Table-content:-

1. Wildcards in Linux
2. Hostname setup
3. Basic network Setup
4. sudo configuration

1.Wildcards

• In Linux wildcards are special character used in the command line to match multiple files or directory. They mostly used with command like ls, cp ,mv,

=> * match any characters, including zero charters

1. ls *.log -> list the log files
2. rm temp*
3. mv *.txt directory/

=> ? match the exact one character like single character
1.ls file?.txt
list the file1 to file9 but to list out file10.txt
2.mv log?.txt logs/
move logs files from log1.txt to log9.txt
=> [] match the one character from set.
1.ls file[12].txt
list out only file1.txt and file2.txt not file3.txt
2.ls [abc]*.txt
list only matching character like a, b, c
=> [!] match any character except those inside brackets

1.ls file[!1].txt
list all .txt file except file1.txt
2.ls [!abc]*.txt
list all .txt except a, b ,c character

=> {}Expand to comma separate the values
1.touch {file1,file2,file3}.txt
create mutiple fiel once
2.delete specific files
rm {error,server,server}.log
3.copy the multiple file types


cp *.{jpg,png,gif,txt} backupdir/

** match file in subdirectories and

1. ls **/*.txt find the all .txt file in all subdirectories
2. rm **/*.log delete .log files inside any folder

Escape character \ prevent wildcard:

• main purpose of using the escape character \ is to disable wildcard expansion or treat special characters as normal text in command.

Example:-

rm *text.txt -> delete all files ending in text.txt
rm \*test.txt-> delete only one fie named as *text.txt

**2. Hostname setup

**

I. Check host name

$ hostname

output:-

cyber.TAMIL.com

II. check hostname full info
**
$ hostnamectl or hostname status

*III. Change hostname temporarily
*
$ hostname Cyber.TAMIL.com

**IV. Change hostname permanently in terminal

$ hostnamectl set-hostname Cyber.TAMIL.com

V. Change hostname permanently in configuration file

$ sudo vim /etc/hostname

Output:-

Centos.TAMIL.com

=> you can press i button (insert mode) then Esc , save :wq then , sudo reboot or exec bash(refresh the shell)

VI. /etc/hosts (hostname <-> ip mapping)

*This file maps hostnames to ip address for local name resolution not applicable for Network-wide Resolution.

Example:-

$ sudo vim /etc/hosts

127.0.0.1 oldhostname

you can modify:-

127.0.0.1 newhostname

For real network-wide hostname resolution:

• Set up a DNS server (like BIND, dnsmasq, or Unbound).Configure all machines to use the DNS server for hostname resolution.

3. Basic network Setup:-

I.# nmclid d -> shows all network iterface.
II. # nmcli d show eth0s3(my interface_name) -> Display details of eth0s3

III. Set the Static ip address

$sudo systemctl restart Networkmanger

or
$nmcli networking off && nmcli networking on

Restart on specific Network interface.

sudo ifdown eth0 && sudo ifup eth0


id=eth0 -> name of the network connection.
uuid=<uniqid> -> unique identifier (auto generated)
type=ethernet -> wired ethernet connection
autoconnect=true -> the system automatically connect the interface on boot
interface-name=eth0 → Ensures the settings apply only to eth0.
permissions=-> Restrict who can modify this connection (empty means restricted)
permissions=username1;username2; -> only can access specific user


timestamp=0 -> last modification timestamp(optional)

2 [ethernet] Section

mac-address= -> mac address of your network card, optional but useful for binding configuration.

3. [ipv4] Section (Static IP Settings)

method=manual -> assign the static ip address
addresses=192.168.1.100/24;
ip address -> 192.168.1.100
sub netmask -> /24
gateway=192.168.1.1 -> Default router IP
dns=8.8.8.8;1.1.1.1;
if you want to dynamic ip set only
[ipv4]
method=auto
dns=8.8.8.8;1.1.1.1;
ignore-auto-dns=true

-> google 8.8.8.8 & 1.1.1.1 Cloudflare servers,ignore-auto-dns=true -> prevent Dhcp fri changing your dns settings, use always manual Dns settings.
may-fail=false -> Forces the system to wait for a network connection before booting. I f network fails ,the system won't start until it's connected, it useful for server , may-fail=true the system will boot even if the network fails, it useful for desktop ,computer.

4. [ipv6] Section (Disabling IPv6)

method=ignore -> Disable the IPv6 completely.
or
method=auto -> automatically get an ipv6 address.

Network manger GUI( CentOS, RHEL, Fedora)

$nmtui(text based gui )
(you can set Ip adress like static and dynamic)

4.Sudo configuration:-

$sudo -l (check the who has the sudo access)
$sudo -l -U username
$sudo useradd username

$sudo gpasswd -a username wheel
or
$sudo usermod -aG wheel username
(add the user to wheel group)
$sudo gpasswd -d username wheel
(disabling the user to wheel group)

whell = defaul admin group of centos/RHEL os
$sudo visudo (edit the sudoers file)
(or)
$sudo visudo /etc/sudoers

• I added user prasanth362k to give allow to root run any commands.

6.ALLOW a user to run only specific sudo commands :-

• Restrict sudo access to only commands improves security and limits risks.
• add the end of the file of /etc/sudoers without #
• Allowing a user to run only specific commands:-
• Tamil ALL= NOPASSWD: /bin/systemctl restart apache2( restart service with out password)
• English ALL= PASSWD: /bin/systemctl restart apache2,/bin/systemctl restart nginx( restart service with password) -Akash ALL = PASSWD /sbin/ifconfig, /sbin/ip -Amala_paul ALL= NOPASSWD: /bin/mount, /bin/umount
• think about ,one multination company is there work over 100000 employ , how it possible each user can set permission level . we can create group then we will restrict the group which command can execute and not execute.

Example:-

$ sudo groupadd it_team

%it_team ALL = NOPASSWD: /bin/systemctl restart apache2
%network_admin = PASSWD: /sbin/ip, /sbin/iptables
%hr_team ALL= NOPASSWD: /bin/cat /etc/payroll.conf
%dev_team ALL= NOPASSWD: /bin/git pull, /bin/systemctl restart app-service

/bin = permit only normal user can execute the small tasks like nano cat ,systemctl.

/sbin= permit only admin user can execute the system level tasks like reboot, ifconfig ,iptables.

Tamil ALL= NOPASSWD: /bin/cat =>only user Tamil environent execute command
Samantha ALL=(root) PASSWD : /bin/cat =>Samantha can run the specified command as root,password require when using sudo.

Trisha ALL=(ALL) PASSWD : /bin/cat => only Trish user can execute the command,She can run the command as any user(not regular human user) (including root) using sudo -u. password required, does not mean any user.

Example:-

sudo -u root /bin/systemctl restart apache2
sudo -u apache /bin/systemctl restart apache2
sudo -u www-data /bin/systemctl restart apache2

• you can aks me question apche ,ww-data is user ?
• This users all system service related specific user.

Example:-

www-data-> user all webservice like apache, nginx (debian/ubuntu)

mysql -> user for MYSQL
postgres-> user for PostgreSQL

=> Validate the sudoers file before applying changes:

$sudo visudo -c ( Check for syntax)