Present Working Directory

pwd

List Contents of a Directory

ls # list files
ls -a # list all files, including hidden files
ls -l # list files in long format
ls <dir_name> # list contents inside the specified directory
ls -d <dir_name> # do not list contents inside the directory, just show the output
# for the specified directory 

echo * # lists the contents of the current working directory
echo m* # lists all contents in the directory starting with m if any.

For help regarding any command

man ls # display the manual of ls command

whatis ls # displays a brief description of ls command

apropos <key_word> # displays all commands that have key_word in their description

info # displays a menu of commands, placing the cursor on the command and hitting
# enter takes you to the manual of the chosen command. You can then come back to main
# menu by hiting left angular bracket (<).

type <command> # displays the type of command, whether it is an alias or an operating
# system command or a shell command

Change Directory

cd <dir path>/<relative_path> # to change the directory
cd - # previous directory
cd .. # one directory up
cd ~ # root directory

Date and Calendar

date
date -R # to display in some format
date +"%d %B %Y" # this displays day month and year of the date
cal # displays the calendar for the current month
cal aug 1947 # displays the calendar for the specified month and year
ncal