usebox macro not expanded?

Question

I'm trying to store the text output of a shell command mktemp and reuse it later in another latex command. I'm attempting to do this using \savebox and \usebox as below. When using \usebox outside a command, the saved text is printed correctly. However, when used inside other commands like \write18 and \includegraphics, the \usebox command appears to not be macroexpanded correctly:

\newcommand{\mktemp}[0]{
  \input{|"mktemp"}
}

\newcommand{\hrefqr}[2]{%
  \tracingall
  \newsavebox{\tmp}
  \savebox{\tmp}{\mktemp}
  %% verify that \tmp is the same file and not repeated mktemp calls
  \usebox{\tmp} \\
  \usebox{\tmp} \\
  \immediate\write18{qrencode #1 -o \usebox{\tmp}}
  \href{#1}{#2 \includegraphics[]{\usebox{\tmp}}}
}
\begin{document}
\hrefqr{www.example.com}{This is example.com}
\end{document}

From the logs:

\write->qrencode www.example.com -o \usebox {\tmp }

\usebox ->\protect \usebox  

\usebox  #1->\leavevmode \copy #1\relax 
#1<-\tmp 

\leavevmode ->\unhbox \voidb@x 
runsystem(qrencode www.example.com -o \protect \unhbox \voidb@x \copy \tmp \rel
ax )...executed safely (allowed).

And later:

! LaTeX Error: File `\usebox {\tmp }' not found.

from the expansion of:

#1<-This is example.com \includegraphics []{\usebox {\tmp }}

What am I doing wrong?

Answer 1

You can't store text in a box and retrieve it by macro expansion. When you do \savebox{\tmp}{some text} the argument is stored in the box register as already typeset.

Here's a working implementation. I create a directory, rather than a file, and in it I create temp.png, otherwise \includegraphics wouldn't be able to determine its size. See https://stackoverflow.com/a/59638023.

If your version of mktemp supports -suffix, you can use it, see later for details.

\documentclass{article}
\usepackage{catchfile}
\usepackage{graphicx}
\usepackage{hyperref}

\ExplSyntaxOn

\NewDocumentCommand{\hrefqr}{mO{}m}
 {
  % get the name of a temporary file
  \sys_get_shell:nnN { mktemp ~ -d } { } \l_tmpa_tl
  \tl_trim_spaces:N \l_tmpa_tl
  % build the QRcode
  \sys_shell_now:x { qrencode ~ #1 ~ -o ~ \l_tmpa_tl/temp.png }
  %
  \href { #1 } { #3 ~ \includegraphics[#2]{ \l_tmpa_tl/temp.png } }
 }

\ExplSyntaxOff

\begin{document}

\hrefqr{www.example.com}{This is example.com}

\hrefqr{www.example.com}[scale=0.5]{This is example.com}

\end{document}

On my system, mktemp doesn't support --suffix, but gmktemp (installed with Homebrew as part of coreutils) does.

\documentclass{article}
\usepackage{catchfile}
\usepackage{graphicx}
\usepackage{hyperref}

\ExplSyntaxOn

\NewDocumentCommand{\hrefqr}{mO{}m}
 {
  % get the name of a temporary file
  \sys_get_shell:nnN { gmktemp ~ --suffix ~.png } { } \l_tmpa_tl
  \tl_trim_spaces:N \l_tmpa_tl
  % build the QRcode
  \sys_shell_now:x { qrencode ~ #1 ~ -o ~ \l_tmpa_tl }
  %
  \href { #1 } { #3 ~ \includegraphics[#2]{ \l_tmpa_tl } }
 }

\ExplSyntaxOff

\begin{document}

\hrefqr{www.example.com}{This is example.com}

\hrefqr{www.example.com}[scale=0.5]{This is example.com}

\end{document}