Read a file and store its contents in a variable

I'm not sure what the right way to do this with the LaTeX macros like \IfFileExists, but using the low-level TeX primitives this is easy to do.

\documentclass{article}
\newread\makerfile
\begin{document}
\openin\makerfile=../DecMaker.txt
\ifeof\makerfile
    Go and buy a computer from ThisRandomBrand!!
\else
    \read\makerfile to\makerline
    \closein\makerfile
    \edef\product{%
        \ifcase\makerline
            Windows%
        \or Linux%
        \or Mac%
        \fi
    }
    \product\ computer bought from ThisRandomBrand is amazing!
\fi
\end{document}

It tries to open ../DecMaker.txt and if that fails, it outputs the "Go and buy..." line. Otherwise, it defines \product to Windows, Linux, or Mac and outputs the "Linux computer..." line (for example).

I don't know (and cannot easily test) what happens if this file is actually run on a Windows machine. In particular, I don't know if ../DecMaker.txt is treated as the path you want.

If DecMaker.txt were moved to the same directory, then this should work correctly everywhere.

Run with lualatex

\documentclass{article}
\usepackage{luacode}
\begin{luacode}
local x = {"Windows","Linux","Darwin"}

function FileTest(filename)
  local f=assert(io.open(filename, "r"))
  local t=f:read()
  f:close()
  if t then
    return x[tonumber(t)].." computer bought from ThisRandomBrand is amazing!"
  else
    return "Go and buy a computer from ThisRandomBrand!!"
  end
end
\end{luacode}

\newcommand\Test[1]{\directlua{tex.print(FileTest("#1"))}}

\begin{document}
  \Test{../DecMaker.txt}
\end{document}

You can use catchfile. It's also better to separate the reading of the auxiliary file from the definition part, so you can easily reuse the code for the OS. The macro \newOScommand has five arguments: the name of the command to define and the four branches as shown in the example.

\usepackage{catchfile}

\IfFileExists{../DecMaker.txt}
 {\CatchFileEdef{\OSnumber}{../DecMaker.txt}{\endlinechar=-1 }}
 {\def\OSnumber{-1}}

\newcommand{\newOScommand}[5]{%
  \ifcase\OSnumber\relax
    \newcommand{#1}{#2}\or
    \newcommand{#1}{#3}\or
    \newcommand{#1}{#4}\else
    \newcommand{#1}{#5}%
\fi

\newOScommand{\product}{Windows}{Linux}{Mac}{No computer}