A guide to the Bash Terminal for Beginners

Below is a list of most common commands:

NAVIGATION

• ls - list directory contents
• pwd - print name of current/working directory
• cd - change working directory
• pushd/popd - put the working directory on a stack
• file - determine the file type
• locate - find files by name
• updatedb - update database for locate
• which - locate a command
• history - display bash command history

GETTING HELP

• whatis - display the on-line manual descriptions
• apropos - search the manual page names and descriptions
• man - an interface to the on-line reference manuals

WORKING WITH FILES

• mkdir - create a directory/make directories
• touch - change file timestamps/create empty files
• cp - copy files and directories
• mv - move (rename) files
• rm - remove files or directories
• rmdir - remove empty directories

TEXT FILES

• cat - concatenate files and print on the standard output
• more/less - file perusal filter for CRT viewing
• nano - command line text editor

USERS

• sudo - execute a command as the superuser
• su - change user ID or become another user
• users - print the usernames of users currently logged in
• id - print real and effective user and group IDs

CHANGING FILE PERMISSIONS

• chmod - change permissions of a file

KILLING PROGRAMS AND LOGGING OUT

• Ctrl+C - kill a running command
• killall - kill processes by name
• exit - log out of bash

How to check if a package is installed on Linux

On RPM based system

on RPM based systems like Red Hat, Cent OS etc, we can use rpm query command like below :

rpm -qa | grep telnet

or 

rpm -q telnet

If system is configured with YUM then it can list all installed packages for you and you can grep out your desired package from it.

yum list installed | grep telnet

On APT based systems 
on APT based systems like Debian, Ubuntu etc, dpkg command can be used to verify if package is installed

dpkg -l | grep telnet