Use ProcessBuilder to separate commands and arguments instead of spaces. This should work regardless of shell used:

Copyimport java.io.BufferedReader;
import java.io.File;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.ArrayList;
import java.util.List;

public class Test {

    public static void main(final String[] args) throws IOException, InterruptedException {
        //Build command 
        List<String> commands = new ArrayList<String>();
        commands.add("/bin/cat");
        //Add arguments
        commands.add("/home/narek/pk.txt");
        System.out.println(commands);

        //Run macro on target
        ProcessBuilder pb = new ProcessBuilder(commands);
        pb.directory(new File("/home/narek"));
        pb.redirectErrorStream(true);
        Process process = pb.start();

        //Read output
        StringBuilder out = new StringBuilder();
        BufferedReader br = new BufferedReader(new InputStreamReader(process.getInputStream()));
        String line = null, previous = null;
        while ((line = br.readLine()) != null)
            if (!line.equals(previous)) {
                previous = line;
                out.append(line).append('\n');
                System.out.println(line);
            }

        //Check result
        if (process.waitFor() == 0) {
            System.out.println("Success!");
            System.exit(0);
        }

        //Abnormal termination: Log command parameters and output and throw ExecutionException
        System.err.println(commands);
        System.err.println(out.toString());
        System.exit(1);
    }
}

Note: You should use ProcessBuilder to launch as it provides better control over I/O streams, and the Runtime.exec call you use is deprecated. However the outcome would be the same in your case.

The issue here is that bash on Windows is expecting a Linux style pathname, and you are passing in Windows OS pathname.

Your Path environment for bash.exe is correct because you don't get IOException : Cannot run program "bash.exe": CreateProcess error=2, The system cannot find the file specified.

Thus bash.exe has run, but it reports exit code 127 which means Command not found, or PATH error. To fix you need to form valid GNU/Linux pathname to the script. If you are using WSL the C drive is mapped as "/mnt/c" so you need to run the translated path:

String[] cmd = new String[]{ "bash.exe", "/mnt/c/Users/abc/Downloads/test.sh" };

// Using "bash" above should also work if "bash.exe" is in the Path and PATHEXT contains ".EXE"
Process p = new ProcessBuilder(cmd).redirectErrorStream(true).start();
p.getInputStream().transferTo(System.out);
System.out.println(p.waitFor());

You could avoid the OS pathname translation by using a relative path to the script and run from it's directory:

File exe = new File("C:\\Users\\abc\\Downloads\\test.sh");
// Set up relative path for the script
String[] cmd = new String[]{"bash", exe.getName()};

ProcessBuilder pb = new ProcessBuilder(cmd).redirectErrorStream(true);
// Run from the directory of the script:
pb.directory(exe.getParentFile());
Process p = pb.start();
p.getInputStream().transferTo(System.out);
System.out.println(p.waitFor());

On Windows to start a command shell from Java program, you can do it as follows:

Copyimport java.io.IOException;

public class Command {
    public static void main(String[] args) {
        try {
            Runtime.getRuntime().exec("cmd.exe /c start");
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

You need to use the same approach for Linux.

Use ProcessBuilder

ProcessBuilder pb=new ProcessBuilder(command);
pb.redirectErrorStream(true);
Process process=pb.start();
BufferedReader inStreamReader = new BufferedReader(
    new InputStreamReader(process.getInputStream())); 

while(inStreamReader.readLine() != null){
    //do something with commandline output.
}

an example. 1. create cmd 2. write to cmd -> call a command.

try {
    // Execute command
    String command = "cmd /c start cmd.exe";
    Process child = Runtime.getRuntime().exec(command);

    // Get output stream to write from it
    OutputStream out = child.getOutputStream();

    out.write("cd C:/ /r/n".getBytes());
    out.flush();
    out.write("dir /r/n".getBytes());
    out.close();
} catch (IOException e) {
}