Java and the Linux Shell

Java and the Linux Command Line

This document instructs you on how to use the Linux shell with Java. For a light-hearted essasy on the command line, you might enjoy Neal Stephenson's light-hearted essay In the Beginning was the Command Line.

Java

You will use the Java compiler javac to compile your Java programs and the Java interpreter java to run them. We'll assume you have already installed these.

Command-line interface

You will type commands in an application known as the shell. Since you're using Linux, we assume you're somewhat familiar with it.

Launch a new shell.
To configure Java, you will need to know which shell you are running. In case you don't know, type the following command:
```
[wayne] ~> echo $SHELL
```
Your shell will likely be bash, tcsh, sh, or ksh.
To make sure Linux can find the Java compiler and interpreter, edit your shell login file according to the shell you are using.
- Bash. If you are using the bash shell, append the following line to your /u/username/.bashrc file. If it doesn't exist, create a new one. Here, j2sdk1.5.16/bin is the location of the javac and java executables.
```
export PATH=$PATH:~/j2sdk1.6.0.18/bin
```
  Then close your shell and open up a new one. If it does not work, in addition, try repeating the same instructions with the file /u/username/.bash_profile.
- Tcsh. If you are using the tcsh shell, append the following line to your /u/username/.tcshrc file:
```
setenv PATH ~/j2sdk1.6.0.18/bin:$PATH
```
  Then close your shell and open up a new one.

To check that you have the right version of Java installed, type the commands in boldface below. You should see something similar to the information printed below.

[wayne] ~> java -version
java version "1.6.0_18"
Java(TM) 2 Runtime Environment, Standard Edition (build 1.6.0_18-112)
Java HotSpot(TM) Client VM (build 1.6.0_18-64, mixed mode, sharing)

Then type the following javac command.

[wayne] ~> javac -version
javac 1.6.0_18
javac: no source files
...

Compile the program

You will use the javac command to convert your Java program into a form more amenable for execution on a computer.

From the shell, navigate to the directory containing your .java files, say ~wayne/introcs/hello, by typing the cd command below.
```
[wayne] ~> cd introcs/hello
[wayne] ~/introcs/hello>
```
Assuming the file, say HelloWorld.java is in the current working directory, type the javac command below to compile it.
```
[wayne] ~/introcs/hello> javac HelloWorld.java
[wayne] ~/introcs/hello>
```
If everything went well, you should see no error messages.

Execute the program

You will use the java command to execute your program.

From the shell, type the java command below.
```
[wayne] ~/introcs/hello> java HelloWorld
Hello, World
```
If all goes well, you should see the output of the program - Hello, World.

Input and Output

If your program gets stuck in an infinite loop, type Ctrl-c to break out.

If you are entering input from the keyboard, you can signify to your program that there is no more data by typing Ctrl-d for EOF (end of file). You should type this character on its own line.

Troubleshooting

When I try to run java I get: Exception in thread "main" java.lang.NoClassDefFoundError. First, be sure that HelloWorld.class is in the current directory. Be sure to type java HelloWorld without a trailing .class or .java. Also, try running your program with the command line

[wayne] ~/introcs/hello> java -cp ./ HelloWorld

If this works, your classpath is set incorrectly.

How do I check that my PATH variable is set correctly? Type echo $PATH.

Where can I learn more about the command line? Here is a short tutorial on the command-line.