How to animate the command line?

Question

I have always wondered how people update a previous line in a command line. a great example of this is when using the wget command in linux. It creates an ASCII loading bar of sorts that looks like this:

[======>                    ] 37%

and of course the loading bar moves and the percent changes, But it doesn't make a new line. I cannot figure out how to do this. Can someone point me in the right direction?

Answer 1

One way to do this is to repeatedly update the line of text with the current progress. For example:

def status(percent):     sys.stdout.write("%3d%%\r" % percent)     sys.stdout.flush() 

Note that I used sys.stdout.write instead of print (this is Python) because print automatically prints "\r\n" (carriage-return new-line) at the end of each line. I just want the carriage-return which returns the cursor to the start of the line. Also, the flush() is necessary because by default, sys.stdout only flushes its output after a newline (or after its buffer gets full).

Answer 2

There are two ways I know of to do this:

• Use the backspace escape character ('\b') to erase your line
• Use the curses package, if your programming language of choice has bindings for it.

And a Google revealed ANSI Escape Codes, which appear to be a good way. For reference, here is a function in C++ to do this:

void DrawProgressBar(int len, double percent) {   cout << "\x1B[2K"; // Erase the entire current line.   cout << "\x1B[0E"; // Move to the beginning of the current line.   string progress;   for (int i = 0; i < len; ++i) {     if (i < static_cast<int>(len * percent)) {       progress += "=";     } else {       progress += " ";     }   }   cout << "[" << progress << "] " << (static_cast<int>(100 * percent)) << "%";   flush(cout); // Required. }