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!!!
Some Useful Commands
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- 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
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.
There are 7 items in this directory. Some of them are files, like HelloWorld.java. Others are directories, like introcs.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 - 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.
To change directories, use the cd command with the name of a directory.C:\> cd C:\
Now, the command prompt will be:C:\> cd introcs
To see what is in this directory type:C:\introcs>
To return to the previous directory, use the cd command, but this time followed by a space and two periods.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)
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.
To see that it actually worked, use the dir command.C:\introcs> mkdir 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 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.
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> 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)
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> 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)
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.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) - 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.
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.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) - 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:
Here the * matches all files in the C:\introcs\hello directory. It copies them to your newly created loops directory.C:\introcs> mkdir loops C:\introcs> copy c:\introcs\hello\* loops
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 programCenterofMass.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:
Then to read the integers from the file instead of the keyboard, we use the redirection symbol "<".C:\introcs\loops> more input.txt 0 0 10 1 1 10
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.C:\introcs\loops> java CenterofMass < input.txt 0.5 0.5 20
- 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 '>'.
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.)C:\introcs\loops> java CenterofMass > output.txt 0 0 10 1 1 10
phoenix.Princeton.EDU% more output.txt
- Redirecting standard input and standard output. It is often useful to use both redirection operations simultaneously.
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.C:\introcs\loops> java CenterofMass < input.txt > output2.txt
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.
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 > temp.txt C:\introcs> more < temp.txt
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.
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