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Terminal texteditors like emacs,vim,joe or even nano have the ability to display arbitrary UI elements inside a command line without completely rewriting the whole UI every single time, but overwriting what currently is their UI.

With regular output streams, you can only use the return character '\r' to jump to the beginning of the current line of output and write over it, but from what I found you can not jump up multiple lines, cout << "\r\r\r\r\r\r"; has the same effect as cout << '\r', so everything followed by a newline is apparently cast in stone.

Other applications do something similar, for example $dpkg-reconfigure ca-certificates on Ubuntu or the aptitude graphical package manager. They also draw outlines for UI elements, which are probably just special characters. But still, they'd have to overwrite multiple lines of console output.

How do they do that? Is the behaviour portable to Windows platforms?

iFreilicht
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  • It is likely that the implementation is different for each terminal emulator. – Anderson Green Oct 06 '14 at 01:45
  • @AndersonGreen From what I've read, there seem to be ANSI escape sequences that allow to set the cursor on the terminal to an arbitrary position. Curses or Ncurses wraps these sequences into a usable interface so you can write more portable code. – iFreilicht Oct 06 '14 at 10:31

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You'll find that these programs depend on a library called ncurses:

http://en.wikipedia.org/wiki/Ncurses

There are builds available for almost all operating systems.

Richard Hodges
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    That was the keyword I needed. A good question on SO regarding this: http://stackoverflow.com/questions/7558348/how-do-ncurses-et-al-work – iFreilicht Oct 05 '14 at 15:30
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curses is a unix library that lets you manipulate the contents of a terminal at arbitrary positions. ncurses is a free, vendor-independent version of curses, and the curses library used on linux.

dpkg-reconfigure uses dialog that builds on top of curses and provides user interface widgets instead of raw terminal access.

ncurses includes the extensions "menu", "forms", "panel" that you could use to implement user interface widgets yourself, though it gets very complex very quickly if you go that route.

Regarding windows, there is pdcurses that runs natively in the "dos box" terminal. You can write portable programs that use pdcurses on windows and ncurses on linux if you restrict yourself to the common subset. Alternatively, you can use ncurses on windows using cygwin.

Another alternative for windows might be Borlands TVision - if you can find an old version of Borland's turbo pascal or c++ compiler, they included a complete application framework for text user interfaces called TVision. Not sure if the code generated by these would still run on modern windows versions, though.

Ludwig Schulze
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The console follows a specification: https://www.xfree86.org/current/ctlseqs.html

This is similar to a client/server. If you application (the client) follows the same specification, it can communicate with the console via the standard input/output to draw anything its wants.

There are many things in the specification:

  • Clear a specific line.
  • Move the cursor
  • Set background/foreground color.
  • Set style: bold, dim, underlined, blink, ...
  • Request sending mouse events, receive mouse events.
  • Configure the console to nowrap/wrap lines.
  • Configure the console to use the "alternate" screen.
  • etc..
ArthurS
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