UX issue with nix-shell: No clean way to preserve my $PS1

[drewm1980]

I have customized $PS1 with my personal notion of ideal usability, but nix-shell replaces it with one that is hard coded.

Is there some clean way to tell nix-shell which environment variables it should preserve? If not maybe nix-shell could benetif from flags like:

nix-shell --preserve-variable PS1 --ignore-variable FOO --do-what-you-want-with-variable BAR

I think there is a way to preserve my customized PS1 variable with a --command 'export PS1=...' flag, but I'm already putting all of these nix-shell commands into a bash alias in my .bashrc, and I don't want to invest the time needed to figure out the proper escaping, and I don't want to have to edit and compile recompile nix-shell either.

This is only my second day with nix, not counting a few previous failed attempts to get into it over the years, so maybe this is something that is trivial to fix if you've already invested the time to learn the nix language...

[ben0x539]

I think this isn't even intentional. But maybe alias nix-shell='nix-shell --command "$(declare -p PS1); return"' approximates a workaround.

[pjones]

I'm using PROMPT_COMMAND in my ~/.bashrc as a workaround:

function restore_prompt_after_nix_shell() {
  if [ "$PS1" != "$PROMPT" ]; then
    PS1=$PROMPT
    PROMPT_COMMAND=""
  fi
}

PROMPT_COMMAND=restore_prompt_after_nix_shell
PROMPT='$ '
export PS1=$PROMPT

[brprice]

Digging into this a little, I see that in Release 1.6 (2013-09-10):

nix/doc/manual/release-notes/rl-1.6.xml

Lines 35 to 37 in 6924bdf

	<para><command>nix-shell</command> now sets the shell prompt
	(<envar>PS1</envar>) to ensure that Nix shells are distinguishable
	from your regular shells.</para>

and in Release 2.0 (2018-02-22)

nix/doc/manual/release-notes/rl-2.0.xml

Lines 566 to 573 in 906afed

	<para><command>nix-shell</command> <link
	xlink:href="https://github.com/NixOS/nix/commit/ea59f39326c8e9dc42dfed4bcbf597fbce58797c">now</link>
	sets the <varname>IN_NIX_SHELL</varname> environment variable
	during evaluation and in the shell itself. This can be used to
	perform different actions depending on whether you’re in a Nix
	shell or in a regular build. Nixpkgs provides
	<varname>lib.inNixShell</varname> to check this variable during
	evaluation.</para>

The relevant code appears to be (from inspection, I haven't understood or tested this yet)

nix/src/nix-build/nix-build.cc

Line 449 in ea5bcfb

R"s([ -n "$PS1" ] && PS1='\n\[\033[1;32m\][nix-shell:\w]\$\[\033[0m\] '; )s"

Is there any appetite for changing this to not set PS1 now that there is another way to tell if you are in a nix-shell, or are we more concerned with backward compatability?
(I'm happy to (try to) make a patch if so!)
If we choose to not set PS1, we could potentially give a /etc/bashrc that gives different default prompts depending on if $IN_NIX_SHELL is set.