---
layout: page
topic: The grep command
author: Domenico Votta
category_title: Advanced Commands
category_page: Advanced
tags: search for occurences
title: grep
---
<p>The <code>grep</code>  is a command that permits to search occurences of a keyword or more in a file or more. Through some flags you can decide the search criteria. The command grep is case sensitive (robot is different from Robot), but we will see how to ignore it</p>
Here we have two files named <code>example1.txt</code> and <code>example2.txt</code> that contain 5 elements, to test this command:

<code>example1.txt</code>
{% highlight ruby linenos %}
Car
Computer
Robot
Smartphone
Videogame
{% endhighlight %}

<code>example2.txt</code>
{% highlight ruby linenos %}
Apple
Computer
Robot
Microsoft
Huawei
{% endhighlight %}

The syntax command is:
<pre>grep [flag] [keyword] [file]</pre>
You can put different flags together to refine the search.

{% highlight ruby linenos %}
grep Robot example1.txt example2.txt (if you write robot you won't have the correspondence.)
example1.txt: Robot
example2.txt: Robot
{% endhighlight %}
<p>Your output will be this because grep found the keyword Robot in both files</p>

<h2>Flags</h2>
<p>Here a list that contains the main flags of these command:

<ul>
  <li> <b>-i</b>: this flag ignore the case sensitive. Here we can write Robot or robot that grep will find the correspondence. </li>
  {% highlight ruby linenos %}
grep -i robot example1.txt example2.txt
example1.txt: Robot
example2.txt: Robot
  {% endhighlight %}

  <li> <b>-v</b>: this flag ignore the keyword from the search. </li>
  {% highlight ruby linenos %}
grep -i -v robot example1.txt example2.txt
example1.txt:Car
example1.txt:Computer
example1.txt:Smartphone
example1.txt:Videogame
example2.txt:Apple
example2.txt:Computer
example2.txt:Microsoft
example2.txt:Huawei
  {% endhighlight %}

  <li> <b>-c</b>: this flag indicates the number of times that the keyword appears in the file. </li>
  {% highlight ruby linenos %}
grep -i -c robot example1.txt
1
  {% endhighlight %}
<p> If in the file you would have had two times the keyword Robot, the output would have been 2</p>

<li> <b>-h</b>: Remove from the output the file name where it found the keyword</li>
{% highlight ruby linenos %}
grep -i -h apple example1.txt example2.txt
Apple
{% endhighlight %}
<p>How you can see the output doesn't show tha file name <code>example2.txt</code></p>
</ul>

</p>