Historically, computer systems have offered two very different kinds of user interface: a command-line based interface, and a graphical interface (GUI). In the former, users interact with the computer system by issuing commands - e.g. in the command language used on Unix systems a file called "foo.bar" might be deleted by issuing a command like the following:
while on a GUI system the file might be deleted by dragging its icon to the trash can.
Modern desktop computer systems often make both interfaces available to the user. The graphical interface is typically what computer users learn first, because it is simple and intuitive. However, being able to use a command-line interface is important for skilled users, for at least two reasons:
This document is intended to introduce the use of the command-line interface for users who are already familiar with a graphical user interface. The examples will be specific to the computers in our Macintosh lab, running OS-X, which offers a full-featured Unix command-line interface through the Terminal application. However, the principles are equally applicable to other command line interfaces, such as that presented by Linux or the DOS interface of Windows.
Regardless of the interface that is used, every file on the computer can be uniquely specified by giving a path specification that includes the name of the file and enclosing folders (directories). If the file is located on a removeable or network volume, the path also includes information about the volume on which it is located.
For example, suppose a user in our lab mounts a network volume named "aardvark" that contains a folder named "cs211", which in turn contains a folder named "lab1", which in turn contains a file named "Lab1Main.java". Then the full name of that file would be:
In the graphical interface, one would follow this path by opening the icon for the volume "aardvark" on the desktop (where all the icons for mounted volumes appear), then opening the "cs211" folder within it, and then opening the "lab1" folder within that - which would result in the Lab1Main.java icon becoming visible. On the command line, one would specify this path by literally typing the full name given above - or by using one of the shortcuts to be discussed shortly.
It should be evident that having to type the full path for a file every time one
wanted to access it from the command line could quickly become an unpleasant and
error-prone process. One important way around this is the notion of a "current
directory". When interacting with the operating system through the command-line
interface, there is always a current directory. If one types a path
without preceeding it by a "/", the path is taken as being relative to
the current directory. For example, suppose the user's current directory were the
lab1 directory (
/Volumes/aardvark/cs211/lab1). Then if the
user were to type
a command specifying
Lab1Main.java, that would be interpreted
as relative to the
current directory, and would be equivalent to the full path of the file:
/Volumes/aardvark/cs211/lab1/Lab1Main.java. (The latter full
path, which begins
with a "/", is called an absolute path.)
There are, then, two important principles of interpretation of a file specification typed on the command line:
cs211directory on aardvark, then the file we have been using for our examples would have to be specified by the relative path
lab1is not part of the current path.
In addition to the relative path specification form we have considered, it is also
possible to use "../" in a relative path. The occurrence of ".." means "go up
(backwards) one level in the path". For example, suppose the aardvark volume also
contains a folder called
cs112, which contains a file called
if the current directory were
would be possible
to refer to
paper.txt in either of the following ways:
Even though relative paths can save typing, there is still the matter of typing a directory or file name correctly - even a one character error, including a capitalization error, is enough to mess up the specification. To save typing, it is usually possible, while typing a file name on the command line, to press the tab key - and if the word that is currently being typed can be unambiguously expanded (i.e. only one directory/file in the current directory begins with the letters typed so far) then the command interpreter will complete the name by typing the missing characters.
For example, if one were trying to type the complete path to Lab1Main.java, it might be possible to proceed as follows:
/V then press tab. The system completes this to /Volumes/.
aa, then press tab. The system complete the partial path to
/Volumes/aardvark/. Now type
cs2 (c or cs alone would be ambiguous
if there were also a cs112 folder on the volume). Pressing tab would
cause this to be
/Volumes/aardvark/cs211/. At this point, one might type
lab1/Lab1M and press tab, which would complete the path:
The following are some command-line commands you will likely find useful:
cd- change the current directory.
/Volumes/aardvark/cs211/lab1the current path, you could type
/Volumes/aardvark/cs211/lab1, then the following cd command would make the working directory be /Volumes/aardvark/cs111:
ls- list the contents of the current directory.
ls- list information about a specific file or directory anywhere on the system.
rm- remove (delete) a file.
javac- compile a java program. The current directory must be the root of the package tree. (Note: the file must be a java source file - hence the end of the file name specified in the path will the ".java").
java- run a previously compiled java program. The current directory must be the root of the package tree. (Note: the file must be a java class file - but you do not include ".class" in the path!)