This command prints its entire argument as output on the command-line.
It can be used with variables, like in the following example:
example="this is an example" echo $example this is an example
CatThis command prints the content of a certain file as output on the command-line.
For example, we could imagine a simple text file in nano named "Hello", which contains the line "Hello World".
So, our command example will look like this:
nano Hello cat Hello Hello World
GrepThis 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 Dragon Dinosaur Dog Dolphin