theshell.ch/site/pages/cmd/advanced/grep.html

---
layout: page
author: Domenico Votta
category-page: advanced
category-title: Advanced commands
tags: search for occurencies
title: grep
previous-page: pages/cmd/advanced/emacs.html
next-page: pages/cmd/advanced/head-tail.html
---
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.<br>

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 bash %}
Car
Computer
Robot
Smartphone
Videogame
{% endhighlight %}

<code>example2.txt</code>
{% highlight bash %}
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.

<pre>
grep Robot example1.txt example2.txt (if you write robot you won't have the correspondence.)
    example1.txt: Robot
    example2.txt: Robot
</pre>

Your output will be this because grep found the keyword Robot in both files

<h2>Flags</h2>
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.
<pre>
grep -i robot example1.txt example2.txt
    example1.txt: Robot
    example2.txt: Robot
</pre>
    </li>

    <li> <b>-v</b>: this flag ignore the keyword from the search.
<pre>
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
</pre>
    </li>

    <li> <b>-c</b>: this flag indicates the number of times that the keyword appears in the file.
<pre>
grep -i -c robot example1.txt
    1
</pre>
    (If in the file you would have had two times the keyword Robot, the output would have been 2)
    </li>

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