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L i n u x H e l p
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Permissions explained
Linux permissions seem a bit cryptic at first glance, but once you begin to understand how they work, it's a breeze. The first thing we need to understand in an effort to get permissions under control is binary to octal conversion. Take a look at the following output of the command ' ls -l '.
drwxr-xr-- 1 krnl nobody 5274 Oct 3 21:51 somedir
Permissions owner group size date filename
Now, the permissions column is broken down as follows:
The first character, in this case 'd', tells the file's type. There are several possible entries in this field.
- = regular file
d = directory
c = character special file ( /dev/ttyp0 )
b = block special file ( /dev/fd0 )
l = symbolic link
So, in the example above, the owner (krnl) can read, write and execute; the group (nobody) can read and execute;
and all other users can read this directory. Let's say that we want to give the group 'nobody'
write access to this directory.
# ls -l
drwxr-xr-- 1 krnl nobody 1024 Oct 3 21:51 somedir
# chmod 774 somedir
# ls -l
drwxrwxr-- 1 krnl nobody 1024 Oct 3 21:51 somedir
Notice how we added the write capability to the group that the directory belongs to. Using the command above with the various number combinations allowed by chmod will allow you to change file permissions to what you need.
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000 = ---------
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OWNER
001 = --------x
002 = -------w-
003 = -------wx
004 = ------r--
005 = ------r-x
006 = ------rw-
007 = ------rwx
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GROUP
010 = -----x---
020 = ----w----
030 = ----wx---
040 = ---r-----
050 = ---r-x---
060 = ---rw----
070 = ---rwx---
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USER
100 = --x------
200 = -w-------
300 = -wx------
400 = r--------
500 = r-x------
600 = rw-------
700 = rwx------
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Hopefully you have the point now ;-)
The fourth bit
The fourth bit can be optionally used to set userid, groupid and sticky bit (save text). The following
example shows how to set the SUID bit. SUID or Set User ID is used when you want an executable to run as
the file's owner regardless of who executes it. The following example shows how to use chmod to make a file
SUID root, which is generally a bad idea (anyone who executes the file executes it as root).
# ls -l
-rwxr-xr-x 1 root nobody 49358 Oct 7 14:39 filename
# chmod 4755 filename
# ls -l
-rwsr-xr-x 1 root nobody 49358 Oct 7 14:39 filename
SGID is very similar to SUID except that when executed, the program runs with the permissions of the group that
it belongs to, regardless of who executes it. Take the following example in which we change the permissions so
that 'filename' is executed with the group permissions of 'nobody':
# ls -l
-rwxr-xr-x 1 root nobody 49358 Oct 7 14:39 filename
# chmod 2755 filename
# ls -l
-rwxr-sr-x 1 root nobody 49358 Oct 7 14:39 filename
The purpose of the sticky bit or save text bit (t) is to cause the operating system to not delete a program's text
from swap space when all user processes finish. This allows the next user to run the process to run it with the
image already in swap or physical memory, therefore making process startup faster. Here is how we would set the sticky bit:
# ls -l
-rwxr-xr-x 1 root nobody 49358 Oct 7 14:39 filename
# chmod 1755 filename
# ls -l
-rwxr-xr-t 1 root nobody 49358 Oct 7 14:39 filename
I hope you have found this helpful.
-Krnl
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L i n u x W o r l d N e w s
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What in the hell is a KrnlPanic? Well, a KrnlPanic is me! Actually, let's start with "What is a kernel?". The
kernel is the core of your operating system (OS), whether your OS is Linux, Unix or windows. The kernel takes
care of all process management (what program runs and when), memory management (which parts of memory get used for what) and
also, the kernel takes care of interfacing the OS with your computer's hardware (disk drives, sound card, modem, network card, etc).
Now...since the kernel is doing all of these extremely important jobs, it stands to reason
that if it has an error, it will be a Bad Thing(tm).
If you use windows, you know a kernel panic as a "BSOD" or Blue Screen of Death. Or how about "Invalid Page
Fault in KERNEL32.DLL". I'm sure you've probably seen both of those. In Linux, a Kernel Panic is normally plainly stated
so. At boot time is when you will see most linux panics. I have yet to see a linux kernel panic while the system is running.
You may have seen "Kernel Panic: init not found" or "Kernel Panic: VFS unable to mount root fs on 2:00". All of these previously
listed errors are because of something that happened to the kernel that it couldn't handle, whether it was an access to an
invalid memory location or the inability to find the initialization files it requires.
I hope that sufficiently explains things. Oh yeah...KrnlPanic is also my name ;-)
- Rick
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S l a s h d o t / F r e s h m e a t
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