Does tex know its own command line arguments?

Question

Is it possible to test within the document what commandline arguments were passed to tex? The particular case that prompts this is the question Tabs in output file written by xelatex and pdflatex are different where it would be useful to know if the -8bit argument has been passed to tex and issue a warning if not. That is, if pgfplots could contain the code:

\ifxetex
 \ifarg{8bit}
 \else
  \pkgwarning{You're using xe(la)tex without the -8bit argument.  Some external files might go wrong.  If things don't work as you expect, try using the -8bit argument.}
 \fi
\fi

then it might make it easier to spot when the situation of the linked question occurs.

(A year after asking the above question, it happened to me again and it took me some time to realise what the problem was and find the solution, even though it had been me who found it last time!)

Answer 1

I'm afraid the answer is "not directly".

One can define such a test:

\input catchfile.sty
\newif\ifeightbit
\begingroup
  \immediate\openout15=tab.tst
  \catcode`\^^I=12
  \immediate\write15{^^I}
  \immediate\closeout15
  \CatchFileDef\atab{tab.tst}{\catcode`\^=12 \catcode`I=12 }
  \edef\btab{\string^\string^\string I }
  \ifx\atab\btab
    \global\eightbitfalse
  \else
    \global\eightbittrue
  \fi
\endgroup
\show\ifeightbit

With xetex test (test.tex is the file name) we'll get \iffalse while with xetex -8bit test we'll get \iftrue.

It seems to be impossible to do this without writing a file, because \scantokens avoids the "translation" via the xchr tables. So while we can catch the three distinct characters when reading back the file, if written without the -8bit option, we can't do it with \scantokens.

Answer 2

With --shell-escape:

(because of some permission issue this does not work on Overleaf, but otherwise should work on most POSIX systems)

\ExplSyntaxOn
\sys_get_shell:nnN {ps~-o~args=~-p~$(ps~-o~ppid=~-p~$$)} {\cctab_select:N \c_other_cctab} \l_data_tl
\ExplSyntaxOff

Quoted from my other answer in https://tex.stackexchange.com/a/621510/250119.

Then you need to parse the arguments somehow -- but be warned that parsing the arguments correctly is really difficult. A simple way is

\str_if_in:VnTF \l_data_tl {-8} {
  \typeout{>> probably~8bit}
} {
  \typeout{>> not~8bit}
}

but it's not foolproof, I run this simple bash file:

g(){
    echo "========" "$@"
    "$@" </dev/null | grep '>>\|!'
}
f(){
    g pdflatex --shell-escape "$@"
    g xelatex --shell-escape "$@"
}

mkdir -p -- --8bit a "a a"  # create auxiliary directories named literally "--8bit" and "a" and "a a"
ln -rsf a.tex "a --8bit b.tex"  # link "a.tex" -> a file literally named "a --8bit b.tex"
ln -rsf a.tex "b.tex"

# ======== these 3 cases return false:
f  a.tex
f  --output-di a "a --8bit b.tex"
f  --output-di --8bit a.tex
# ======== these 7 cases return true:
f  --output-di "a a" --8bit b.tex
f  -8bit a.tex
f  -8b a.tex
f  --8bit a.tex
f  --8bi a.tex
f  --8b a.tex
f  --8 a.tex
f  -8 a.tex

with a.tex contains:

% ======== copy from egreg's test ========
\input catchfile.sty
\newif\ifeightbit
\begingroup
  \immediate\openout15=tab.tst
  \catcode`\^^?=12
  \immediate\write15{^^?}
  \immediate\closeout15
  \CatchFileDef\atab{tab.tst}{\catcode`\^=12 \catcode`?=12 }
  \edef\btab{\string^\string^\string ? }
  \ifx\atab\btab
    \global\eightbitfalse
  \else
    \global\eightbittrue
  \fi
\endgroup




\ExplSyntaxOn
\cs_generate_variant:Nn \str_if_in:nnTF {V}
\sys_get_shell:nnN {ps~-o~args=~-p~$(ps~-o~ppid=~-p~$$)} {\cctab_select:N \c_other_cctab} \l_data_tl
\typeout{>> \l_data_tl}

\str_if_in:VnTF \l_data_tl {-8} {
  \typeout{>> probably~8bit}
} {
  \typeout{>> not~8bit}
}

\typeout{>> ifeightbit=\meaning\ifeightbit}
\ExplSyntaxOff

\stop

As you can see, the simple test outputs "probably 8bit" for the last 2 cases of the first block even though it isn't 8bit.

In particular, the 2nd and 4nd case makes the output identical with the provided command-line argument (you can fix this by not using ps -o args but I'm not sure how...)

---

With LuaLaTeX there's the arg table, but it looks like you'll still have to manually parse it:

\directlua{for _, v in ipairs(arg) do print(v) end}
\stop

From the other question in In Luatex, how to know effective output directory?.

But... in this specific case LuaTeX always ignore --8bit argument anyway: