Bash non scrolling terminal output

Question

Some programs produce output to bash in a way that the user experience like strings that changes values rather than rows printed(bash scrolls down to be able to show all the changing output). What is this phenomena called? How is it achieved in e.g a java program or a bash script. How can the output be parsed? Is it just printing new lines but using some special function?

top is one program that uses this "phenomena" of outputting data

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Okey cursors are used.

Then how can i parse them in java? I tried this code but it does not print anything

public static void exeTest(String [] args) throws IOException{
        if (args.length <= 0) {
            System.out.println("empty command");
            return;
        }

       Process process = new ProcessBuilder(args).start();
       InputStream is = process.getInputStream();
       InputStreamReader isr = new InputStreamReader(is);
       BufferedReader br = new BufferedReader(isr);


       System.out.printf("Output of running %s is:", 
          Arrays.toString(args));
       String line;
       while ((line = br.readLine()) != null) {
            System.out.println(line);
       }

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Thanks for all good answers, im now more familiar at this topic even thou I still cant parse a program output like the one from top in java. Ill close this question and do more research about how to parse this in java. Then I may start another more specific one about just parsing the output in java.

Answer 1

Instead of printing whole lines (as with println or printing %n), write backspace characters (\b) or bare carriage returns without linefeeds (\r) to move the cursor back in front of what you want to print over.

Answer 2

Since you have tagged the question with bash, I am presuming that you are running linux/cygwin. Hence it's likely that the terminal is ANSI compatible. In that case, you could also use ANSI escape sequences...

e.g.:

echo -e "Hello world\rHi"
# Produces "Hillo world" since the line is not cleared before printing "Hi".
echo -e "Hello world\r\e[KHi"
#produces "Hi" (prints "Hello world", goes to 1st column, clears to last column, prints "Hi")
# For more details, refer above link.

If you need to run it in loop (which is the most obvious case), make sure that either

1. You don't put a newline after the message string, OR
2. You revert the vertical cursor movement (due to "\n") using ANSI escape sequence - "\e[A" (or "\e[<N>A", where N=number of lines.)

NOTE:

This code given above are bash commands. Sorry, I am not much familiar with java syntax, but to my knowledge,

1. echo would get translated to System.out.println
2. echo without last newline would be achievable with System.out.printf
3. & \e refers to <ESC> character (ascii = 27), can be achieved with \033 in java string.