sudo reboot
sudo shutdown -r now
sudo reboot -f --> force reboot
ssh root@remote-server /sbin/reboot --> remote server restart
cat /sbin/reboot  --> not in readable form

help help
help cd
help -d help
help -d ls
help -d cd
help -m help
help -m pwd

d --> short description
m --> pseudo-manpage format

help -m ls  [ not working , why ? ]
help -d cat [ not working , why ? ]
help cat [ not working , why ? ]

sudo apt install at
at
date
at 23.33
at> df -Th > df.txt
at> CTRL+D
atq --> to list the jobs in queue
atrm <job_number> --> cancel the jobs in queue

sudo useradd -s /sbin/nologin user
cat /etc/passwd | grep nologin
tail -2 /etc/passwd

sudo chsh -s /bin/bash user4
tail -2 /etc/passwd
chsh ( just typing this will be helpful for current user logged in or To change the login shell for user) 

crontab -l
crontab -e

Eg., create a script name sample.sh to display amount of free
and used memory in the system with timestamp

cat > sample.sh
#!/bin/bash
free -h
echo "this is memory available"
current_time=$(date)
echo "time is: $current_time"

To execute this script for every 2 minutes and write to
file called free.log
create free.log file

touch free.log
crontab -e
*/2 * * * * /bin/bash /home/venus/sample.sh >>
/home/venus/free.log

Restart the service which is very important,
sudo systemctl restart cron.service
crontab -l

wget http://path/to/url
wget https://www.kaniyam.com/download/Learn%20GNU
%20Linux%20in%20Tamil%20-%20Part%201.pdf
wget https://www.kaniyam.com/download/Learn-GNU-Linux-
in-Tamil-Part-2.pdf

cat downloads.txt  --> download multiple sites
http://path/to/url1
http://path/to/url2
http://path/to/url3

wget -i downloads.txt
wget http://path/to/url1 http://path/to/url2

du /etc   --> it will show all files or folders size
du /home
du -h /etc  --> size in human readable format
du -h /home/testing
du -sh /etc --> total usage size of a particular directory
du -hs * --> size of current folder
du -chs *
du -hsc /home/testing --> grand total of the folder

sudo systemctl stop mariadb.service
sudo systemctl start mariadb.service
sudo systemctl status mariadb.service
sudo systemctl enable mariadb.service --> this will help to start the service when the system gets rebooted.
sudo systemctl disable mariadb.service

sudo systemctl list-units --type=service --> status of all service

sudo systemctl list-units --type=service --state=active/running/stopped/enabled/disabled/failed

sudo systemctl kill -s 9 <service_name> ( kill the service too)
systemctl daemon-reload ( reload all daemon services )

mkdir tar_examples && cd tar_examples
mkdir files && cd files
touch file{0..1000}.txt
cd ..
tar cf myfiles.tar files  --> archiving using tar
tar tf myfiles.tar  --> list the files in tar
tar -xvf myfiles.tar --> to extract
tar cf myfiles.tar.gz files --> archive using gzip
tar tf myfiles.tar.gz  --> to list
tar -xvzf myfiles.tar.gz  --> to extract

sudo apt update
sudo apt install vsftpd apache2 mariadb-server --> install packages
sudo apt depends bind9  --> check dependencies
sudo apt search apache2 --> search packages
sudo apt show apache2   --> information about the package
sudo apt upgrade        --> mostly don't try this , until any issues are faced in the current version.
sudo apt autoremove     --> remove unused package
sudo apt autoclean      --> clean the repository

sudo systemctl stop apache2
sudo apt purge apache2
sudo apt list

sudo add-apt-repository ppa:PPA_REPOSITORY_NAME/PPA
sudo add-apt-repository ppa:ondrej/php --> php PPA repository
sudo apt update
sudo apt install php

sudo add-apt-repository --remove ppa:PPA_REPO_NAME/PPA
sudo add-apt-repository --remove ppa:ondrej/php

mkdir -p /home/venus/Documents/backup
rsync -v *.txt /home/venus/Documents/backup

rsync -av --progress *.txt
your_username@remote_server:/home/testing/ --> local to remote
rsync -av --progress testing@ip:/home/testing/*.txt . --> remote to local

scp *.txt testing@remote_server:/home/testing/
scp testing@remote_server:/home/testing/file.txt .  --> server to local
scp -r example_folder testing@remote_server:/home/testing/ --> copy folder
scp -r testing@remote_server:/home/testing/example_folder .  --> remote to local

curl https://www.kaniyam.com/foundation/
curl https://www.kaniyam.com/foundation/ > curl_file.txt
curl -# -O https://www.kaniyam.com/download/Learn%20GNU%20Linux%20in%20Tamil%20-%20Part%201.pdf --> shows the download and transfer rate.

free
free -h   --> human readable
free -g/m/k/b  --> in Bytes/KB/MB/GB
free -s 2 --> every 2 sec
free -h > free.log  --> redirect 

ifconfig -a
ifconfig -s --> short 
ifconfig lo --> to display any one network

ip -a
ip -4 a
ip link ls up  --> shows the running interface
ip addr show
ip route show  --> IP route table

netstat --all
netstat -l     --> list the port
netstat -at    --> list TCP port
netstat -au    --> UDP port alone
netstat -lt    --> TCP port 
netstat -s     --> statistics for all ports
netstat -c     --> continuously 
sudo netstat -tulpn | grep 80
sudo netstat -tulpn | grep https

ping google.com
ping -c 5 google.com  --> limit the number of pings
ping -c 5 -q google.com  --> print the statistics

alias c=clear
alias f=free -h
c
f

Above command won't work with another session.

vi ~/.bashrc
alias c=’clear’
alias u=’uptime’
aliad f=’free -h’
:wq! Save and exit

After adding enter the below command , so that the file gets refreshed.
source ~/.bashrc

clear
control + l
reset

source ~/.bashrc
source /etc/profile

cat > example.txt
free -h
pwd
date
time
uptime

source example.txt

sh
sh sample.sh

ssh user@122.132.123.33
ssh user@server.com
ssh user@my.server.in -p 2222 --> using port

tldr -u --> update the local cache of tldr pages
tldr free