Fun with Numbers in chmod

Remember when we made a reference to the "shorthand" method of chmod? Here's another way to change permissions; it may seem a little complex at first - especially if math isn't your strong suit.

Let's go back to the original permissions for sneakers.txt .

-rw-rw-r--    1 billy billy     150 Mar 19 08:08 sneakers.txt

	  

Each permission setting can be represented by a numerical value:

• r = 4

• w = 2

• x = 1

• - = 0

When these values are added together, the total is used to set specific permissions - more specific than changing permissions with the alphabetical "shorthand."

In sneakers.txt , then, here are the numerical permissions settings:

 -  (rw-)   (rw-)  (r--)
      |       |      |
    4+2+0   4+2+0  4+0+0
	  

The total for the user is six, the total for the group is six and the total for others is four. The permissions setting, then, is read as 664 .

If we want to change sneakers.txt so those in our group didn't have write access, but could still read the file (as shown in Figure 3-20 ), we'll have to remove the access by subtracting 2 from that set of numbers.

The numerical values, then, would become six, four and four -- or 644.

So we can type:

chmod 644 sneakers.txt

	  

Let's check our changes by listing the file ( ls -l sneakers.txt ):

-rw-r--r--    1 billy billy     150 Mar 19 08:08 sneakers.txt

	  

And there it is; now, neither the group nor others have write permission to sneakers.txt . To return the group's write access for the file, we can just add the value of w (2) to the second set of permissions.

chmod 664 sneakers.txt

	  

Here's a list of some common settings, numerical values and their meanings:

• -rw------- (600) -- Only the user has read and write permissions.

• -rw-r--r-- (644) -- Only user has read and write permissions; the group and others can read only.

• -rwx------ (700) -- Only the user has read, write and execute permissions.

• -rwxr-xr-x (755) -- The user has read, write and execute permissions; the group and others can only read and execute.

• -rwx--x--x (711) -- The user has read, write and execute permissions; the group and others can only execute.

• -rw-rw-rw- (666) -- Everyone can read and write to the file. Bad idea.

• -rwxrwxrwx (777) -- Everyone can read, write and execute. Another bad idea.

Here are a couple common settings for directories:

• drwx------ (700) -- Only the user can read, write in this directory.

• drwxr-xr-x (755) -- Everyone can read the directory, but its contents can only be changed by the user.

You've already come quite a distance in learning about your Red Hat Linux system -- from navigation to setting and changing permissions. Now, it's time to learn a little more about managing what you have on your system.