5_users_and_rights.Rmd

title: Users and rights

Users and rights

Objective: Understand how rights works in GNU/Linux

GNU/Linux and other Unix-like OS are multiuser, this means that they are designed to work with multiple users connected simultaneously to the same computer.

There is always at least one user: the root user

• It’s the super user
• he has every right (we can say that he ignores the rights system)
• this account should only be used to administer the system.

There can also be other users who

• have rights
• belong to groups
• the groups also have rights

File rights

Each file is associated with a set of rights:

• - nothing
• r reading right
• w writing right
• x execution right

Check your set of rights on your .bashrc file

ls -l ~/.bashrc

The first column of the ls -l output show the status of the rights on the file

 rwxr-xr--
 \ /\ /\ /
  v  v  v
  |  |  others (o)
  |  |
  |  group (g)
  |
 user (u)

• the 1st character is the type of the file (we already know this one)
• he 3 following characters (2 to 4) are the user rights on the file
• the characters 5 to 7 are the group rights on the file
• the characters 8 to 10 are the others’ rights on the file (anyone not the user nor in the group)

To change the file rights you can use the command chmod

Use the command ls -l to check the effect of the following options for chmod

chmod u+x .bashrc

chmod g=rw .bashrc

chmod o+r .bashrc

chmod u-x,g-w,o= .bashrc

What can you conclude on the symbols + , =, - and , with the chmod command ?

Numeric notation

Another method for representing Unix permissions is an octal (base-8) notation as shown by stat -c %a.

Symbolic notation	Numeric notation	English
----------	0000	no permissions
-rwx------	0700	read, write, & execute only for owner
-rwxrwx---	0770	read, write, & execute for owner and group
-rwxrwxrwx	0777	read, write, & execute for owner, group and others
---x--x--x	0111	execute
--w--w--w-	0222	write
--wx-wx-wx	0333	write & execute
-r--r--r--	0444	read
-r-xr-xr-x	0555	read & execute
-rw-rw-rw-	0666	read & write
-rwxr-----	0740	owner can read, write, & execute; group can only read; others have no permissions

The default group of your user is the first in the list of the groups you belong to. You can use the command groups to display this list. What is your default group ?

The command id show the same information, but with some differences what are they ?

Can you cross this additional information with the content of the file /etc/passwd and /etc/group ?

What is the user id of root ?

When you create an empty file, system default rights and your default groups are used. You can use the command touch to create a file.

touch my_first_file.txt

What are the default rights when you crate a file ?

You can create folders with the command mkdir (make directories).

mkdir my_first_dir

What are the default rights when you create a directory ? Try to remove the execution rights, what appends then ?

You can see the /root home directory. Can you see it’s content ? Why ?

Create a symbolic link (ln -s) to your .bashrc file, what are the default rights to symbolic links ?

Can you remove the writing right of this link ? What happened ?

Users and Groups

We have seen how to change the right associated with the group, but what about changing the group itself ? The command chgrp allows you to do just that:

chgrp audio .bashrc

Now the next step is to change the owner of a file, you can use the command chown for that.

chown ubuntu my_first_file.txt

You can change the user and the group with this command:

chown ubuntu:audio my_first_file.txt

What are the rights on the program mkdir (the command which can help you find where program file are) ?

Can you remove the execution rights for the others ?

The command cp allows you to copy file from one destination to another.

man cp