I’ve written shell scripts using escape codes to move the cursor and change the color. There are however escape codes that return a response.
I’m struggling to figure out how one reads the responses to control codes that query the terminal itself, particularly within a shell script? Codes like <ESC>[6n which is "Query Cursor Position"
It doesn’t seem to be standard out or standard error as far as I can tell. I am confused.
In ZSH I do the following
~ » echo "\e[6n"
~ » 3;1R
the response to the query comes through as the next Terminal command, already typed in for me. I don’t understand how. I also am unclear why bash doesn’t seem to demonstrate this behavior.
What do I read this value from?
The responses come through on the channel from the terminal (or terminal emulator) to the serial port (or other tty device). That’s the same channel used for transmitting characters entered at the terminal keyboard; there is no out-of-band signaling.
Since you didn’t read the response after sending the query, it was interpreted as a series of keypresses by your shell. The different shells have different responses to the unusual keyboard input.
To read the response properly, you have to take the terminal out of line-based (“icanon” or “cooked”) mode and read a byte at a time (from the tty, i.e. possibly stdin, the same place you’d read keyboard input from) until the terminating character is found. And there’s no real way to distinguish the response from any real keypresses that happened to occur at the same time.
It’s an unclean business, and if you’re trying to do it in a shell script you add extra pain.