Linux commands are categorized into different sections according to its usage. The categories of linux are

SYSTEM

uname	Displays  Linux system information
uname -r	Displays  kernel release information
uptime	Displays how long the system has been running including load average
hostname	Shows the system hostname
hostname -i	Displays the IP address of the system
last reboot	Shows system reboot history
date	Displays current system date and time
timedatectl	Query and change the System clock
cal	Displays the current calendar month and day
w	Displays currently  logged in users in the system
whoami	Displays who you are logged in as
finger username	Displays information about the user

HARDWARE

dmesg	Displays bootup messages
cat /proc/cpuinfo	Displays more information about CPU e.g model, model name, cores, vendor id
cat /proc/meminfo	Displays more information about hardware memory e.g. Total and Free memory
lshw	Displays information about system’s hardware configuration
lsblk	Displays block devices related information
free -m	Displays free and used memory in the system (-m flag indicates memory in MB)
lspci -tv	Displays PCI devices in a tree-like diagram
lsusb -tv	Displays USB devices in a tree-like diagram
dmidecode	Displays hardware information from the BIOS
hdparm -i /dev/xda	Displays information about disk data
hdparm -tT /dev/xda <:code>	Conducts a read speed test on device xda
badblocks -s /dev/xda	Tests  for unreadable blocks on disk

USERS

id	Displays the details of the active user e.g. uid, gid, and groups
last	Shows the last logins in the system
who	Shows who is logged in to the system
groupadd “admin”	Adds the group ‘admin’
adduser “Sam”	Adds user Sam
userdel “Sam”	Deletes user Sam
usermod	Used for changing / modifying user information

FILE COMMMANDS AND PERMISSIONS

ls -al	Lists files – both regular &  hidden files and their permissions as well.
pwd	Displays the current directory file path
mkdir ‘directory_name’	Creates a new directory
rm file_name	Removes a file
rm -f filename	Forcefully removes a file
rm -r directory_name	Removes a directory recursively
rm -rf directory_name	Removes a directory forcefully and recursively
cp file1 file2	Copies the contents of file1 to file2
cp -r dir1 dir2	Recursively Copies dir1 to dir2. dir2 is created if it does not exist
mv file1 file2	Renames file1 to file2
ln -s /path/to/file_name   link_name	Creates a symbolic link to file_name
touch file_name	Creates a new file
cat > file_name	Places standard input into a file
more file_name	Outputs the contents of a file
head file_name	Displays the first 10 lines of a file
tail file_name	Displays the last 10 lines of a file
gpg -c file_name	Encrypts a file
gpg file_name.gpg	Decrypts a file
wc	Prints the number of bytes, words and lines in a file
xargs	Executes commands from standard input
chmod octal filename	Change file permissions of the file to octal
Example
chmod 777 /data/test.c	Set rwx permissions to owner, group and others (otherswho has access to the server)
chmod 755 /data/test.c	Set rwx to the owner and r_x to group and everyone
chmod 766 /data/test.c	Sets rwx for owner, rw for group and everyone
chown owner user-file	Change ownership of the file
chown owner-user:owner-group file_name	Change owner and group owner of the file
chown owner-user:owner-group directory	Change owner and group owner of the directory

PROCESS RELATED

ps	Display currently active processes
ps aux | grep ‘telnet’	Searches for the id of the process ‘telnet’
pmap	Displays memory map of processes
top	Displays all running processes
kill pid	Terminates process with a given pid
killall proc	Kills / Terminates all processes named proc
pkill process-name	Sends a signal to a process with its name
bg	Resumes suspended jobs in the background
fg	Brings suspended jobs to the foreground
fg n	job n to the foreground
lsof	Lists files that are open by processes
renice 19 PID	makes a process run with very low priority
pgrep firefox	find Firefox process ID
pstree	visualizing processes in tree model

NETWORK

ip addr show	Displays IP addresses and all the network interfaces
ip address add 192.168.0.1/24 dev eth0	Assigns IP address 192.168.0.1 to interface eth0
ifconfig	Displays IP addresses of all network interfaces
ping  host	ping command sends an ICMP echo request to establish a connection to server / PC
whois domain	Retrieves more information about a domain name
dig domain	Retrieves DNS information about the domain
dig -x host	Performs reverse lookup on a domain
host google.com	Performs an IP lookup for the domain name
hostname -i	Displays local IP address
wget file_name	Downloads a file from an online source
netstat -pnltu	Displays all active listening ports

 INSTALL PACKAGES

rpm -i pkg_name.rpm	Install an rpm package
rpm -e pkg_name	Removes an rpm package
dnf install pkg_name	Install package using dnf utility
./configure	Checks your system for the required software needed to build the program. It will build the Makefile containing the instructions required to effectively build the project
make	It reads the Makefile to compile the program with the required operations. The process may take some time, depending on your system and the size of the program
make install	The command installs the binaries in the default/modified paths after the compilation

FILE TRANSER

scp file1.txt server2/tmp	Securely copy file1.txt to server2 in /tmp directory
rsync -a /home/apps  /backup/	Synchronize contents in /home/apps directory with /backup  directory

SEARCH

grep ‘pattern’ files	Search for a given pattern in files
grep -r pattern dir	Search recursively for a pattern in a given directory
locate file	Find all instances of the file
find /home/ -name “index”	Find file names that begin with ‘index’ in /home folder
find /home -size +10000k	Find files greater than 10000k in the home folder

 LOGIN

ssh user@host	Securely connect to host as user
ssh -p port_number user@host	Securely connect to host using a specified port
ssh host	Securely connect to the system via SSH default port 22
telnet host	Connect to host via telnet default port 23

DISK USAGE

df  -h	Displays free space on mounted systems
df  -i	Displays free inodes on filesystems
fdisk  -l	Shows disk partitions, sizes, and types
du  -sh	Displays disk usage in the current directory in a human-readable format
findmnt	Displays target mount point for all filesystems
mount device-path mount-point	Mount a device

INSTALL SOURCE

./configure	Checks your system for the required software needed to build the program. It will build the Makefile containing the instructions required to effectively build the project
make	It reads the Makefile to compile the program with the required operations. The process may take some time, depending on your system and the size of the program
make install	The command installs the binaries in the default/modified paths after the compilation

COMPRESSION

tar -cf home.tar home<:code>	Creates archive file called ‘home.tar’ from file ‘home’
tar -xf files.tar	Extract archive file ‘files.tar’
tar -zcvf home.tar.gz source-folder	Creates gzipped tar archive file from the source folder
gzip file	Compression a file with .gz extension