Escape non-printing characters in a function for a Bash prompt

Question

In a Bash Prompt (PS1 variable), I'm calling a function to potentially add text to the prompt: export PS1="\u@\h \$(my_function) \$ "

However, the function in the prompt contains ANSI color codes that change based on the output of the function (sometimes red, sometimes green). Adding "\[" to the PS1 variable should escape those codes as non-printing, but if I do an echo in the function, the "\[" get printed literally in the prompt.

How can I escape these ANSI color codes from within a function for use in a bash prompt?

Answer 1

The readline library accepts \001 and \002 (ASCII SOH and STX) as non-printable text delimiters. These also work in any application that uses readline.

From lib/readline/display.c:243 in bash source code:

243 /* Current implementation:
244         \001 (^A) start non-visible characters
245         \002 (^B) end non-visible characters
246    all characters except \001 and \002 (following a \001) are copied to
247    the returned string; all characters except those between \001 and
248    \002 are assumed to be `visible'. */

The bash-specific \[ and \] are in fact translated to \001 and \002 at y.tab.c:7640.

Answer 2

If you want to use them in the prompt, then you do need to do the \[. But if you want to use it in an echo, you have to use \033[.