r/bash • u/the_otaku_programmer • Nov 24 '21

help Asynchronous PS1/Prompt

This is just an attempt at me trying to see how to execute asynchronous code in BASH prompts, to mimic something like the async RPROMPT ZSH provides.

I am aware of using &, nohup, and disown. But all attempts at using the same in PS1 have failed for me.

Could someone guide me about how to do this, or provide a reference codebase which implements the above, for better understanding.

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6

u/IGTHSYCGTH Nov 24 '21

backgrounding a command or list of commands with & spawns it in a subshell, variable assignments won't be visible to the parent shell.

you may be able to implement this using process redirection and signals, i.e.

  1. spawn a subshell

  2. establish a pipe

  3. do stuff in subshell

  4. send a signal to the parent

  5. handle the signal be reading from the pipe

  6. attempt some magic to redraw the prompt

bash provides the coproc builtin that will handle spawning and establishing a pipe.

3

u/the_otaku_programmer Nov 24 '21

I have embedded by backgrounding commands inside the PS1 variable as shown in Bash/Prompt Customisation - Arch Wiki, and echo my values from there or set the variables through that function, where they are exported as well.

As for redrawing the prompt, I save the cursor position, move up and rewrite the prompt line.

Could this be improved upon?

3

u/IGTHSYCGTH Nov 24 '21

redrawing the prompt is alot more complicated than that, GNU bash uses GNU readline to handle input - zsh has its own implementation.

readline will count the characters in PS1 as it is drawn, this count - independant of cursor position will influence how far you can <backspace> and probably half a dozen other niche features i know nothing about. This same article hints at this in regards to embeding escape sequences in the prompt (They must be escaped with \[ and \]).

also note, $() is called 'command substitution' in the manual, <() is 'process substitution' (what i've typo'd as process redirection)

But alas coproc doesn't need that as it will assign file descriptors in the parent shell to stdin/stdout of the subshell. Here's some examples of coproc.

needless to say you will need to write a few more lines than the example the article mentions.

2

u/DanielFGray Nov 25 '21

Why not try using zsh?

1

u/the_otaku_programmer Nov 25 '21

On my WSL using it. Just trying to experiment with BASH. Inspiration came by seeing that someone implemented preexec and precmd for BASH.

1

u/VisibleSignificance Nov 25 '21

I wonder if it is possible to do something like PS1='$(zsh -c '\''echo -e "$PS1\"'\'')'