If you are new to Linux, or have used it for a few months, then you must have heard or read statements such as “In Linux, everything is a File”.
Read Also: 5 Useful Commands to Manage Linux File Types
That is in fact true although it is just a generalization concept, in Unix and its derivatives such as Linux, everything is considered as a file. If something is not a file, then it must be running as a process on the system.
To understand this, take for example the amount of space on your root (/) directory is always consumed by different types of Linux files. When you create a file or transfer a file to your system, it occupies some space on the physical disk and it is considered to be in a specific format (file type).
And also the Linux system does not differentiate between files and directories, but directories do one important job, that is store other files in groups in a hierarchy for easy location. All your hardware components are represented as files and the system communicates with them using these files.
The idea is an important description of a great property of Linux, where input/output resources such as your documents, directories (folders in Mac OS X and Windows), keyboard, monitor, hard-drives, removable media, printers, modems, virtual terminals and also inter-process and network communication are streams of bytes defined by file system space.
A notable advantage of everything being a file is that the same set of Linux tools, utilities and APIs can be used on the above input/output resources.
Although everything in Linux is a file, there are certain special files that are more than just a file for example sockets and named pipes.
What are the different types of files in Linux?
In Linux there are basically three types of files:
- Ordinary/Regular files
- Special files
- Directories
Ordinary/Regular Files
These are files data contain text, data or program instructions and they are the most common type of files you can expect to find on a Linux system and they include:
- Readable files
- Binary files
- Image files
- Compressed files and so on.
Special Files
Special files include the following:
Block files : These are device files that provide buffered access to system hardware components. They provide a method of communication with device drivers through the file system.
One important aspect about block files is that they can transfer a large block of data and information at a given time.
Listing block files sockets in a directory:
# ls -l /dev | grep "^b"
Sample Output
brw-rw---- 1 root disk 7, 0 May 18 10:26 loop0 brw-rw---- 1 root disk 7, 1 May 18 10:26 loop1 brw-rw---- 1 root disk 7, 2 May 18 10:26 loop2 brw-rw---- 1 root disk 7, 3 May 18 10:26 loop3 brw-rw---- 1 root disk 7, 4 May 18 10:26 loop4 brw-rw---- 1 root disk 7, 5 May 18 10:26 loop5 brw-rw---- 1 root disk 7, 6 May 18 10:26 loop6 brw-rw---- 1 root disk 7, 7 May 18 10:26 loop7 brw-rw---- 1 root disk 1, 0 May 18 10:26 ram0 brw-rw---- 1 root disk 1, 1 May 18 10:26 ram1 brw-rw---- 1 root disk 1, 10 May 18 10:26 ram10 brw-rw---- 1 root disk 1, 11 May 18 10:26 ram11 brw-rw---- 1 root disk 1, 12 May 18 10:26 ram12 brw-rw---- 1 root disk 1, 13 May 18 10:26 ram13 brw-rw---- 1 root disk 1, 14 May 18 10:26 ram14 brw-rw---- 1 root disk 1, 15 May 18 10:26 ram15 brw-rw---- 1 root disk 1, 2 May 18 10:26 ram2 brw-rw---- 1 root disk 1, 3 May 18 10:26 ram3 brw-rw---- 1 root disk 1, 4 May 18 10:26 ram4 brw-rw---- 1 root disk 1, 5 May 18 10:26 ram5 ...
Character files : These are also device files that provide unbuffered serial access to system hardware components. They work by providing a way of communication with devices by transferring data one character at a time.
Listing character files sockets in a directory:
# ls -l /dev | grep "^c"
Sample Output
crw------- 1 root root 10, 235 May 18 15:54 autofs crw------- 1 root root 10, 234 May 18 15:54 btrfs-control crw------- 1 root root 5, 1 May 18 10:26 console crw------- 1 root root 10, 60 May 18 10:26 cpu_dma_latency crw------- 1 root root 10, 203 May 18 15:54 cuse crw------- 1 root root 10, 61 May 18 10:26 ecryptfs crw-rw---- 1 root video 29, 0 May 18 10:26 fb0 crw-rw-rw- 1 root root 1, 7 May 18 10:26 full crw-rw-rw- 1 root root 10, 229 May 18 10:26 fuse crw------- 1 root root 251, 0 May 18 10:27 hidraw0 crw------- 1 root root 10, 228 May 18 10:26 hpet crw-r--r-- 1 root root 1, 11 May 18 10:26 kmsg crw-rw----+ 1 root root 10, 232 May 18 10:26 kvm crw------- 1 root root 10, 237 May 18 10:26 loop-control crw------- 1 root root 10, 227 May 18 10:26 mcelog crw------- 1 root root 249, 0 May 18 10:27 media0 crw------- 1 root root 250, 0 May 18 10:26 mei0 crw-r----- 1 root kmem 1, 1 May 18 10:26 mem crw------- 1 root root 10, 57 May 18 10:26 memory_bandwidth crw------- 1 root root 10, 59 May 18 10:26 network_latency crw------- 1 root root 10, 58 May 18 10:26 network_throughput crw-rw-rw- 1 root root 1, 3 May 18 10:26 null crw-r----- 1 root kmem 1, 4 May 18 10:26 port crw------- 1 root root 108, 0 May 18 10:26 ppp crw------- 1 root root 10, 1 May 18 10:26 psaux crw-rw-rw- 1 root tty 5, 2 May 18 17:40 ptmx crw-rw-rw- 1 root root 1, 8 May 18 10:26 random
Symbolic link files : A symbolic link is a reference to another file on the system. Therefore, symbolic link files are files that point to other files, and they can either be directories or regular files.
Listing symbolic link sockets in a directory:
# ls -l /dev/ | grep "^l"
Sample Output
lrwxrwxrwx 1 root root 3 May 18 10:26 cdrom -> sr0 lrwxrwxrwx 1 root root 11 May 18 15:54 core -> /proc/kcore lrwxrwxrwx 1 root root 13 May 18 15:54 fd -> /proc/self/fd lrwxrwxrwx 1 root root 4 May 18 10:26 rtc -> rtc0 lrwxrwxrwx 1 root root 8 May 18 10:26 shm -> /run/shm lrwxrwxrwx 1 root root 15 May 18 15:54 stderr -> /proc/self/fd/2 lrwxrwxrwx 1 root root 15 May 18 15:54 stdin -> /proc/self/fd/0 lrwxrwxrwx 1 root root 15 May 18 15:54 stdout -> /proc/self/fd/1
You can make symbolic links using the ln utility in Linux as in the example below.
# touch file1.txt # ln -s file1.txt /home/tecmint/file1.txt [create symbolic link] # ls -l /home/tecmint/ | grep "^l" [List symbolic links]
In the above example, I created a file called file1.txt in /tmp directory, then created the symbolic link, /home/tecmint/file1.txt to point to /tmp/file1.txt.
Pipes or Named pipes : These are files that allow inter-process communication by connecting the output of one process to the input of another.
A named pipe is actually a file that is used by two process to communicate with each and it acts as a Linux pipe.
Listing pipes sockets in a directory:
# ls -l | grep "^p"
Sample Output
prw-rw-r-- 1 tecmint tecmint 0 May 18 17:47 pipe1 prw-rw-r-- 1 tecmint tecmint 0 May 18 17:47 pipe2 prw-rw-r-- 1 tecmint tecmint 0 May 18 17:47 pipe3 prw-rw-r-- 1 tecmint tecmint 0 May 18 17:47 pipe4 prw-rw-r-- 1 tecmint tecmint 0 May 18 17:47 pipe5
You can use the mkfifo utility to create a named pipe in Linux as follows.
# mkfifo pipe1 # echo "This is named pipe1" > pipe1
In the above example, I created a named pipe called pipe1, then I passed some data to it using the echo command, after that the shell became un-interactive while processing the input.
Then I opened another shell and run the another command to print out what was passed to pipe.
# while read line ;do echo "This was passed-'$line' "; done<pipe1
Socket files : These are files that provide a means of inter-process communication, but they can transfer data and information between process running on different environments.
This means that sockets provide data and information transfer between process running on different machines on a network.
An example to show the work of sockets would be a web browser making a connection to a web server.
# ls -l /dev/ | grep "^s"
Sample Output
srw-rw-rw- 1 root root 0 May 18 10:26 log
This is an example of a socket create in C by using the socket() system call.
int socket_desc= socket(AF_INET, SOCK_STREAM, 0 );
In the above:
AF_INETis the address family(IPv4)SOCK_STREAMis the type (connection is TCP protocol oriented)0is the protocol(IP Protocol)
To refer to the socket file, use the socket_desc, which is the same as the file descriptor, and use read() and write() system calls to read and write from the socket respectively.
Directories
These are special files that store both ordinary and other special files and they are organized on the Linux file system in a hierarchy starting from the root (/) directory.
Listing sockets in a directory:
# ls -l / | grep "^d"
Sample Output
drwxr-xr-x 2 root root 4096 May 5 15:49 bin drwxr-xr-x 4 root root 4096 May 5 15:58 boot drwxr-xr-x 2 root root 4096 Apr 11 2015 cdrom drwxr-xr-x 17 root root 4400 May 18 10:27 dev drwxr-xr-x 168 root root 12288 May 18 10:28 etc drwxr-xr-x 3 root root 4096 Apr 11 2015 home drwxr-xr-x 25 root root 4096 May 5 15:44 lib drwxr-xr-x 2 root root 4096 May 5 15:44 lib64 drwx------ 2 root root 16384 Apr 11 2015 lost+found drwxr-xr-x 3 root root 4096 Apr 10 2015 media drwxr-xr-x 3 root root 4096 Feb 23 17:54 mnt drwxr-xr-x 16 root root 4096 Apr 30 16:01 opt dr-xr-xr-x 223 root root 0 May 18 15:54 proc drwx------ 19 root root 4096 Apr 9 11:12 root drwxr-xr-x 27 root root 920 May 18 10:54 run drwxr-xr-x 2 root root 12288 May 5 15:57 sbin drwxr-xr-x 2 root root 4096 Dec 1 2014 srv dr-xr-xr-x 13 root root 0 May 18 15:54 sys drwxrwxrwt 13 root root 4096 May 18 17:55 tmp drwxr-xr-x 11 root root 4096 Mar 31 16:00 usr drwxr-xr-x 12 root root 4096 Nov 12 2015 var
You can make a directory using the mkdir command.
# mkdir -m 1666 tecmint.com # mkdir -m 1666 news.tecmint.com # mkdir -m 1775 linuxsay.com
Summary
You should now be having a clear understanding of why everything in Linux is a file and the different types of files that can exit on your Linux system.
You can add more to this by reading more about the individual file types and they are created. I hope this find this guide helpful and for any questions and additional information that you would love to share, please leave a comment and we shall discuss more.
Hi, I am just understanding all these stuff. What is the relation between socket files and ports (e.g. port:80) in regards of “everything is a file” philosophy?
@Leopoldo
Ports are communication end-points that are associated with different applications on an operating system such as 80 for HTTP server, 22 for SSH etc. Applications listen on a particular port and clients that want to request services or connect to them do that via the port.
On the other hand, a socket file allows communication between processes over a network after a connection between two applications via a port.
In conclusion, a client must first access an application running on a network via a port, before two or more processes on the client and application can communicate via a socket: a socket is bound to a port number.
Thank you for the clarification Aaron.
Once the communication is stablished through the port (btw, these ports are (physically) associated only to the CPU? the CPU of the cliente or of the server?) then how does the socket file knows that every task or process must be conducted through this specific port? How these tasks avoid to get lost in the network or motherboard? (A kind of header mentioning the port in each socket files just created, for example?)
Sorry for my dummies questions.
Thank you in advance for your gently explanations.
@Leopoldo
As i mentioned before, all sockets in an application are bound to the port used by the application. Considering a connection-oriented(over a network) client-to-server model, when the server is running, the socket on it waits for requests from a client for a service(s).
The server first of all establishes (binds) an address that clients can use to find the server process; then it waits for clients to requests.
Most importantly, the exchange of data between the server and a client practically happens when the client connects to the server through a socket; the server performs the client’s request and sends a response back to the client —- as simple as that.
You can read more about sockets from IBM Knowledge center: https://www.ibm.com/support/knowledgecenter/en/ssw_ibm_i_71/rzab6/howdosockets.htm
Lastly, about this: “Sorry for my dummies questions.” – Never be afraid of asking, it helps you to understand better and learn.
Did I miss the comparison between soft and hard links?
Great explanation of the different types of files and some useful examples of how they can be used. Thank you!
@Humberto
Welcome, hope you enjoyed reading this article and many thanks for the feedback.