The Command Prompt program allows you to work in an environment that looks
more like a traditional operating system as opposed to the icon based
Windows environment. In Command Prompt, you will use your keyboard. You
won't use your mouse at all.
Command Prompt works at a lower level than Windows. This means that you
will have more control over the machine.
The disadvantage is that it is less user-friendly.
You will need the command prompt in COS 126 to compile and execute
your Java programs.
Learning the Command Prompt also provides a gradual transition to
Unix and Linux systems, which are prevalent in science, engineering,
and industry.
To launch Command Prompt select
Start -> Run
and type
cmd in the box.
The Command Prompt shows up as a black terminal window.
The
command prompt should look something like:
C:\>
This is where you type commands.
The boldface type below (that follows the command prompt) is what you
should type as you work through this tutorial. Windows does not care
if you use upper or lower case. That means that command
cd is
the same as
CD. It also means that, in Windows, file
HelloWorld.java is the same as
helloworld.java.
This is NOT true in the system to which you will be submitting your files.
Be very careful!!!
- javac: To compile a Java program, use the
javac
command.
Your program
should compile without any errors or warnings (or if there are warnings be
absolutely sure that they do not indicate a flaw in your program).
C:\>javac HelloWorld.java
- java: To run a successfully compiled Java program, use
the java command.
C:\>java HelloWorld
- more: Display the contents of a file one screenful at a time.
C:\>more HelloWorld.java
- exit: Exit the Command Prompt program and close the
terminal window.
C:\>exit
|
Working with Files and Directories
|
You can also use Command Prompt commands to organize files into a directory
hierarchy. These commands are equivalent to corresponding commands
that you access via the Windows point-and-click interface.
It is useful to be familiar with both interfaces for managing files.
- dir: To view the contents of a directory, type
dir. This command will list all the files and directories within
the current directory. It is analogous to clicking on a Windows folder to
see what's inside.
C:\> dir
Volume in drive C has no label.
Volume Serial Number is C8C7-BDCD
Directory of C:\
10/26/2004 01:36 PM 0 AUTOEXEC.BAT
10/26/2004 01:36 PM 0 CONFIG.SYS
02/10/2005 01:36 PM 126 HelloWorld.java
12/09/2004 12:11 AM DIR Documents and Settings
02/10/2005 08:59 PM DIR introcs
11/02/2004 08:31 PM DIR j2sdk1.4.2_06
12/29/2004 07:15 PM DIR Program Files
01/13/2005 07:33 AM DIR WINDOWS
3 File(s) 126 bytes
5 Dir(s) 32,551,940,096 bytes free
There are 7 items in this directory. Some of them are files,
like HelloWorld.java.
Others are directories, like introcs.
- cd: It is frequently useful to know in which
directory you are currently working. In order to find out, type
cd at the command prompt.
C:\> cd
C:\
To change directories, use the cd command with the name of a
directory.
C:\> cd introcs
Now, the command prompt will be:
C:\introcs>
To see what is in this directory type:
C:\introcs> dir
Volume in drive C has no label.
Volume Serial Number is C8C7-BDCD
Directory of C:\introcs
02/10/2005 08:59 PM DIR .
02/10/2005 08:59 PM DIR ..
02/03/2005 11:53 PM 126 HelloWorld.java
01/17/2005 01:16 AM 256 readme.txt
2 File(s) 382 bytes
2 Dir(s)
To return to the previous directory, use the cd command,
but this time followed by a space and two periods.
C:\introcs> cd ..
C:\>
- mkdir: To create a new directory, use the command mkdir.
The following command creates a directory named hello,
which you can use to to store all of your files associated with
the Hello World assignment.
C:\introcs> mkdir hello
To see that it actually worked, use the dir command.
C:\introcs> dir
Volume in drive C has no label.
Volume Serial Number is C8C7-BDCD
Directory of C:\introcs
02/10/2005 08:59 PM DIR .
02/10/2005 08:59 PM DIR ..
02/11/2005 02:53 PM DIR hello
02/03/2005 11:53 PM 126 HelloWorld.java
01/17/2005 01:16 AM 256 readme.txt
2 File(s) 382 bytes
3 Dir(s)
- move: Now, move the two files HelloWorld.java and
readme.txt into the hello directory using the move
command.
C:\introcs> move HelloWorld.java hello
C:\introcs> move readme.txt hello
C:\introcs> dir
Volume in drive C has no label.
Volume Serial Number is C8C7-BDCD
Directory of C:\introcs
02/10/2005 08:59 PM DIR .
02/10/2005 08:59 PM DIR ..
02/11/2005 02:53 PM DIR hello
0 File(s) 0 bytes
3 Dir(s)
The two files are no longer visible from the current directory.
To access the two files,
change directories with the cd command. Then use the dir
command to see what is in this new directory.
C:\introcs> cd hello
C:\introcs\hello> dir
Volume in drive C has no label.
Volume Serial Number is C8C7-BDCD
Directory of C:\introcs\hello
02/10/2005 08:59 PM DIR .
02/10/2005 08:59 PM DIR ..
02/03/2005 11:53 PM 126 HelloWorld.java
01/17/2005 01:16 AM 256 readme.txt
2 File(s) 382 bytes
2 Dir(s)
You can also use move to rename a file. Simply specify a new
filename instead of a directory name. Suppose you accidentally messed up
the upper and lower case and had saved HelloWorld.java as helloworld.java.
Use two move commands to fix it.
C:\introcs\hello> dir
Volume in drive C has no label.
Volume Serial Number is C8C7-BDCD
Directory of C:\introcs\hello
02/10/2005 08:59 PM DIR .
02/10/2005 08:59 PM DIR ..
02/03/2005 11:53 PM 126 helloworld.java
01/17/2005 01:16 AM 256 readme.txt
2 File(s) 382 bytes
2 Dir(s)
C:\introcs\hello> move helloworld.java temp.java
C:\introcs\hello> move temp.java HelloWorld.java
C:\introcs\hello> dir
Volume in drive C has no label.
Volume Serial Number is C8C7-BDCD
Directory of C:\introcs\hello
02/10/2005 08:59 PM DIR .
02/10/2005 08:59 PM DIR ..
02/03/2005 11:53 PM 126 HelloWorld.java
01/17/2005 01:16 AM 256 readme.txt
2 File(s) 382 bytes
2 Dir(s)
It takes two moves because Windows won't let you move to an already
existing filename and, to Windows, helloworld.java is the same as
HelloWorld.java.
- copy: To make a copy of a file,
use the copy command. The following command
creates a backup copy of our HelloWorld.java program. This is especially
useful when you
modify a working program, but might want to revert back to the original
version if
your modifications don't succeed.
C:\introcs\hello> copy HelloWorld.java HelloWorld.bak
C:\introcs\hello> dir
Volume in drive C has no label.
Volume Serial Number is C8C7-BDCD
Directory of C:\introcs\hello
02/10/2005 08:59 PM DIR .
02/10/2005 08:59 PM DIR ..
02/03/2005 11:53 PM 126 HelloWorld.java
01/17/2005 01:16 AM 256 readme.txt
2 File(s) 382 bytes
3 Dir(s)
- del: Subsequently, you might want to clean up useless
files. The del command deletes a file.
C:\introcs\hello> del HelloWorld.bak
C:\introcs\hello> dir
Volume in drive C has no label.
Volume Serial Number is C8C7-BDCD
Directory of C:\introcs
02/10/2005 08:59 PM DIR .
02/10/2005 08:59 PM DIR ..
02/03/2005 11:53 PM 126 HelloWorld.java
01/17/2005 01:16 AM 256 readme.txt
2 File(s) 382 bytes
3 Dir(s)
WARNING: When you revise a file in jEdit, the jEdit program
will automatically save a backup copy of your original file in the same
directory. The name of the backup file will be the name of the original
file with a ~ at the end. When you submit your program be
careful to
submit HelloWorld.java and not HelloWorld.java~ which
is an old version of the file and has the wrong name.
- wildcards:
You can also apply the copy, del, and move
commands to several files (or directories) at once.
To create a new directory called loops,
and copy all of the files in the hello directory
C:\introcs\hello\
into this newly created directory type:
C:\introcs> mkdir loops
C:\introcs> copy c:\introcs\hello\* loops
Here the * matches all files in the
C:\introcs\hello directory. It copies
them to your newly created loops directory.
Two important abstractions in Command Prompt are
standard input
and
standard
output. By default standard input is your keyboard, and standard
output is your computer screen. For example, in Assignment 1, we write a
program
CenterofMass.java that reads input using StdIn.java and
writes
output using
System.out.println(). To run our
program, the user types the command "
java CenterofMass"
and enters double type values in triplets:
xposition yposition
mass
from the keyboard. The results appear in the terminal window.
C:\introcs\loops> java CenterofMass
0 0 10
1 1 10
0.5 0.5 20
- Redirecting standard input.
As an alternative, we can create a file
that consists of the same six input numbers.
Using a text editor (like jEdit), create a file named input.txt,
and type in the six numbers.
After saving the file in the loops directory, type the following
command to verify that you
entered the integers correctly:
C:\introcs\loops> more input.txt
0 0 10
1 1 10
Then to read the integers from the file instead of the keyboard, we use the
redirection symbol "<".
C:\introcs\loops> java CenterofMass < input.txt
0.5 0.5 20
This produces exactly the same result as if the user had typed the numbers,
except that the user has no opportunity to enter numbers from the
keyboard. This is especially useful for two reasons. First, if there are lots of
input values (there are over 700 inputs for Assignment 2) it would be tedious to
retype them in each time we run our program. Second, it allows programs to be automated,
without waiting for user interaction. This means that your grader can process
your homework programs without typing in the input values by hand each
time.
- Redirecting standard output. Similarly it is possible to redirect
the output to a file instead of to the screen. Continuing with the same
example, if we want to save the output permanently, we can use the output
redirection symbol '>'.
C:\introcs\loops> java CenterofMass > output.txt
0 0 10
1 1 10
The user still types in the input values from the keyboard, but instead of
sending the output to the screen, it is sent to the file named
output.txt.
Note that all printf output is sent to the file, even the
statement that tells the user what to do. Be careful, if the file
output.txt already exists, it will be overwritten. (To append
use '>>' instead.)
phoenix.Princeton.EDU% more output.txt
- Redirecting standard input and standard output. It is often useful to
use both redirection operations simultaneously.
C:\introcs\loops> java CenterofMass < input.txt > output2.txt
After executing this command, no output appears on the screen, but the file output2.txt
now contains exactly the same data as
output.txt above.
Another useful abstraction is
piping. Piping is when the
output of one program is used as the input of another program.
For example,
suppose we want to view the output of a program, but there is so much that it
whizzes by on the screen too fast to read. (The program
RandInts.java prints out a
bunch of random integers.) One possible way to accomplish this
is to type the following two commands.
C:\introcs> java RandInts > temp.txt
C:\introcs> more < temp.txt
Note that
more will work by redirecting the file temp.txt to
standard input (as
is done here) or by simply using the filename (as is done at the beginning of
the document). Instead, we could do this in one line using the pipe symbol '|'
C:\introcs> java RandInts | more
This is often useful when debugging a program, especially if your program goes
into an infinite loop and you want to see the first few values that it prints.
