That is actually four questions:
- How do I know their code points?
- Some sort of regex?
- How do I count how many character cells a string takes?
- How do I erase everything that was outputted?
OP mentions xterm, but only the last two are possibly specific to xterm.
For (1) and (2), the echo command is not much help. You are better off using
printf, which recognizes backslash escapes. In some implementations (e.g., GNU coreutils), that includes Unicode constants, e.g.,
although for regular expressions, you are yet again better off using a scripting language such as Perl (which can handle UTF-8).
Questions (3) and (4) are more interesting. First, the script cannot really tell how many character cells a string takes in advance, but can only measure it after the fact. That is because the width is based on a combination of behavior of the terminal and the kernel.
- xterm uses wcwidth to decide how wide the character should be, with some issues about "wide" (double-width) fonts, and implementations of wcwidth reflecting developer's biases regarding the ambiguous-width Unicode values. xterm can be configured (at runtime) to use its copy of Markus Kuhn's implementation of
wcwidth; you are cautioned that it may not be complete nor match the actual system locale information.
- when xterm is told to erase part of a double-width character (as in the given example), it replaces the other part with a space. Most other terminals imitating xterm do this (though in a quick check, I noticed one simply moving the cursor, causing the wide character and ASCII text to overlap). If you knew that the value was double-width, you could simply adjust your notion of where the cursor was.
- the Linux kernel does not know anything about
wcwidth. Linux-based systems since 2004 have a feature in
which tells the kernel
that input is encoded in UTF-8, for proper editing support in canonical
- the Linux kernel feature is useful for editing input, because it helps the terminal driver do something reasonable when a backspace deletes the previous character. However, there is no comparable feature for output.
You can, as suggested, use the cursor-position report (an escape sequence) to find the position of the cursor at different points. But if you are going to use that to decide how to clear the line, it might seem more direct to just move to the position before printing the Unicode value, and start clearing from that point.
Alternatively, you could tell the terminal to save the cursor position before printing the wide character, and restore it (moving back). That might seem cleaner and more predictable. After restoring the cursor position, you could clear the line. All three could be done using escape sequences — or
Other than as a demonstration, this does have the drawback that the terminal would have only one saved-position for the cursor, and that to ensure clearing a single/double-width character, it is clearing an entire line. But the cursor would end up in the "right" place.