# Find what you need with Linux

Now that we covered: pwd, ls, and cd and are familiar with these basic commands for navigating the Linux file system, let's look at a couple of ways to find what you need within this system.

As a security analyst, your work will likely involve filtering for the information you need.

Filtering means searching your system for specific information that can help you solve complex problems.

For example, imagine that your team determines a piece of malware contains a string of characters.

You might be tasked with finding other files with the same string to determine if those files contain the same malware.

Later, we'll learn more about how you can use SQL to filter a database, but Linux is a good place to start basic filtering.

First, we'll start with grep.

The grep command searches a specified file and returns all lines in the file containing a specified string.

Here's an example of this.

Let's say we have a file called updates.txt, and we're currently looking for lines that contain the word: OS.

If the file is large, it would take a long time to visually scan for this.

Instead, after navigating to the directory that contains updates.txt, we'll type the command: grep OS updates.txt into the shell.

Notice how the grep command is followed by two arguments.

The first argument is the string we're searching for; in this case, OS.

The second argument is the name of the file we're searching through, updates.txt.

When we press enter, Bash returns all lines containing the word OS.

Now let's talk about piping.

Piping is a Linux command that can be used for a variety of purposes.

In a moment, we'll focus on how it can be used for filtering.

But first, let's talk about the general idea of piping.

The piping command sends a standard output of one command as standard input into another command for further processing.

It's represented by the vertical bar character.

In our context, we can refer to this as the pipe character.

Take a moment and imagine a physical pipe.

Physical pipes have two ends.

On one end, for example, water might enter the pipe from a hot water tank.

Then, it travels through the pipe and comes out on the other end in a sink.

Similarly, in Linux, piping also involves redirection.

Output from one command is sent through the pipe and then is used on the other side of the pipe.

Earlier in this video, I explained how grep can be used to filter for strings of characters within a file.

Grep can also be incorporated after a pipe.

Let's focus on this example.

The first command, ls, instructs the operating system to output the file and directory contents of their reports subdirectory.

But because the command is followed by the pipe, the output isn't returned to the screen.

Instead, it's sent to the next command.

As we just learned, grep searches for a specified string of characters;

in this case, it's users.

But where is it searching?

Since grep follows a pipe, the output of the previous command indicates where to search.

In this case, that output is a list of files and directories within the reports subdirectory.

It will return all files and directories that contain the word: users.

Let's explore this in Bash.

So we can better understand how the filter works, let's first output everything in the reports directory.

If we were already in the directory, we would just need to input ls.

But since we're not, we'll also specify the path to this directory.

When we press enter, the output indicates there are seven files in the reports directory.

Because we want to return only the files that contain the word users, we'll combine this ls command with piping and the grep command.

As the output demonstrates, Linux has been instructed to return only files that contain the word users.

The two files that don't contain this string no longer appear.

So now you have two different ways that you can filter in Linux while working as an analyst.

Navigating through files and filtering are just part of what you need to know.

Let's keep exploring the Linux command line.