List

ls -> list folders and files  
-a -> shows everything (also hidden files)  
-h -> easy way to understand
-R -> shows a list recursively  
-r -> shows reverse listing  
-t -> shows last modified  
-S -> shows sorting by size  
-l -> lists, longer form with more details  

ls usr/bin         -> executable binaries on the system  
ls usr/bin | wc -l -> number of executables (commands)