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:
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--)
| | |
0+4+2 0+4+2 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
39), 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
Figure 39: Removing group write permissions
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
Tip: Beware 666 and 777. Biblical implications aside, either of
these settings will allow everyone to read and write to a file or
directory. Such settings as these could allow tampering with sensitive
files, so in general, it's not a good idea to allow these settings.
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.
Summary: You can change permissions with the chmod
command by using letters or numbers. Type chmod (permissions) file
to change permissions of a file or directory.
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. The following chapter wil help you to understand a little more
about file types and how to work with a variety of commands.
