Echo

This command print as output its entire argument on the command-line.
It could be used with variables, like in the following example:

    $ example:"this is an example"
    

    $ echo $example

    this is an example
    
    

    
    
Cat
    
This command prints the content of a certain file as an output on the command-line.
    
As example, we could imagine a simple text file in pages named "Hello", which contains the line "Hello World".

      So, our command example will look like this:
    
    $ pages Hello
    $ cat Hello

      Hello World

    
    

    
    
Grep
    
This one behaves very similarly to the previus one, but in this case it prints only the lines matching a certain pattern.

      In this we could imagine a certain text file "animals", which contains a list of animal-names.
    
If we want to select only the animals whose name begins with the letter "d", we will write this:

    

    
$ grep d animals

      Deer

      Dog

      Dolphin

      ...