The Unix file system is a hierarchical structure that allows users to store information
by name. At the top of the hierarchy is the root
directory, which always has the name /. A typical Unix file
system might look like:
The location of a file in the file system is called its
path. Since the root directory is at the
top of the hierarchy, all paths start from /.
Using the sample file system illustrated above, let us determine the path for
quiz1.txt:
- All paths start from the root directory, so the path begins with
/
- We need to go to the home subdirectory,
and the path becomes /home
- In the home directory, we go into the
ian subdirectory. The path is now
/home/ian
- In the ian directory, we go into the
cpsc124 subdirectory. The path is now
/home/ian/cpsc124
- The file quiz1.txt is in the
cpsc124 directory. Appending the filename to
the path, the path becomes
/home/ian/cpsc124/quiz1.txt
Unix assigns a special "shorthand" notation for specifying commonly used paths. These
special paths and their shorthand names are listed below:
- /:
A single slash / specifies the root directory. As we have seen,
all paths begin with / because root is at the top of the file
system structure.
- ~: A
tilde ~ specifies the current user's home directory. Every Unix
user has a home directory, where his/her personal files are stored. When a user logs into
Unix, he/she is automatically moved to this directory. This notation is not always
available in the Bourne Shell.
- ~user: A
tilde ~ followed by a user-id specifies the home directory of the
given user. For example, at UBC, ~a1a1 is the home directory of
the user who has the user-id a1a1. This shorthand notation is
also not always available in the Bourne Shell.
- .: This
single dot . specifies the current working directory, that is,
the directory that the user is currently in.
- ..: A
pair of dots .. refers to the parent of the current working
directory.
Now that we know how Unix organizes files, let us attempt to define what we mean by the
term file.
Files and Directories
There are 3 kinds of files: ordinary files
(files), directory files
(directories), and special files.
- Files: Files are containers for data.
This data can be anything, including the text of a report, an image (picture) of a house,
an executable program like a word processor, or any arbitrary data. Files are created by
users or programs in order to save data for future use. For example, a user might save the
report she wrote using a text editor into a text file, or she might save an image from a
drawing program into an image file.
- Directories: Each directory
contains a number of files. A directory can contain other directories, be contained in
another directory, or both. A directory that contains a file is called that file's
parent directory. Similarly, if directory A contains
directory B, then directory A is directory B's parent directory. A directory that is
contained in another directory is called a
subdirectory.
- Special files: A special file is
much like an ordinary file, and shares the same basic interface. However, special files
are not stored in the file system because they represent input/output devices. The I/O
device provides the data directly. You will not responsible for knowing how special files
work or even how to use them. Simply be aware that they are present.
Every file has an associated data structure that contains important information about
that file. This data structure is known as an i-node. An i-node has information about the
length of the file, the time the file was most recently modified or accessed, the time the
i-node itself was most recently modified, the owner of the file, the group identifications
and access permissions, the number of links to the file (we will discuss these shortly),
and pointers to the location on the disk where the data contained in the file is stored. A
filename is used to refer to an i-node. A directory is a file that contains a list of
names, and for each name there is a pointer to the i-node for the file or directory.
The following picture offers a graphical interpretation of files, their i-nodes, and
the blocks that they occupy on disk:
Filenames
The name given to a file or directory can contain almost any character that can be typed
on the keyboard and be of almost any length. It is common to use only letters and digits
for the first character of a filename, and use only letters, digits, periods [.], hyphens
[-] and underscores [_] for the remainder of the filename. However, the following
characters should never be used in filenames:
- quotes ["], ['] or
[`]
- spaces
- question marks [?]
- asterisks [*]
- slashes [/] or [\]
- greater than [>] or less than
[<] signs
These characters all have special meaning to Unix. Also, Unix is case sensitive. For
example, the files hello.cpp,
Hello.cpp and HELLO.cpp
are three different filenames representing three different files in Unix.
In the Macintosh and Windows operating systems, an extension is automatically assigned
to each file when it is created (an extension is a
special string that is added at the end of a filename to indicate its type, and is
separated from the rest of the file name by a period, such as
.doc or .xls). By
default, Unix filenames do not have any extension. Therefore, Unix users often add this
extension themselves to indicate the type of data a file contains. Some examples are
.cpp for C++ program files (e.g.,
hello.cpp), .txt for
text files (e.g., document.txt), and
.ps for PostScript documents (e.g.,
outline.ps).
Unix Online Help
File and Directory Management
Back to the introduction