Linux

 # See directory structure 
 tree

 # Check in which folder currently	
 pwd
   
 # List files in the current directory
 ls

 # Go inside subdirectory 'folder'
 cd folder

 # Go one step up
 cd ..
 
 # Go back to your home dir	
 cd

 # Searching for file 'fff'
 locate fff
 find . -name fff

 # Create file named 'fff'
 touch fff

 # Edit file	
 nano fff

 # Rename 'fff' as 'goodname'
 mv fff goodname

 # Delete file 'goodname'
 rm goodname

 # Create another copy of 'fff' at 'another-fff'
 cp fff another-fff

 # Create folders 'myfolder' 'bigfolder'
 mkdir myfolder
 mkdir bigfolder

 # Move file 'fff' inside 'myfolder'
 mv fff myfolder

 # Move 'myfolder' inside 'bigfolder'
 mv myfolder bigfolder

 # Delete folder	
 rmdir myfolder

 # Type content of file 'fff' on screen
 cat fff

 # Type first/last 10 lines
 head fff
 tail fff

 # Type content of file - page by page
 #
 # http://helpdeskgeek.com/linux-tips/more-less-command-linux-unix/
 #
 less fff
 more fff

 # Count number of lines in file 'fff'
 wc fff

 # Print lines of file matching the word 'dog'	
 grep 'dog' fff

 # Print lines of file without the word 'dog'	
 grep -v 'dog' fff

 # PERL-like regular expression
 grep -P 'cat' fff

 # Sorting file 'fff'
 sort fff

 # Extracting second column of file	'fff'
 cut -f 2 fff

 # Joining two files fff1 and fff2
 paste fff1 fff2

 # Printing unique lines only	
 uniq fff

 # unique with count
 uniq -c fff

 # Sending output to file instead of screen
 grep -P 'cat' fff > file2

 cut -f 1 fff | sort | uniq -c

 # Processor and operating system
 uname -a
 
 # Disk space
 df -h

 # Mounted file systems
 mount

 # RAM 
 free -g

 # Network 
 netstat -a