Chapter 3. Access Linux [6 Hrs]

Excellent introduction to UNIX anatomy (similar to linux): http://ibgwww.colorado.edu/~lessem/psyc5112/usail/concepts/anatomy-of-unix/anatomy.html (If it doesn’t work, make sure your browser didn’t substitute http for https. Copy pasting may work.)

Shell

A shell provides a command line.
It acts as intermediary between kernel and an user.
Default shell in most linux (like rocky linux) is GNU Bourne-Again Shell (bash).
Root user’s prompt with a #.
```
  [root@rocky-linux ~]# _
```
Normal user’s prompt start with a $.
```
  [lisa@rocky-linux ~]$ _
```

Some common shells are:

csh: C like shell
sh: Posix standard shell
tcsh: Improved shell
zsh: Improved bourne shell
fish: Friendly interactive shell
nushell: A structured shell with support for data types and functional approach.

Shell working loop

The loop of the shell can be summarized in following steps:

Display the prompt.
Read the command.
Analyze the syntax.
Substitute special characters
Execute the command

Display the prompt
...
and so on

Command Structure

Command may contain options and arguments.

Options
- Option modify behavior of the command.
- Option preceed with a dash (-, mostly for single character) or a double dash (--, mostly for a word).
- Example: ls -a or ls --all
- Some commands allows combining multiple single character options into a single one. Example ls -a -l or ls -al mean same thing.
Arguments
- Arguments are the input that the commadn will act upon
- Example: cat -n /etc/hosts

Essential linux commands:

cd: Change directory
ls: List files and directories.
ln: Create symbolic links.
pwd: Print working directory.
cp: Copy
mv: Move
scp: Securely copy files.
chown: Change ownership of a file.
chmod: Change file access permissions.
lsattr: List attributes
chattr: Change attributes

Man page

man pages can be used to get usage of any commands.
man pages are indexed in mandb
man page is divided into 8 sections, can be remembered by the following sentence: Good Students Learn Keenly For Great Marks, Sometimes Knowing.
- General Commands Manual (default)
- System Calls Manual
- Library Function Manual
- Kernel Interfaces Manual
- File Formats Manual
- Games Manual
- Miscellaneous Information Manual
- System Manager’s Manual
- Kernel Developer’s Manual
To find the right man page: man -a passwd man -k passwd or apropos passwd

OS specific

To see the version of the rocky linux
```
  cat /etc/rocky-release
```

To see the information about the system

  uname -a

Options of unames

Options:
  -s : Kernel's name
  -n : Nodename (name of the system)
  -v : Kernel's version
  -o : Operating system
  -r : Kernel release

To see the user id of an user:
```
  id -u username
```
Output: Uid of the user named username.

Example: Output of id -u root will always be 0

If you omit the username, you will get the uid of current user.
```
  id -u
```
Output: Uid of the current user.

Super user (root)

The user called root has the highest level of right or access capability.
Avoid accessing the system as the root user unless necessary.
Root user’s UID is 0.

sudo vs su

sudo was originally meant to be a command that allowed any user to execute a super user’s command, thus its name orinally stood for super user do.
However, later version of sudo also allowed to execute commands as other users, making its new full form substitue user do.
With sudo -i we can even open interactive shell for root users.
With sudo -u username command we can launch command for any user username.
Most likely, sudo will ask for your password. (The user currently logged in.)
Sudo may not ask password in following conditions:
- Timeout for previously entered password is not reached.
- In sudoers file, an user is allwoed to run sudo without password with :NOPASSWD
su means switch user (or substitue user)
It allows us to log in as another user without closing current user.
With su username -c command we can execute a command as the user username
su will always ask for the password of the user you are switching as. (Unlike sudo which asks the password of current user.)

Example:

    [ramesh@rocky-linux ~]$ su ram -c ls

The above command will ask the password of ram, and then execute the command as ram.

    [ramesh@rocky-linux ~]$ sudo -u ram ls

The above command will ask the password of ramesh, and then execute the command as ram.

sudoers file

The file /etc/sudoers is known as the sudoers file. It can be accessed by the command:

    visudo

Structure of sudoers file:

The structure of sudoers file is:

user hostlist = (userlist) commandlist

Meaning: An user user logged in from one of the host from hostlist can run any command from commandlist as any user form the userlist.

If group instead of user is to be used, preceed it with a % as in

%group hostlist = (userlist) commandlist

Examples

# Ram can execute any command as any user, but only from rocky-peeps host
ram rocky-peeps = (ALL) ALL

# Ram can execute any command from any host but only as user lisa
ram ALL = (lisa) ALL

# Ram can execute only `/usr/bin/ls` and `/usr/bin/cat` command but as any user and from any host
ram ALL = (ALL) /usr/bin/ls, /usr/bin/cat

# Ram can execute any command as any user and from any host
ram ALL = (ALL) ALL

# Ram can execute any command as any user and from any host and no password is prompted when doing so
ram ALL = (ALL) NOPASSWD: ALL

When an user uses sudo to attempt something he cannot do, it may say “This incident will be reported.” To see the report, you can simply observe the file /var/log/secure as:

    less -f /var/log/secure