-2016, 10:32 AM

Quote:

Originally Posted by linux4evr5581

[7=rwx|6=rw-|5=r-x|4=r--|3=-wx|2=-w-|1=--x] Just so you know the numerical version of chmod is for setting permissions absolutely, and should used when creating new files. The letter version adds or subtracts permissions as opposed to setting absolute values, for example:
chmod ugo+rwx or chmod u-rw or chmod u=rwx,g=rwx,o=rwx or a=rwx (a means all)

At the risk of scaring with a short shell script, it is possible to view all the permissions minus the sticky bit, the SetGUID bit, and the SetUID bit using some loops:

Code:

#!/bin/bash

temp=$(mktemp || exit);

touch $temp;

{ 
    for o in {0..7}; do 
        for g in {0..7}; do 
            for u in {0..7}; do 

                chmod $u$g$o $temp; 
                echo chmod $u$g$o : $(stat --printf "%A" $temp); 

            done; 
        done; 
    done; 
} | less  

rm -f $temp;

linux4evr5581 · 10-28-2016, 10:46 AM

Sounds useful, I have no idea how to read that but I get the idea

kakistocrat · 10-28-2016, 10:59 AM

"stat" is the useful part there, if only one part should be highlighted.

Code:

man stat

linux4evr5581 · 10-28-2016, 11:04 AM

stat as in status I guess.. I really need to lean shell scripting, sed, awk and all that, im just so overwhelmed with everything else computer related. But one day...

rtmistler · 10-28-2016, 11:38 AM

Quote:

Originally Posted by leeondet

My understanding is that you have a 9 bit permission and you add up numbers (r=4, w=2, x=1)) to set each of the 3 parts of the permission(owner, group, and others)

I would have thought that the permission 755 would read rwx-xr-xr not rwxr-xr-x

Hi and Welcome to LQ!

Pretty much all of the answers given are valid, not changing them. However please look at how your perceived this because even I got confused with your thinking.

"Weighting is r=4, w=2, x=1" - Yes, that's fine.

But you interpreted 755 to mean rwx -xr -xr. By the convention you mentioned, w=2 and x=1, well that adds up to 3. I believe this is where you confused yourself, right from the get-go.

Shadow_7 · 10-28-2016, 02:53 PM

binary octal sets. AKA 3 bit groups.

000 000 000 == 000
111 111 111 == 777

421 421 421
rwx rwx rwx

r+x == 4+1 == 5
r+w+x == 4+2+1 == 7

Most chmod setups let you use the +x and similar flags now instead of doing the math. Although the math is more predictable and definitive.

suicidaleggroll · 10-28-2016, 04:29 PM

Quote:

Originally Posted by rtmistler

But you interpreted 755 to mean rwx -xr -xr. By the convention you mentioned, w=2 and x=1, well that adds up to 3. I believe this is where you confused yourself, right from the get-go.

He still has the letters write, x and r for 5, he just rearranged them, putting "-xr" instead of "r-x".

OP - the order in the permissions is ALWAYS "r w x", in that order, no exceptions. If the bit is set, you put the letter, if it's not, you put a "-". "5" is "4+1", which means r (read) and x (execute) are set, while w (write) is not, so you would write it as "r-x". That's not "r minus x" or "r and x", it's "r <blank> x", where <blank> would be "w", if "w" was set. Similarly, permission 0 is written "---", permission 1 is written "--x", permission 2 is written "-w-", and so on.

leeondet · 10-28-2016, 09:18 PM

Thanks for taking the time to respond. It makes sense now that i know that there is a specific order that can not be changed, and that the permission is represented by a letter for on, or a dash for off. I was viewing the dash as a separator rather than an indicator of a permission.

leeondet · 10-28-2016, 09:22 PM

Hi Jjanel, i'm using Fedora workstation 24 and just booting it from a USB stick at the moment. I want to work through the book before deciding whether to completely change over from Windows

Jjanel · 10-29-2016, 12:04 AM

Cool! Maybe I can 'sell' you dozens of free&EASY&safe VirtualBox'es, with DistroWatch's .iso

Looking forward to your next question! Best Wishes... (p.s. what computer do you use?)