Command Line: Introduction

Terminal runs a Bourne Again Shell (BASH — one type of Unix interface). It allows you to use command line arguments to execute various operations on your computer.


Basic Commands

If you know the command name which you want to know how to use, you can normally get a detailed information by reading the man pages; type in : 

man cmd
where cmd is the command name, and the man page infromation will be dipslayed in the Terminal window. To return back to the BASH, type 'q'.

Here are some common UNIX commands:

help [command]
Display on screen help.
This will list the commands on which help is provided. If you specify command then help specific to that command will be displayed.

cd [path]
Change your present working directory.
This command is used to change the current directory. path is substituted for either an absolute or relative folder path.

ls [directory]
List the contents of a directory.
This command will list the contents of the specified directory to the screen. If no directory is specified then the contents of the current folder is listed.

pwd
Present working directory.
This command will display the path of the current directory.

rm [files]
remove (delete) files.
This command will remove the specified files if you have write access to them. (DANGEROUS - try "rm -i" instead to get a confirmation prompt.)

exit
Close Terminal

echo [variable]
Output to the terminal
This command will display the value stored in the specified variable.

"Tab"
Command completion.
Terminal is capable of filename and command completion. When specifying a path just press the tab key and Terminal will complete the path as much as it can (up to any ambiguity). If Terminal cannot complete the path the computer will beep. Press tab again to see a list of all the possible completions. This can be used any time a filename or command is expected. (Very useful.)

*
wildcard.
This character is used to match everything (with some limitations). For example "rm *" will delete all files in the current directory (DANGEROUS), where as 'rm *.class' will remove every file with a .class extension in the current directory.


File System

For the most part Terminal follows UNIX file conventions, and as such uses the forward slash '/' not the standard windows back slash '\' as file separators. A notable exception to this is Java's CLASSPATH variable which must be set using the Windows file convention.


The symbol '.' refers to the current directory. Thus the next 2 commands are identical
ls
ls .
(Both list the contents of the current folder to the screen.)

The '..' directory refers to the parent of the current directory.

The '/' directory is the root directory and is the folder that Terminal is installed in.

To specify a drive on your computer you must type '//x/' where x is substituted for a drive letter.
For example to change the directory to drive d type 'cd //d/' and press Enter.


Miscellaneous

When reading help files, or other BASH tutorials, you may see commands such as 'C-x'. 'C' stands for the control key, so 'C-x' would mean to hold the control key down and press 'x'.

C-c (Control + c)
Aborts the current program. We should never have to use this, since every program should be able to exit by itself, but if a program crashes or is unable to exit, we need to abort it - and you will use it lots once you get into GUI programming in Java.

Arrow up/down
Cycles through the history of entered commands.

Arrow left/right
Moves the cursor in the current line.


Java Commands (from the Sun Java Development Kit - jdk)

javac [file].java
Compile a .java code file into byte code.
After you have written and saved your java application as a .java file you must use this command to compile it before it can be executed. In most cases you will use the cd command to change the current directory to the one the java file is located in, then use the javac command to compile it. If there is a syntax error in your code the javac command will fail and it will notify you of the error, telling you the line number and what is wrong. Usually the compiler is quite good, but don't always believe what it says, sometimes something as simple as forgetting a closing bracket or semi-colon can cause the compiler to report several errors when there is only one. If you have multiple java classes that need to be compiled it is a good idea to use 'javac *.java' which will compile all the .java files in the current directory without you having to specify them all individually.

java [class] [input arguments]
Execute a Java class file.
When using this command you only specify the name of the class file, not its extension. ie you don't include the .class extension. If you get an error running this command make sure you have compiled your classes and that the class you are trying to run has a main method. Any input arguments will be picked up by your java application and placed in the args string array.

jar [options] [jar file] [files or directories]
Construct archives of files after compressing them. List and extract files from archives.
jar is similar to the Unix tar or zip commands. To compress files, substitute [options] with 'cvf', [jar file] with the name of the compressed file you want to create and [files or directories] with a list of files or directories you want to compress. To decompress files substitute [options] with 'xvf' and [jar file] with the name of the compressed file you want to decompress.


Java Miscellaneous

Terminal itself doesn't run the javac or java command (or almost all of the other commands you give it) itself. Terminal looks for these commands when you give them in the directories specified in the PATH variable. To see what directories are in the variable type 'echo $PATH'. If you go looking through those directories you will find a java.exe and javac.exe file. These are the applications Terminal calls when you compile or execute java code.

Similarly there is a CLASSPATH variable (you can see this by typing echo $CLASSPATH) that is used to specify where Java packages are stored.

Both the PATH and CLASSPATH variable can be set like this:

PATH=$PATH:'additional path'; export PATH

or like this: 

export CLASSPATH=$HOME/'my_java_classpath':.