Switch based on whether or not an environment variable is set

Question

TLDR: Problem: kpsewhich outputs \par for unset environment variables and I do not know how to strip it.

I was inspired from the question Can I access system/environment variables from LaTeX? For instance, $HOME after finding out that kpsewhich works without --shell-escape and am trying to create a switch based on whether or not a given environment variable is set. Then I can check if e.g. DARK is set and swap text and background color.

However the problem emerged that even for unset environment variables kpsewhich apparently outputs a newline which leads to the captured output not being empty. I already tried to strip it using \tl_trim_spaces:N (as is already the case in the linked answer) but to no avail.

Is it somehow possible to either (a) instruct kpsewhich not to print a newline for unset environment variables, (b) read the return code from sys_get_shell:nnN as kpsewhich fails with a non-zero exit code if the variable is not set or (c) strip the resulting \par from the variable?

Credit goes to @Phelype Oleinik, check out his anwer for an explanation and also check out @egreg's answer for a solution without changing \endlinechar
I could now implement what I wanted using the following code:

\documentclass{article}
\usepackage{fontspec}
\usepackage{xparse}
\usepackage{xcolor}
\ExplSyntaxOn
\NewDocumentCommand{\ifenvset}{m m}
{
  \sys_get_shell:nnN { kpsewhich ~ --var-value ~ #1 }
    { \int_set:Nn \tex_endlinechar:D { -1 } }
    \l_tmpa_tl
  \tl_if_empty:NTF { \l_tmpa_tl } {} { #2 }
}
\ExplSyntaxOff
\ifenvset{DARK}{
  \pagecolor{black!90}
  \color{white!90}
}
\begin{document}
\noindent
\verb|lualatex darkmode.tex|\
\verb|env DARK=1 lualatex darkmode.tex|\
\end{document}

Answer 1

When you execute a shell command, TeX sees it as if you had written the output of the command to a temporary (pseudo) file, and then read that file in. While reading that file, the usual rules apply: a line end is turned into a space, and an empty line is turned into a \par token. When the output of the command is empty, it is as if the file had an empty line, and that is turned into a \par. The usual trick is to set the \endlinechar parameter to -1, so that TeX doesn't insert anything at the end of a line, neither a \par token.

The second argument in \sys_get_shell:nnN is the <setup>, in which you can change these types of parameters before the command is executed and the pseudo file is read in, so you can do:

\sys_get_shell:nnN { kpsewhich ~ --var-value ~ #2 }
  { \int_set:Nn \tex_endlinechar:D { -1 } }
  \l_septatrix_env_tl

Putting that in egreg's \getenv code:

\documentclass{article}
\usepackage{xparse}
\ExplSyntaxOn
\tl_new:N \l_septatrix_env_tl
\NewDocumentCommand \getenv { o m }
  {
    \sys_get_shell:nnN { kpsewhich ~ --var-value ~ #2 }
      { \int_set:Nn \tex_endlinechar:D { -1 } }
      \l_septatrix_env_tl
    \IfNoValueTF {#1}
      { \tl_use:N \l_septatrix_env_tl }
      { \tl_set_eq:NN #1 \l_septatrix_env_tl }
  }
\ExplSyntaxOff
\begin{document}
\getenv[\HOME]{HOME}\show\HOME
\getenv[\HOMER]{HOMER}\show\HOMER
\end{document}

Answer 2

You can define \ifenvsetTF (this is a better name, in my opinion} by

\tl_const:Nn \c_getenv_par_tl { \par }
\NewDocumentCommand{\ifenvsetTF}{mmm}
 {
  \sys_get_shell:nnN { kpsewhich ~ --var-value ~ #1 } { } \l_tmpa_tl
  \tl_if_eq:NNTF \l_tmpa_tl \c_getenv_par_tl { #3 } { #2 }
 }

Full example:

\documentclass{article}
\usepackage{xparse}
\ExplSyntaxOn
\NewDocumentCommand{\getenv}{om}
 {
  \sys_get_shell:nnN { kpsewhich ~ --var-value ~ #2 } { } \l_tmpa_tl
  \tl_trim_spaces:N \l_tmpa_tl
  \IfNoValueTF { #1 }
   {
    \tl_use:N \l_tmpa_tl
   }
   {
    \tl_set_eq:NN #1 \l_tmpa_tl
   }
 }
\tl_const:Nn \c_getenv_par_tl { \par }
\NewDocumentCommand{\ifenvsetTF}{mmm}
 {
  \sys_get_shell:nnN { kpsewhich ~ --var-value ~ #1 } { } \l_tmpa_tl
  \tl_if_eq:NNTF \l_tmpa_tl \c_getenv_par_tl { #3 } { #2 }
 }
\ExplSyntaxOff
\begin{document}
\getenv{LC_CTYPE}
\ifenvsetTF{LC_CTYPE}{SET}{UNSET}
\ifenvsetTF{BOZO}{SET}{UNSET}
\end{document}

However, note that variables set to empty value will trigger “unset” anyway (with this method and also with the \endlinechar method).

How to define an expl3 conditional?

\prg_new_protected_conditional:Nnn \septatrix_if_setenv:n {T,F,TF}
 {
  \sys_get_shell:nnN { kpsewhich ~ --var-value ~ #1 } { } \l_tmpa_tl
  \tl_if_eq:NNTF \l_tmpa_tl \c_getenv_par_tl
   { \prg_return_false: }
   { \prg_return_true: }
 }

Full code

\documentclass{article}
\usepackage{xparse}
\ExplSyntaxOn
\NewDocumentCommand{\getenv}{om}
 {
  \sys_get_shell:nnN { kpsewhich ~ --var-value ~ #2 } { } \l_tmpa_tl
  \tl_trim_spaces:N \l_tmpa_tl
  \IfNoValueTF { #1 }
   {
    \tl_use:N \l_tmpa_tl
   }
   {
    \tl_set_eq:NN #1 \l_tmpa_tl
   }
 }
\tl_const:Nn \c_getenv_par_tl { \par }
\prg_new_protected_conditional:Nnn \septatrix_if_setenv:n {T,F,TF}
 {
  \sys_get_shell:nnN { kpsewhich ~ --var-value ~ #1 } { } \l_tmpa_tl
  \tl_if_eq:NNTF \l_tmpa_tl \c_getenv_par_tl
   { \prg_return_false: }
   { \prg_return_true: }
 }
\NewDocumentCommand{\ifenvsetTF}{mmm}
 {
  \septatrix_if_setenv:nTF { #1 } { #2 } { #3 }
 }
\ExplSyntaxOff
\begin{document}
\getenv{LC_CTYPE}
\ifenvsetTF{LC_CTYPE}{SET}{UNSET}
\ifenvsetTF{BOZO}{SET}{UNSET}
\end{document}