Linux Commands - Useful

#http://cb.vu/unixtoolbox.xhtml Unix toolbox - collection of Unix/Linux/BSD commands

Linux Administration: The Complete Linux Bootcamp 2021
=========================================================
##########################
## Getting Help in Linux
##########################

# MAN Pages
man command     # => Ex: man ls

# The man page is displayed with the less command
# SHORTCUTS:
# h         => getting help
# q         => quit
# enter     => show next line
# space     => show next screen
# /string   => search forward for a string
# ?string   => search backwards for a string
# n / N     => next/previous appearance

# checking if a command is shell built-in or executable file
type rm        # => rm is /usr/bin/rm
type cd        # => cd is a shell builtin

# getting help for shell built-in commands
help command    # => Ex: help cd
command --help  # => Ex: rm --help

# searching for a command, feature or keyword in all man Pages
man -k uname
man -k "copy files"
apropos passwd


##########################
## Bash History
##########################

# removing a line (ex: 100) from the history
history -d 100

# removing the entire history
history -c

# printing the no. of commands saved in the history file (~/.bash_history)
echo $HISTFILESIZE

# printing the no. of history commands saved in the memory
echo $HISTSIZE


# printing the last command starting with abc
!abc:p

# reverse searching into the history
CTRL + R

# recording the date and time of each command in the history
HISTTIMEFORMAT="%d/%m/%y %T "

##########################
## Running commands as root (sudo, su)
##########################

# running a command as root (only users that belong to sudo group [Ubuntu] or wheel [CentOS])
sudo command

# becoming root temporarily in the terminal
sudo su      # => enter the user's password

# setting the root password
sudo passwd root

# changing a user's password
passwd username

# becoming root temporarily in the terminal
su     # => enter the root password


##########################

## The ls Command

## ls [OPTIONS] [FILES]

##########################


# -l => long listing
ls -l ~

# -a => listing all files and directories including hidden ones
ls -la ~

# -d => displaying information about the directory, not about its contents
ls -ld /etc

# -h => displaying the size in human readable format
ls -h /etc

# -S => displaying sorting by size
ls -Sh /var/log

# Note: ls does not display the size of a directory and all its contents. Use du instead
du -sh ~

# -X => displaying sorting by extension
ls -lX /etc

# --hide => hiding some files
ls --hide=*.log /var/log

# -R => displaying a directory recursively
ls -lR ~

# -i => displaying the inode number
ls -li /etc

##########################
## File Timestamps and Date
##########################

# displaying atime
ls -lu

# displaying mtime
ls -l
ls -lt

# displaying ctime
ls -lc

# displaying all timestamps
stat file.txt

# displaying the full timestamp
ls -l --full-time /etc/

# creating an empty file if it does not exist, update the timestamps if the file exists
touch file.txt

# changing only the access time to current time
touch -a file

# changing only the modification time to current time
touch -m file

# changing the modification time to a specific date and time
touch -m -t 201812301530.45 a.txt

# changing both atime and mtime to a specific date and time
touch -d "2010-10-31 15:45:30" a.txt

# changing the timestamp of a.txt to those of b.txt
touch a.txt -r b.txt


# setting the date and time
date --set="2 OCT 2020 18:00:00"

# displaying the modification time and sorting the output by name.
ls -l

# displaying the output sorted by modification time, newest files first
ls -lt

# displaying and sorting by atime
ls -ltu

# reversing the sorting order
ls -ltu --reverse


##########################
## Viewing files (cat, less, more, head, tail, watch)
##########################



# displaying the line numbers
can -n filename

# concatenating 2 files
cat filename1 filename2 > filename3


# showing the last 10 lines of a file
tail filename

# showing the last 15 lines of a file
tail -n 15 filename

# showing the last lines of a file starting with line no. 15
tail -n +5 filename

# showing the last 10 lines of the file in real-time
tail -f filename


# showing the first 10 lines of a file
head filename

# showing the first 15 lines of a file
head -n 15 filename

# running repeatedly a command with refresh of 3 seconds
watch -n 3 ls -l


##########################
## Piping and Command Redirection
##########################

## Piping Examples:

ls -lSh /etc/ | head            # see the first 10 files by size
ps -ef | grep sshd              # checking if sshd is running
ps aux --sort=-%mem | head -n 3  # showing the first 3 process by memory consumption

## Command Redirection
# output redirection
ps aux > running_processes.txt
who -H > loggedin_users.txt

# appending to a file
id >> loggedin_users.txt

# output and error redirection
tail -n 10 /var/log/*.log > output.txt 2> errors.txt

# redirecting both the output and errors to the same file
tail -n 2 /etc/passwd /etc/shadow > output_errors.txt 2>&1

cat -n /var/log/auth.log | grep -ai "authentication failure" | wc -l
cat -n /var/log/auth.log | grep -ai "authentication failure" > auth.txt     # => piping and redirection


##########################
## Finding Files (find, locate)
##########################

## LOCATE ##
# updating the locate db
sudo updatedb

# displaying statistics
locate -S

# finding file by name
locate filename # => filename is expanded to *filename*
locate -i filename # => the filename is case insensitive
locate -b '\filename' # => finding by exact name

# finding using the basename
locate -b filename

# finding using regular expressions
locate -r 'regex'

# checking that the file exists
locate -e filename

# showing command path
which command
which -a command

 ##########################
## Account Management
##########################

## IMPORTANT FILES
# /etc/passwd # => users and info: username:x:uid:gid:comment:home_directory:login_shell
# /etc/shadow # => users' passwords
# /etc/group # => groups

# creating a user account
useradd [OPTIONS] username
# OPTIONS:
# -m => create home directory
# -d directory => specify another home directory
# -c "comment"
# -s shell
# -G => specify the secondary groups (must exist)
# -g => specify the primary group (must exist)

Exemple:
useradd -m -d /home/john -c "C++ Developer" -s /bin/bash -G sudo,adm,mail john

# changing a user account
usermod [OPTIONS] username # => uses the same options as useradd
Example:
usermod -aG developers,managers john # => adding the user to two secondary groups

# deleting a user account
userdel -r username # => -r removes user's home directory as well

# creating a group
groupadd group_name

# deleting a group
groupdel group_name

# displaying all groups
cat /etc/groups

# displaying the groups a user belongs to
groups

# creating admin users
# add the user to sudo group in Ubuntu and wheel group in CentOS
usermod -aG sudo john


## Monitoring Users ##
who -H # => displays logged in users
id # => displays the current user and its groups
whoami # => displays EUID

# listing who’s logged in and what’s their current process.
w
uptime

# printing information about the logins and logouts of the users
last
last -u username



##########################
## File Permissions
##########################

## LEGEND
u = User
g = Group
o = Others/World
a = all

r = Read
w = write
x = execute
- = no access

# displaying the permissions (ls and stat)
ls -l /etc/passwd
    -rw-r--r-- 1 root root 2871 aug 22 14:43 /etc/passwd

stat /etc/shadow
    File: /etc/shadow
    Size: 1721          Blocks: 8          IO Block: 4096   regular file
    Device: 805h/2053d  Inode: 524451      Links: 1
    Access: (0640/-rw-r-----)  Uid: (    0/    root)   Gid: (   42/  shadow)
    Access: 2020-08-24 11:31:49.506277118 +0300
    Modify: 2020-08-22 14:43:36.326651384 +0300
    Change: 2020-08-22 14:43:36.342652202 +0300
    Birth: -

# changing the permissions using the relative (symbolic) mode
chmod u+r filename
chmod u+r,g-wx,o-rwx filename
chmod ug+rwx,o-wx filename
chmod ugo+x filename
chmod a+r,a-wx filename

# changing the permissions using the absolute (octal) mode
PERMISSIONS      EXAMPLE
u   g   o
rwx rwx rwx     chmod 777 filename
rwx rwx r-x     chmod 775 filename
rwx r-x r-x     chmod 755 filename
rwx r-x ---     chmod 750 filename
rw- rw- r--     chmod 664 filename
rw- r-- r--     chmod 644 filename
rw- r-- ---     chmod 640 filename

# setting the permissions as of a reference file
chmod --reference=file1 file2

# changing permissions recursively
chmod -R u+rw,o-rwx filename

## SUID (Set User ID)

# displaying the SUID permission
ls -l /usr/bin/umount
    -rwsr-xr-x 1 root root 39144 apr  2 18:29 /usr/bin/umount

stat /usr/bin/umount
    File: /usr/bin/umount
    Size: 39144         Blocks: 80         IO Block: 4096   regular file
    Device: 805h/2053d  Inode: 918756      Links: 1
    Access: (4755/-rwsr-xr-x)  Uid: (    0/    root)   Gid: (    0/    root)
    Access: 2020-08-22 14:35:46.763999798 +0300
    Modify: 2020-04-02 18:29:40.000000000 +0300
    Change: 2020-06-30 18:27:32.851134521 +0300
    Birth: -

# setting SUID
chmod u+s executable_file
chmod 4XXX executable_file      # => Ex: chmod 4755 script.sh


## SGID (Set Group ID)

# displaying the SGID permission
ls -ld projects/
    drwxr-s--- 2 student student 4096 aug 25 11:02 projects/

stat projects/
    File: projects/
    Size: 4096          Blocks: 8          IO Block: 4096   directory
    Device: 805h/2053d  Inode: 266193      Links: 2
    Access: (2750/drwxr-s---)  Uid: ( 1001/ student)   Gid: ( 1002/ student)
    Access: 2020-08-25 11:02:15.013355559 +0300
    Modify: 2020-08-25 11:02:15.013355559 +0300
    Change: 2020-08-25 11:02:19.157290764 +0300
    Birth: -

# setting SGID
chmod 2750 projects/
chmod g+s projects/


## The Sticky Bit

# displaying the sticky bit permission
ls -ld /tmp/
    drwxrwxrwt 20 root root 4096 aug 25 10:49 /tmp/

stat /tmp/
    File: /tmp/
    Size: 4096          Blocks: 8          IO Block: 4096   directory
    Device: 805h/2053d  Inode: 786434      Links: 20
    Access: (1777/drwxrwxrwt)  Uid: (    0/    root)   Gid: (    0/    root)
    Access: 2020-08-22 14:46:03.259455125 +0300
    Modify: 2020-08-25 10:49:53.756211470 +0300
    Change: 2020-08-25 10:49:53.756211470 +0300
    Birth: -

# setting the sticky bit
mkdir temp
chmod 1777 temp/
chmod o+t temp/
ls -ld temp/
    drwxrwxrwt 2 student student 4096 aug 25 11:04 temp/


## UMASK
# displaying the UMASK
umask

# setting a new umask value
umask new_value     # => Ex: umask 0022

## Changing File Ownership (root only)

# changing the owner
chown new_owner file/directory      # => Ex: sudo chown john a.txt

# changing the group owner
chgrp new_group file/directory

# changing both the owner and the group owner
chown new_owner:new_group file/directory

# changing recursively the owner or the group owner
chown -R new-owner file/directory

# displaying the file attributes
lsattr filename

#changing the file attributes
chattr +-attribute filename     # => Ex: sudo chattr +i report.txt



##########################
## Killing processes (kill, pkill, killall)
##########################

# listing all signals
kill -l

# sending a signal (default SIGTERM - 15) to a process by pid
kill pid        # => Ex: kill 12547

# sending a signal to more processes
kill -SIGNAL pid1 pid2 pid3 ...

# sending a specific signal (SIGHUP - 2) to a process by pid
kill -2 pid
kill -HUP pid
kill -SIGHUP pid

# sending a signal (default SIGTERM - 15) to process by process name
pkill process_name          # => Ex: pkill sleep
killall process_name
kill $(pidof process_name)  # => Ex: kill -HUP $(pidof sshd)

# running a process in the background
command &   # => Ex: sleep 100 &

# Showing running jobs
jobs

# Stopping (pausing) the running process
Ctrl + Z

# resuming and bringing to the foreground a process by job_d
fg %job_id

# resuming in the background a process by job_d
bg %job_id

# starting a process immune to SIGHUP
nohup command &     # => Ex: nohup wget http://site.com &



#########################
## Getting info about the network interfaces (ifconfig, ip, route)
##########################

# displaying information about enabled interfaces
ifconfig

# displaying information about all interfaces (enabled and disabled)
ifconfig -a
ip address show

# displaying info about a specific interface
ifconfig enp0s3
ip addr show dev enp0s3

# showing only IPv4 info
ip -4 address

# showing only IPv6 info
ip -6 address

# displaying L2 info (including the MAC address)
ip link show
ip link show dev enp0s3

# displaying the default gateway
route
route -n    # numerical addresses
ip route show

# displaying the DNS servers
systemd-resolve --status


##########################
## Setting the network interfaces (ifconfig, ip, route)
##########################
# disabling an interface
ifconfig enp0s3 down
ip link set enp0s3 down

# activating an interface
ifconfig enp0s3 up
ip link set enp0s3 up

# checking its status
ifconfig -a
ip link show dev enp0s3

# setting an ip address on an interface
ifconfig enp0s3 192.168.0.222/24 up
ip address del 192.168.0.111/24 dev enp0s3
ip address add 192.168.0.112/24 dev enp0s3

# setting a secondary ip address on sub-interface
ifconfig enp0s3:1 10.0.0.1/24

# deleting and setting a new default gateway
route del default gw 192.168.0.1
route add default gw 192.168.0.2

# deleting and setting a new default gateway
ip route del default
ip route add default via 192.168.0.1

# changing the MAC address
ifconfig enp0s3 down
ifconfig enp0s3 hw ether 08:00:27:51:05:a1
ifconfig enp0s3 up

# changing the MAC address
ip link set dev enp0s3 address 08:00:27:51:05:a3


##########################
## Network Static configuration using Netplan (Ubuntu)
##########################

# 1. Stop and disable the Network Manager

sudo systemctl stop NetworkManager
sudo systemctl disable NetworkManager
sudo systemctl status NetworkManager
sudo systemctl is-enabled NetworkManager

# 2. Create a YAML file in /etc/netplan

network:
  version: 2
  renderer: networkd
  ethernets:
    enp0s3:
      dhcp4: false
      addresses:
        - 192.168.0.20/24
      gateway4: "192.168.0.1"
      nameservers:
        addresses:
          - "8.8.8.8"
          - "8.8.4.4"

# 3. Apply the new config
sudo netplan apply

# 4. Check the configuration
ifconfig
route -a



##########################
## OpenSSH
##########################

# 1. Installing OpenSSH (client and server)
# Ubuntu
sudo apt update && sudo apt install openssh-server openssh-client

# CentOS
sudo dnf install openssh-server openssh-clients

# connecting to the server
ssh -p 22 username@server_ip        # => Ex: ssh -p 2267 john@192.168.0.100
ssh -p 22 -l username server_ip
ssh -v -p 22 username@server_ip     # => verbose

# 2. Controlling the SSHd daemon
# checking its status
sudo systemctl status ssh       # => Ubuntu
sudo systemctl status sshd      # => CentOS

# stopping the daemon
sudo systemctl stop ssh       # => Ubuntu
sudo systemctl stop sshd      # => CentOS

# restarting the daemon
sudo systemctl restart ssh       # => Ubuntu
sudo systemctl restart sshd      # => CentOS

# enabling at boot time
sudo systemctl enable ssh       # => Ubuntu
sudo systemctl enable sshd      # => CentOS

sudo systemctl is-enabled ssh       # => Ubuntu
sudo systemctl is-enabled sshd      # => CentOS

# 3. Securing the SSHd daemon
# change the configuration file (/etc/ssh/sshd_config) and then restart the server
man sshd_config

a) Change the port
Port 2278

b) Disable direct root login
PermitRootLogin no

c) Limit Users’ SSH access
AllowUsers stud u1 u2 john

d) Filter SSH access at the firewall level (iptables)

e) Activate Public Key Authentication and Disable Password Authentication

f) Use only SSH Protocol version 2

g) Other configurations:
ClientAliveInterval 300
ClientAliveCountMax 0
MaxAuthTries 2
MaxStartUps 3
LoginGraceTime 20



##########################
## Copying files using SCP and RSYNC
##########################

### SCP ###
# copying a local file to a remote destination
scp a.txt john@80.0.0.1:~
scp -P 2288 a.txt john@80.0.0.1:~       # using a custom port

# copying a local file from a remote destination to the current directory
scp -P 2290 john@80.0.0.1:~/a.txt .

# copying a local directory to a remote destination (-r)
scp -P 2290 -r projects/ john@80.0.0.1:~


### RSYNC ###
# synchronizing a directory
sudo rsync -av /etc/ ~/etc-backup/

# mirroring (deleting from destination the files that were deleting from source)
sudo rsync -av --delete /etc/ ~/etc-backup/

# excluding files
rsync -av --exclude-from='~/exclude.txt' source_directory/ destination_directory/
# exclude.txt:
# *.avi
# music/
# abc.mkv

rsync -av --exclude='*.mkv' --exclude='movie1.avi' source_directory/ destination_directory/

# synchronizing a directory over the network using SSH
sudo rsync -av -e ssh /etc/ student@192.168.0.108:~/etc-backup/

# using a custom port
sudo rsync -av -e 'ssh -p 2267' /etc/ student@192.168.0.108:~/etc-backup/


##########################
## WGET
##########################
# installing wget
apt install wget        # => Ubuntu
dnf install wget        # => CentOS

# download a file in the current directory
wget https://cdimage.kali.org/kali-2020.2/kali-linux-2020.2-installer-amd64.iso

# resuming the download
wget -c https://cdimage.kali.org/kali-2020.2/kali-linux-2020.2-installer-amd64.iso

# saving the file into a specific directory
mkdir kali
wget -P kali/ https://cdimage.kali.org/kali-2020.2/kali-linux-2020.2-installer-amd64.iso

# limiting the rate (bandwidth)
wget --limit-rate=100k -P kali/ https://cdimage.kali.org/kali-2020.2/kali-linux-2020.2-installer-amd64.iso

# downloading more files
wget -i urls.txt      # urls.txt contains urls

# starting the download in the background
wget -b -P kali/ https://cdimage.kali.org/kali-2020.2/kali-linux-2020.2-installer-amd64.iso
tail -f wget-log        # => checking its status

# getting an offline copy of a website
wget --mirror --convert-links --adjust-extension --page-requisites --no-parent http://example.org
wget -mkEpnp http://example.org


##########################
## NETSTAT and SS
##########################
# displaying all open ports and connections
sudo netstat -tupan
sudo ss -tupan
netstat -tupan | grep :80   # => checking if port 80 is open

##########################
## LSOF
##########################
# listing all files that are open
lsof

# listing all files opened by the processes of a specific user
lsof -u username

# listing all files opened by a specific process
lsof -c sshd

# listing all files that have opened TCP ports
lsof -iTCP -sTCP:LISTEN
lsof -iTCP -sTCP:LISTEN -nP


##########################
## Scanning hosts and networks using nmap
##########################
##** SCAN ONLY YOUR OWN HOSTS AND SERVERS !!! **##
## Scanning Networks is your own responsibility ##

# Syn Scan - Half Open Scanning (root only)
nmap -sS 192.168.0.1

# Connect Scan
nmap -sT 192.168.0.1

# Scanning all ports (0-65535)
nmap -p- 192.168.0.1

# Specifying the ports to scan
nmap -p 20,22-100,443,1000-2000 192.168.0.1

# Scan Version
nmap -p 22,80 -sV 192.168.0.1

# Ping scanning (entire Network)
nmap -sP 192.168.0.0/24

# Treat all hosts as online -- skip host discovery
nmap -Pn 192.168.0.0/24

# Excluding an IP
nmap -sS 192.168.0.0/24 --exclude 192.168.0.10

# Saving the scanning report to a file
nmap -oN output.txt 192.168.0.1

# OS Detection
nmap -O 192.168.0.1

# Enable OS detection, version detection, script scanning, and traceroute
nmap -A 192.168.0.1

# reading the targets from a file (ip/name/network separated by a new line or a whitespace)
nmap -p 80 -iL hosts.txt

# exporting to out output file and disabling reverse DNS
nmap -n -iL hosts.txt -p 80 -oN output.txt

##########################
## Software Management (dpkg and apt)
##########################

### DPKG ###
# getting info about a deb file
dpkg --info google-chrome-stable_current_amd64.deb

# installing an application from a deb file
sudo dpkg -i google-chrome-stable_current_amd64.deb

# list all installed programs
dpkg --get-selections
dpkg-query -l

# filtering the output
dpkg-query -l | grep ssh

# listing all files of an installed package
dpkg-query -l | grep ssh
dpkg -L openssh-server

# finding to which package a file belongs
which ls
dpkg -S /bin/ls
    coreutils: /bin/cp

# removing a package
sudo dpkg -r google-chrome-stable

# purging a package
sudo dpkg -P google-chrome-stable

### APT ###
# updating the package index (doesn't install/uninstall/update any package)
sudo apt update
# installing or updating a package named apache2
sudo apt install apache2

# listing all upgradable packages
sudo apt list --upgradable

# upgrading all applications
sudo apt full-upgrade
sudo apt full-upgrade -y        # => assume yes to any prompt (useful in scripts)

# removing a package
sudo apt remove apache2

# removing a package and its configurations
sudo apt purge apache2

# removing dependencies that are not needed anymore
sudo apt autoremove

# removing the saved deb files from the cache directory (var/cache/apt/archives)
sudo apt clean

# listing all available packages
sudo apt list
sudo apt list | wc -l

# searching for a package
sudo apt list | grep nginx

# showing information about a package
sudo apt show nginx

# listing all installed packages
sudo apt list --installed


##########################
## Task Scheduling using Cron
##########################

# editing the current user’s crontab file
crontab -e

# listing the current user’s crontab file
crontab -l

# removing the current user’s crontab file
crontab -r

## COMMON EXAMPLES ##
# run every minute
* * * * * /path_to_task_to_run.sh

# run every hour at minute 15
15 * * * * /path_to_task_to_run.sh

# run every day at 6:30 PM
30 18 * * * /path_to_task_to_run.sh

# run every Monday at 10:03 PM
3 22 * * 1 /path_to_task_to_run.sh

# run on the 1st of every Month at 6:10 AM
10 6 1 * * /path_to_task_to_run.sh

# run every hour at minute 1, 20 and 35
1,20,35 * * * * /path_to_task_to_run.sh

# run every two hour at minute 10
10 */2 * * * /path_to_task_to_run.sh

# run once a year on the 1st of January at midnight
@yearly     /path_to_task_to_run.sh

# run once a month at midnight on the first day of the month
@monthly    /path_to_task_to_run.sh

# run once a week at midnight on Sunday
@weekly      /path_to_task_to_run.sh

# once an hour at the beginning of the hour
@hourly     /path_to_task_to_run.sh

# run at boot time
@reboot     /path_to_task_to_run.sh

All scripts in following directories will run as root at that interval:
/etc/cron.hourly
/etc/cron.daily
/etc/cron.hourly
/etc/cron.monthly
/etc/cron.weekly




##########################
## Getting System Hardware Information
##########################

# displaying full hardware information
lshw
lshw -short     # => short format
lshw -json      # => json format
lshw -html      # => html format

inxi -Fx
# displaying info about the CPU
lscpu
lshw -C cpu
lscpu -J    => json format

# displaying info about the installed RAM memory
dmidecode -t memory
dmidecode -t memory | grep -i size
dmidecode -t memory | grep -i max

# displaying info about free/used memory
free -m

# getting info about pci buses and about the devices connected to them
lspci
lspci | grep -i wireless
lspci | grep -i vga

# getting info about USB controllers and about devices connected
lsusb
lsusb -v

# getting info about hard disks
lshw -short -C disk
fdisk -l
fdisk -l /dev/sda
lsblk
hdparm -i /dev/sda
hdparm -I /dev/sda

# benchmarking disk read performance
hdparm -tT --direct /dev/sda

# getting info about WiFi cards and networks
lshw -C network
iw list
iwconfig
iwlist wlo1 scan

# Getting hardware information from the /proc virtual fs
cat /proc/cpuinfo
/proc/partitions
cat /proc/meminfo
cat /proc/version
uname -r    # => kernel version
uname -a

acpi -bi    # battery information
acpi -V

## Working directly with device files (dd)

# backing up the MBR (the first sector of /dev/sda)
dd if=/dev/sda of=~/mbr.dat bs=512 count=1

# restoring the MBR
dd if=~/mbr.dat of=/dev/sda bs=512 count=1

# cloning a partition (sda1 to sdb2)
dd if=/dev/sda1 of=/dev/sdb2 bs=4M status=progress



##########################
## Service Management using systemd and systemctl
##########################
# showing info about the boot process
systemd-analyze
systemd-analyze blame

# listing all active units systemd knows about
systemctl list-units
systemctl list-units | grep ssh

# checking the status of a service
sudo systemctl status nginx.service

# stopping a service
sudo systemctl stop nginx

# starting a service
sudo systemctl start nginx

# restarting a service
sudo systemctl restart nginx

# reloading the configuration of a service
sudo systemctl reload nginx
sudo systemctl reload-or-restart nginx

# enabling to start at boot time
sudo systemctl enable nginx

# disabling at boot time
sudo systemctl disable nginx

# checking if it starts automatically at boot time
sudo systemctl is-enabled nginx

# masking a service (stopping and disabling it)
sudo systemctl mask nginx

# unmasking a service
sudo systemctl unmask nginx


##########################
## Bash Aliases
##########################

# listing all Aliases
alias

# creating an alias:  alias_name="command"
alias copy="cp -i"

# to make the aliases you define persistent, add them to ~/.bashrc

# removing an alias: unalias alias_name
unalias copy

## Useful Aliases ##
alias c="clear"
alias cl="clear;ls;pwd"
alias root="sudo su"
alias ports="netstat -tupan"
alias sshconfig="sudo vim /etc/ssh/sshd_config"
alias my_server="ssh -p 3245-l user100 80.0.0.1"
alias update=”sudo apt update && sudo apt dist-upgrade -y && sudo apt clean”
alias lt="ls -hSF --size -1"
alias ping='ping -c 5'

# Interactive File Manipulation
alias cp="cp -i"
alias mv="mv -i"
alias rm="rm -i"

## Important alias ##
# This may look a bit confusing, but essentially,
# it makes all of the other aliases you define function correctly when used with sudo
alias sudo='sudo '      # use single quotes, not double quotes.

##########################
## Bash Variables
##########################

# defining a variable: variable_name=value
# variable names should start with a letter or underscore and can contain letters, digits and underscore
os="Kali Linux"
version=10

# referencing the value of a variable (getting the variable value): $variable_name
echo $os
echo $version

# defining a read-only variable (constant)
declare -r temperature=100

# removing (unsetting) a variable
unset version

# listing all environment variables
env
printenv

# searching for an environment variable
printenv PATH
env | grep -i path

# creating new environment variables for the user: in ~/.bashrc add export MYVAR=”value”
export IP="80.0.0.1"

# changing the PATH
export PATH=$PATH:~/scripts     # in ~/.bashrc

# getting user input
read MY_VAR
echo $MY_VAR

# displaying a message
read -p "Enter the IP address: " ip
ping -c 1 $ip

read -s -p "Enter password:" pswd
echo $pswd


### SPECIAL VARIABLES AND POSITIONAL ARGUMENTS ###
./script.sh filename1 dir1

$0 => the name of the script itself (script.sh)
$1 => the first positional argument (filename1)
$2 => the second positional argument (dir1)
...
${10} => the tenth argument of the script
${11} => the eleventh argument of the script

$# => the number of the positional arguments
"$*" => string representation of all positional argument
$? => the most recent foreground command exit status


# if [ some_condition_is_true ]
# then
#   //execute this code
# elif [ some_other_condition_is_true ]
# then
#   //execute_this_code
# else
#   //execute_this_code
# fi
## Examples:

i=1
if [[ $i -lt 10 ]]
then
   echo "i is less than 10."
fi
#################
i=100
if [[ $i -lt 10 ]]
then
   echo "i is less than 10."
else
   echo "i is greater than or equal to 10."
fi
################
i=10
if [[ $i -lt 10 ]]
then
   echo "i is less than 10."
elif [[ $i -eq 10 ]]
then
   echo "i is 10"
else
   echo "i is greater than or equal to 10."
fi

################
### TESTING CONDITIONS => man test ###

### For numbers (integers) ###
# -eq   equal to
# -ne   not equal to
# -lt   less than
# -le   less than or equal to
# -gt   greater than
# -ge   greater than or equal to

# For files:
# -s    file exists and is not empty
# -f    file exists and is not a directory
# -d    directory exists
# -x    file is executable by the user
# -w    file is writable by the user
# -r    file is readable by the user

# For Strings
# =     the equality operator for strings if using single square brackets [ ]
# ==    the equality operator for strings if using double square brackets [[ ]]
# !=    the inequality operator for strings
# -n $str   str is nonzero length
# -z $str   str is zero length

# &&  => the logical and operator
# ||  => the logical or operator