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
+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
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
+ ...
+
+