Linux with Operating System Concepts

Information Technology: An 
Introduction for Today’s Digital World
,
 
FL: CRC Press, 2013.)

---

The Linux File System
◾
395
some special-purpose programs that are treated like files (for instance, a random number 
generator). Let us consider each of these in turn.
10.3.1 Files versus Directories
The directory should be familiar to the reader by now. It is a named entity that contains 
files and subdirectories (or devices, links, etc.). The directory offers the user the ability to 
organize their files in some reasonable manner, giving the file space a hierarchical struc-
ture. Directories can be created just about anywhere in the file system and can contain just 
about anything from empty directories to directories that themselves contain directories.
The directory differs from the file in a few significant ways. First, we expect directories 
to be executable. Without that permission, no one (including the owner) can cd into the 
directory. Second, the directory does not store content like a file; instead it merely stores 
other items. That is, whereas the file ultimately is a collection of blocks of data, the direc-
tory contains a list of pointers to objects. Third, there are some commands that operate on 
directories and not files (e.g., cd, pwd, mkdir) and some commands that operate on files but 
not directories (e.g., wc, diff, less, more). We do find that most Linux file commands will 
operate on directories themselves, including for instance cp, mv, rm (using the recursive 
version), and wildcards apply to both files and directories.
10.3.2 Nonfile File Types
Many devices are treated as files in Linux. These devices are listed under the /dev directory. 
We categorize these devices into two subcategories: character devices and block devices. 
Character devices are those that input or output streams of characters. These will include 
the keyboard, the mouse, a terminal (as in terminal window), and serial devices such as 
older MODEMs and printers. Block devices communicate via blocks of data. The term 
“block” is traditionally applied to disk drives where the files are broken into fixed-sized 
blocks. However, here, block is applied to any device that communicates by transmitting 
chunks of data at a time (as opposed to the previously mentioned character type). Aside 
from hard disk, block devices include optical disk and flash drive.
Aside from the quantity of data movement, another differentiating characteristic 
between character and block devices is how input and output are handled. For a character 
device, a program executing a file command must wait until the character is transferred 
before resuming. For a block device, blocks are buffered in memory so that the program 
can continue once the instruction has been issued. Further, as blocks are only portions 
of entire files, it is typically the case that a file command can request one portion of a file. 
This is often known as 
random access
. The idea is that we do not have to request block 1 
before obtaining block 2. Having to read blocks in order is known as 
sequential access
. But 
in random access, we can obtain any block desired and it should take no longer to access 
block j than block i.
Another type of file construct is the 
domain socket
, also referred to as a local socket. 
This is not to be confused with a network socket. The domain socket is used to open com-
munication between two local 
processes
. This permits interprocess communication (IPC) 
so that the two processes can share data. We might, for instance, want to use IPC when 

---

396
◾
Linux with Operating System Concepts
one process is producing data that another process is to consume. This would be the case 
when some application software is going to print a file. The application software produces 
the data to be printed, and the printer’s device driver consumes the data. The IPC is also 
used to create a rendezvous between two processes where process B must wait for some 
event from process A.
There are several distinctions between a network and domain socket. The network 
socket is not treated as a file (although the network itself is a device that can interact via file 
system commands) while the domain socket is. The network socket is created by the oper-
ating system to maintain communication with a remote computer while domain sockets 
are created by users or running software. Network sockets provide communication lines 
between