Most Useful Linux Commands You Can Learn Today

1) 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

2) 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

3) 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

4) File Commands

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

5) 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

6) File Permission

chmod octal filename         Change file permissions of the file to octal
chmod 777 /data/test.c       Set rwx permissions to owner, group and everyone (everyone else who 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

7) Network

ip addr show                    Displays IP addresses and all the network interfaces
ip address add dev eth0    Assigns IP address 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           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

8) Compression/Archives

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

9) 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

10) Install Source (Compilation)

./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

11) 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

12) 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

13) File Transfer

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

14) 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

15) Directory Traverse

cd ..              Move up one level in the directory tree structure
cd                 Change directory to $HOME directory
cd /test  Change directory to /test directory