I have 2 questions
1) how do I i invoke a unix shell from a java.runtime library to run a command like this
Process p = Runtime.getRuntime().exec(commands);
cat alias > bias
2) How can I read and write to a unix pipe a steady stream of data from java. Do i have to make all the system calls like open read write to a pipe
I basically want to replicate this command
cat alias > bias
where the steady stream of data will be come from java program to bias and not cat alias.
Since JDK7, there is a convinience way to do this:
ProcessBuilder pb = new ProcessBuilder("cat", "alias");
File bias = new File("bias");
pb.redirectOutput(Redirect.appendTo(bias));
pb.start();
Reference: ProcessBuilder#redirectOutput
You can invoke a Unix shell like you would invoke any other program. Use the -c option to pass the command to run as a parameter. Also make sure to use the exec(String[]) method, and not exec(String), to avoid the command being tokenized the wrong way:
String[] cmd = {"/bin/sh", "-c", "/bin/cat alias > bias"};
Process p = Runtime.getRuntime().exec(cmd);
To read and write from/to the process, get the input, output, and error streams from the Process instance you just created:
InputStream in = p.getInputStream();
InputStream err = p.getErrorStream();
OutputStream out = p.getOutputStream();
Then read from/write to these streams as usual.
Note that streams are named relative to the Java application. Anything written to the OutputStream is piped to the standard input of the process you created. Anything written by the process to stdout/stderr is piped to the InputStreams obtained from p.getInputStream() and p.getErrorStream() respectively.
Reference: http://download.oracle.com/javase/6/docs/api/java/lang/Process.html
You can't use ">" directly in a Runtime.exec() command. It won't have the expected effect, and will not redirect the output to the file.
You have to get the input stream of your process and redirect it to the file to write:
Process p = Runtime.getRuntime().exec(new String[] {"cat", "alias"});
InputStream in = p.getInputStream();
FileOutputStream fos = new FileOutputStream("yourFile");
// Write in into fos...
// handle the process streams
I don't detail the writing method but:
Runtime.exec().