output as input


To redirect the output of a command we use the symbol ">".
In this case, we will use a file named hello.txt, in which we want to insert a certain output ("Sun" in this case):
echo "Sun" > hello.txt
So if we print the content of the file in our command-line, the output will be the following: 
cat hello.txt
    Sun
If we want to append a certain output to an existing file, we just have to use ">>" symbols: 
cat hello.txt
    Sun
echo "Moon" >> hello.txt
cat hello.txt
    Sun
    Moon

input as output


To redirect input from a file for a command, the symbol "<" is used. 
echo < $(cat hello.txt)
    Sun
    Moon
This is particularly useful when chaining commands.

Chaining (or piping)

Chaining, also called Piping because it works with the pipe symbol "|", takes the output of a certain command and feeds it to another command. 
cat hello.txt | grep Mo
Moon

A simple example

Now let's say that we want to combine those commands in a more complex operation. Let's say we want to take the content of a certain file and put it into another file.
We want to sort the animals whose name begins with letter "d" from the file animals.txt and put it into d_animals.txt.
grep d < animals.txt > d_animals.txt
    cat d_animals.txt
    Deer
    Dog
    Dolphin
    ...