Everything is a file in Linux

The following post is a section of the book 'Just Enough Linux'.  The entire book can be downloaded in pdf format for free from Leanpub or you can read it online here.
Since this post is a snapshot in time. I recommend that you download a copy of the book which is updated frequently to improve and expand the content.

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A phrase that will often come up in Linux conversation is that;

Everything is a file

For someone new to Linux this sounds like some sort of ‘in joke’ that is designed to scare off the unwary and it can sometimes act as a barrier to a deeper understanding of the philosophy behind the approach taken in developing Linux.

The explanation behind the statement is that Linux is designed to be a system built of a group of interacting parts and the way that those parts can work together is to communicate using a common method. That method is to use a file as a common building block and the data in a file as the communications mechanism.

The trick to understanding what ‘Everything is a file’ means, is to broaden our understanding of what a file can be.

Traditional Files

The traditional concept of a file is an object with a specific name in a specific location with a particular content. For example, we might have a file named foo.txt which is in the directory /home/pi/ and it could contain a couple of lines of text similar to the following;

This is the first line
This is the second line

Directories

As unusual as it sounds a directory is also a file. The special aspect of a directory is that is is a file which contains a list of information about which files (and / or sub-directories) it contains. So when we want to list the contents of a directory using the ls command what is actually happening is that the operating system is getting the appropriate information from the file that represents the directory.

System Information

However, files can also be conduits of information. The /proc/ directory contains files that represent system and process information. If we want to determine information about the type of CPU that the computer is using, the file cpuinfo in the /proc/ directory can list it. By running the command `cat /proc/cpuinfo’ we can list a wealth of information about our CPU (the following is a subset of that information by the way);

pi@raspberrypi ~ $ cat /proc/cpuinfo
processor       : 0
model name      : ARMv7 Processor rev 5 (v7l)
BogoMIPS        : 57.60
Features        : half thumb fastmult vfp edsp neon vfpv3 tls vfpv4 idiva idi\
vt vfpd32 lpae evtstrm
CPU implementer : 0x41
CPU architecture: 7
CPU variant     : 0x0
CPU part        : 0xc07
CPU revision    : 5

Hardware        : BCM2709
Revision        : a01041
Serial          : 000000002a4ea712

Now that might not mean a lot to us at this stage, but if we were writing a program that needed a particular type of CPU in order to run successfully it could check this file to ensure that it could operate successfully. There are a wide range of files in the /proc/ directory that represent a great deal of information about how our system is operating.

Devices

When we use different devices in a Linux operating system these are also represented as a file. In the /dev/ directory we have files that represent a range of physical devices that are part of our computer. In larger computer systems with multiple disks they could be represented as /dev/sda1 and /dev/sda2, so that when we wanted to perform an action such as formatting a drive we would use the command mkfs on the /dev/sda1 file.

The /dev/ directory also holds some curious files that are used as tools for generating or managing data. For example /dev/random is an interface to the kernels random number device. /dev/zero represents a file that will constantly stream zeros (while this might sound weird, imagine a situation where you want to write over an area of disk with data to erase it). The most well known of these unusual files is probably /dev/null. This will act as a ‘null device’ that will essentially discard any information sent to it.

The post above (and heaps of other stuff) is in the book 'Just Enough Linux' that can be downloaded for free (or donate if you really want to :-)).