Proposal: spawn()

For this feature the JDK has a solution already, though not supported in JNode yet, the Runtime.exec() methods. I think they could be used for this functionality. I would avoid the usage of the name spawn for such function anyway. The example above would look much better in a scripting language. Writing a new command in Java in that manner doesn't look very good anyway, instead I would recommand either a script, or normal Java code which could probably reuse other commands directly by refrering to the command classes without the help of the shell.

Currently the default shell of JNode doesn't support scripting. For convenience it has only a simple command called 'run' for executing a set of commands which are listed in a file but there are no constructs for any other programing laguage feature.

Here the primordial question is if we really need a new scripting laguage for this or we could support and existing one. The Bjorne Shell implementation could be an answer for this with the added benefit of supporting many existing bash scripts under JNode. Other options are BeanShell, JavaScript, Groovy, JRuby and many other scripting languages based on the JVM which are already quite good for creating scripts. The support for easily running such scripts under JNode is in progress and it will be based on the scripting support of the Java 6 platform.

It is among the goals of the default shell to be small and user freindly and to efficiently serve the usual daily needs of a Java developer in a command line based environment. So, many of the fancy commands which can come up in a thought experiment might be simply outside of its scope.

I think a command can almost do this already by creating an instance of CommandLine and passing it as an argument to a CommandShell method. But I guess there is value in providing helper methods in the Command or AbstractCommand API to make it easy to do this. There are even potential "value adds" like making the spawned command a subordinate to the one that spawned it.

How do we implement shell scripting? A shell reads from standard input.

$ shell < myscript.sh

It reads a line from standard input, passes the line of text to the spawn() method. The spawn() method parses the command line, handles piping and redirection.

 try {
   for (;;) {
     String line = readLine();
     spawn( line );
   }
 }
 catch( Throwable e ) {
 }

Unfortunately, the execute( CommandLine, InputStream, OutputStream, OutputStream ) signature makes this difficult implement, but not impossible.

The full complexity of command line interpreter should be moved from a shell into a spawn() method. We simplify a shell. We enable any command to invoke any other command.

protected void spawn( String text );

This signature parses the command line--as if it were typed at a shell prompt. It handles piping, redirection, etc..