The file system organizes the files on a storage medium (usually a magnetic disk.) Here we discuss how files are named, next, how they are organized, and, finally, how you refer to them on a command line.
File names can be constructed from nearly any characters, but to avoid
confusion it is best to limit use to only letters (Unix is case
sensitive, so upper and lower case are different), numbers, and a few
special characters, such as underscore
. (dot). Do not use spaces in file names, as is common
Files with names beginning with dot
. are treated as “hidden”
and are usually ignored unless you take special steps to include them.
For example, the command
ls does not list hidden files, but the
ls -a does.
Unix can operate on several files of the same name through the use of
“wild card” characters. The wild card
* stands for any or no
characters. Thus the command
ls s* lists all files with names
beginning with the letter
s. The wild card
? stands for
exactly one character. It can be repeated. Thus the command
ls s?? lists all files with three-character names beginning with
Since practically everything you do with a computer involves working with a file, it is important to understand how the file system is organized. It is arranged in a hierarchical fashion like the branches on an inverted tree. These branches are called directories (known as folders in the Macintosh and Windows operating systems). Any directory can contain files as well as subdirectories. (It is the same in Windows: any folder can contain files as well as folders.) Each user has a personal home directory. By default when you log on you are “connected” to your home directory. After logging on, if you type the Unix command pwd, you will see a complete specification of your current working directory. Try it! For example you might see
% pwd /u/course/u0123456A slash / separates the names of the directories. They are listed in order with top level directory first and lower level directories next. In this case the user's name is u0123456 and this user has a home directory with the same name. This directory is a subdirectory under the directory course (which happens to contain the home directories for all students in physics classes). The course directory is in turn a subdirectory under the u directory, that contains all departmental user accounts for research and education among other directories. Finally the u directory is under the very top of the inverted tree, sometimes referred to as just /, and sometimes called the “root” directory, which “contains” the entire file tree for your machine, including all system files, applications files, and user accounts.
A path to a file or directory is a sequence of directories that take you to the file. Sometimes the term “path” also includes the name of the file itself.
/u/course/u0123456/ specify the path to all
the files and subdirectories in your home directory. It is called an
“absolute” path, because it starts from a fixed location, namely the
root directory, so it means the same thing regardless of what the
current working directory may be. All paths that start from the root
(top level) directory
/ are called “absolute”. (Please note
~u0123456 are convenient abbreviations for your
home directory, so any path starting with
~ is also absolute.)
Unix keeps track of your current working directory. The “relative”
path specifies a location relative to the current working directory.
Its meaning depends on where you are in the file system, i.e. what
the current working directory may be. Extending the above example,
suppose you (u0123456) had subdirectories
asst02 in your home directory and your home directory is your
current working directory. Then the shorthand
the relative path to all the files in your
asst02 specifies the relative path to all the files in the
The special directory
.. means “the parent of the current
directory”. It is used in relative paths to go back up in the tree.
Continuing with the above example, suppose that
/u/class/u0123456/asst01 is the current working directory.
../asst02 is the relative path to all the files in the
For completeness, we note that the special directory
“the current directory”.