LinuxIntro: permissions-exercises.txt

Linux System Administration and IP Services

Exercises: Permissions
----------------------

# Notes

* Commands preceded with "$" imply that you should execute the command as
  a general user - not as root.
* Commands preceded with "#" imply that you should be working as root with
  "sudo"
* Commands with more specific command lines (e.g. "RTR-GW>" or "mysql>")
  imply that you are executing commands on remote equipment, or within
  another program.

REFERENCE

If you look at files in a directory using "ls -al" you will see the
permissions for each file and directories. Here is an example:

drwxrwxr-x    3 hervey   hervey       4096 Feb 25 09:49 directory
-rwxr--r--   12 hervey   hervey       4096 Feb 16 05:02 file

The left column is important. You can view it like this: 

Type User    Group Other Links  owner  group  size   date   hour  name
d    rwx     rwx   r-x   3      hervey hervey 4096   Feb 25 09:49 directory
-    rwx     r     r     12     hervey hervey 4096   Feb 16 05:02 file

So, the directory has r (read), w (write), x (execute) access for the
User and Group. For Other it has r (read) and x (execute) access. The
file has read/write/execute access for User and read only access for
everyone else (Group and Other).

To change permissions you use the "chmod" command. chmod uses a base
eight (octal) system to configure permissions. Or, you can use an
alternate form to specify permissions by column (User/Group/Other) at a
time.

Permissions have values like this: 
Letter  Permission   Value

R       read         4
W       write        2
X       execute      1
-       none         0

Thus you can give permissions to a file using the sum of the values for
each permission you wish to give for each column. Here is an example:

Letter  Permission                   Value

---     none                         0
--x     execute                      1
-w-     write only (rarely used)     2
-wx     write and execute (rare)     3
r--     read only                    4
r-x     read and execute             5
rw-     read and write               6
rwx     read, write, and execute     7


This is just one column. Since we have three areas of permissions (User,
Group, Other), it looks like this will all 3 sets:

Permissions  Numeric      Description
             equivalent  

-rw-------   600          User has read & execute permission.
-rw-r--r--   644          User has read & execute.
                          Group and Other have read permission.
-rw-rw-rw-   666          Everyone (User, Group, Other) have read & write
                          permission (dangerous?)
-rwx------   700          User has read, write, & execute permission.
-rwxr-xr-x   755          User has read, write, & execute permission.
                          Rest of the world (Other) has read & execute
                          permission (typical for web pages or 644).
-rwxrwxrwx   777          Everyone has full access (read, write, execute).
-rwx--x--x   711          User has read, write, execute permission.
                          Group and world have execute permission.
drwx------   700          User only has access to this directory.
                          Directories require execute permission to access.
drwxr-xr-x   755          User has full access to directory. Everyone else
                          can see the directory.
drwx--x--x   711          Everyone can list files in the directory, but Group
                          and Other need to know a filename to do this. 

1.) CHANGING FILE PERMISSIONS


If you are logged in as the root user on your machine please do the following:

        # exit

To become a normal user, like sysadm. Your prompt should change to include a “$” sign.
        
        $ 

Once logged in we'll create a file and set permissions on it in various ways.

        $ cd
        $ echo “test file” > working.txt
        $ chmod 444 working.txt

What does that look like?

        $ ls -lah working.txt

In spite of the fact that the file does not have write permission
for the owner, the owner can still change the file's permissions so
that they can make it possible to write to it. 

        $ chmod 644 working.txt 

Or, you can do this by using this form of chmod: 

        $ chmod u+w working.txt 

Note: when you type these command you should be able to use the tab key for 
command completion once you've typed the "w" in the file name "working.txt" -
This will save you quite a bit of time. It's highly recommended! :-)

To remove the read permission for the user on a file you would do

        $ chmod u-r working.txt 

Or, you can do something like: 

        $ chmod 344 working.txt 

You probably noticed that you can use the "-" (minus) sign to remove
permissions from a file. Try reading your file:

        $ cat working.txt

What happened? Uh oh! You can't read your file. Please make the file readable
by you !

        $ chmod ??? working.txt

Ask your instructor for help if you don't know what to put in for
“???”. Or, look at your reference at the start of these exercises to
figure this out.


2. PROGRAM EXECUTION, PRIVILEGES & SUDO

As a general user you can see that there is a file called “/etc/shadow”:

        $ ls /etc/shadow

But, you cannot see its contents:

        $ less /etc/shadow

What permissions does this file have? Use the examples above to figure this out. Fill
in the blanks below once you know the permissions. We've filled in one item to get you
stated:

-___R_____

As a general user, however, you can see the /etc/shadow file if you do the following:

        $ sudo less /etc/shadow

What is sudo? Read about it:

        $ man sudo



3. CREATE A NEW GROUP

        $ sudo groupadd team1

Prove that it really exists:

        $ grep team1 /etc/group

Now let's place our sysadm user in this new group:

        $ whoami

Just to be sure we really are the "sysadm" user right now:

        $ groups

You can see that sysadm is a member of the groups:

        sysadm adm cdrom plugdev lpadmin sambashare admin

Let's add our user to the team1 group - the '-a' is important!

        $ sudo usermod -a -G team1 sysadm

You won't be able to use your new group until you have logged in and out from
your account, or have simulated this process by doing this:

        $ su - sysadm

        (type your own password)

Now try typing:

        $ groups

You should see something like this:

        sysadm adm cdrom plugdev lpadmin sambashare admin team1

sysadm is now a member of the team1 group.

Using groups like this can be useful for working in teams on a project,
giving access to web directories, etc.


4. GIVE GROUP ACCESS TO A FILE

Do the following:

        $ cd
        $ echo “This is our group test file” > group.txt
        $ chgrp team1 group.txt
        
What permissions does the file have now?

        $ ls -l group.txt

You should see something like:

        -rw-r--r-- 1 sysadm team1 28 2012-04-16 01:32 group.txt

How would you give members of the group team1 read/write access to this
file? Before you look below try solving this on your own.

We'll use the numeric chmod functionality.

        $ chmod 664 group.txt

Alternatively you could have typed:

        $ chmod g+w group.txt

Look at the file's permissions:

        $ ls -l group.txt

You should see something like:

        -rw-rw-r-- 1 sysadm team1 28 2012-04-16 01:32 group.txt

By the way
 Did you remember to just type the "g" in the filename "group.txt" 
and then use the tab key to save time in the exercises above? If not, try using 
tab in upcoming exercises. It's really worth it!


5. MAKE A FILE EXECUTABLE 

Do this exercise as the sysadm user.

        $ cd
        $ touch hello

Now add a single line to the file that reads:

        echo 'Hello, world!'

        $ echo "echo 'Hello, world'" > hello

NOTE: We'll use file editors for operations like this after our next session.

Let's try to run this file:

        $ ./hello

You'll probably see something like:

        bash: ./hello: Permission denied

This implies that the file is not executable. We need to set the file's permission to be executable by our sysadm user. How would you do this?

        $ chmod 755 hello

would work. Now try running the file:

        $ ./hello 

You should see ...

        Hello, world!

... on your screen. 

Congratulations: you've just written your first script!

Now set your hello file to be readable by everyone, NOT executable by
the sysadm user, and executable by the Group and by Other. Can you
figure out how to do this on your own?

Look at the file's permissions to get started:

        $ ls -l hello

        -rwxr-xr-x 1 sysadm sysadm 20 2012-04-16 01:38 hello

You want the permission to be:

        -rw-r-xr-x 1 sysadm sysadm 20 2012-04-16 01:38 hello

There are several ways you can do this with the chmod command. 

Once you have set the permissions like this, what happens if you now type?

        $ ./hello

Why does this happen? If you execute the file as a different user it
will still work! Does this seem odd? (Hint: think “left to right”)

You can get the file to execute, for example, by typing:

        $ sudo ./hello

Now set the file back so that the sysadm can execute it. Verify that this
works.

CONCLUSION

What's the “./” about? 

In our example above when you typed “hello” the file “hello”
is in your home directory. Your home directory is not in your default
path as configured for the bash shell. Thus, bash will not find the
hello file, even though it's in the same directory where you are typing
the command. By using “./” before the filename we tell bash to
explicitly look in the same directory for the file to execute.