theshell.ch/site/pages/scripts/8-redirection.html

---
layout: page
category-page: scripts
category-title: Scripting
tags: redirect output input
author: Dario Rasic
title: Redirection
previous-page: pages/scripts/7-if.html
---
<h3>output as input</h3>
<br>
To redirect the output of a command we use the symbol "&gt;".<br>

In this case, we will use a file named <i>hello.txt</i>, in which we want to insert
a certain output ("Sun" in this case):<br>

<pre>
echo "Sun" &gt; hello.txt
</pre>

So if we print the content of the file in our command-line, the output will be the following:

<pre>
cat hello.txt
    Sun
</pre>

If we want to append a certain output to an existing file,
we just have to use "&gt;&gt;" symbols:

<pre>
cat hello.txt
    Sun
echo "Moon" &gt;&gt; hello.txt
cat hello.txt
    Sun
    Moon
</pre>

<!-- Input redirection text -->
<h3>input as output</h3>
<br>
To redirect input from a file for a command, the symbol "&lt;" is used.

<pre>
echo &lt; $(cat hello.txt)
    Sun
    Moon
</pre>

This is particularly useful when chaining commands.<br>

<!-- Chaining command text -->
<h3>Chaining (or piping)</h3>

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

<h3>A simple example</h3>

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.<br>

We want to sort the animals whose name begins with letter "d" from the file
<i>animals.txt</i> and put it into <i>d_animals.txt</i>.<br>

<pre>
grep d &lt; animals.txt &gt; d_animals.txt
    cat d_animals.txt
    Deer
    Dog
    Dolphin
    ...
</pre>