Shelling Out From Latex

2016-10-30*
programming, tex

The Problem

For years, whenever I wanted to interact with the outer shell environment from Latex, I would use one of two methods:

  1. perform some shell step separately from Latex and inject it into the Latex sources, or
  2. save it somewhere else and pull it in from Latex with \input{foo}.

But I learned recently that it can be done much more simply, if you are willing to use Luatex!

Enter Luatex

Let’s assume that you keep your Latex document in source control and want to inject the Git hash into the document. First, define a new command called \shell.

% Call shell! See http://tex.stackexchange.com/a/114939/30920.
\newcommand\shell[1]{\directlua{
    local handle, err = io.popen([[#1]])
    if not handle then
        tex.print(err)
        os.exit(1)
    end
    local result = handle:read("*a")
    handle:close()
    tex.print(result)
}}

Then use it like this:

\shell{TZ='America/Los_Angeles' date}

\shell{git describe --always}%
\shell{(( $(git --no-pager diff 2>/dev/null | wc -l) + $(git --no-pager diff --cached 2>/dev/null | wc -l) > 0 )) && echo '*'}

. I then use lualatex --shell-escape foo.tex to compile it. This is actual code from a Luatex document of mine.

I am not sure which shell program gets invoked, but for most things it should not matter much.

Conclusion

Now you know how to shell out from Latex!

Happy hacking!