How can I run my own personalized bash shell in Java?

Question

My goal is to execute a personalized prompt in a bash shell in linux and to execute orders. The result should be like in the image.

So I suppose I should start with something like:

ProcessBuilder builder = new ProcessBuilder();
builder.command("sh");
builder.directory(new File(System.getProperty("user.home")));
Process process = builder.start();

or

String homeDirectory = System.getProperty("user.home");
Process process;
try {
    System.out.print("# MyShell> ");
    process = Runtime.getRuntime().exec(String.format("sh", homeDirectory));
} catch (IOException e) {
    System.err.println("Error en el método exec()");
    e.printStackTrace();
}

Controlling the exceptions.

So basically is to start a personalized functional bash prompt in java and control the error messages (like changing its color). Also controlling the exit with a personalized word, like 'quit' instead of 'exit'.

Note: I am not trying to execute a script or some commands like 'tree' or 'ls' like in many other examples around here. I'm looking for a way to run my personalized bash and do everything that the system allows me in bash or close to this idea.

Answer 1

This problem is more complex than it appears due to the interplay of the shell, the terminal, and your program.

Given that the requirements are:

1. Custom shell prompt
2. Custom commands
3. Have the shell work normally
4. Intercept the program output

My suggestion is:

1. Do the customizations from the shell side
2. Create a pty to fool programs into thinking they're writing to a terminal. In Java on Linux/Mac, a simple way to do this is via the script tool.

Here's an example shell configuration file myrc:

export PS1='# MyShell> '
quit() { exit "$@"; }

and here's Shell.java to start the custom shell:

import java.io.*;
import java.nio.charset.*;
import java.lang.ProcessBuilder.Redirect;

class Shell {
  public static void main(String[] args) throws IOException {
    ProcessBuilder builder = new ProcessBuilder(
        // Use 'script' to fool programs into thinking stdout is a tty.
        "script", "-q", "/dev/null",
        // Run bash with prompt and command customizations
        "bash", "--rcfile", "myrc"
        );
    builder
      // Get input from the current TTY
      .redirectInput(Redirect.INHERIT)
      // But pipe stdout/stderr to intercept it
      .redirectErrorStream(true)
      .redirectOutput(Redirect.PIPE);

    Process proc = builder.start();
    InputStream input = proc.getInputStream();
    byte[] buffer = new byte[65536];
    int read;

    String search = "world";

    while((read = input.read(buffer, 0, buffer.length)) != -1) {
      // Simplify by assuming we only get text data, and only
      // in buffers large enough to contain the entire search string
      String s = new String(buffer, 0, read, StandardCharsets.UTF_8);
      // Replace search word with a highlighted/colored string.
      s = s.replaceAll(search, "\u001B[35m<<$0>>\u001B[m");
      System.out.print(s);
    }

    System.out.println("Java program is exiting...");
  }
}

Example session showing custom prompt, working text interception (making the word "world" purple and surrounded by angle brackets), and finally the custom quit command:

$ javac Shell.java && java Shell
# MyShell> echo "hello world"
hello <<world>>
# MyShell> quit
exit
Java program is exiting...
$