Running a system command (pdflatex) within my Rails Controller - ruby-on-rails

Question

So I have a Rails application that upon user submission should generate some kind of a .tex file based on the user input, compile it into a pdf, and deliver the pdf. Through process of elimination, I am pretty positive that everything is working except for one line; the one where pdflatex is called.
Here's the vital snippet of code:
(If it matters, its located in the Questions controller under the generate action, which is called after the form sends the relevant information. Though this may not be the best way, I'm pretty certain its not the cause of the error)
texHeader = 'app\assets\tex\QuestionsFront.txt'
texOut = 'app\assets\tex\Questions.tex'
#copy latex header to new file
FileUtils.cp(texHeader, texOut)
File.open(texOut, 'a+') do |fout|
fout.write("\n")
# a loop writes some more code to fout (its quite lengthy)
fout.write("\\end{enumerate}\n")
fout.write("\\end{document}")
#The problem line:
puts `pdflatex app/assets/tex/Questions.tex --output-directory=app/assets/tex`
end
filename = 'Questions.pdf'
filelocation = "app\\assets\\tex\\" + filename
File.open(filelocation, 'r') do |file|
send_file file, :filename => filename, :type => "application/pdf", :disposition => "attachment"
end
end
Here's my reasoning: It generates the .tex file correctly, and given a pre-created Questions.pdf file it sends it just fine. When the command in the puts is copied to the terminal, it runs without a hitch (the file begins with \nonstopmode so no worries about small errors). But for some reason, when I run the above script, not even a log file is created with an error.
What am I overlooking? Any ideas? Any way of seeing what output the puts line is creating?
Thanks so much in advance!

Answer 1

Figured out my own problem. The error is pretty interesting. You'll see I'm calling
puts `pdflatex app/assets/tex/Questions.tex --output-directory=app/assets/tex`
within the block
File.open(texOut, 'a+') do |fout|
where just a few lines prior
texOut = 'app\assets\tex\Questions.tex'
Basically, I'm trying to get latex to compile a document while the file is still open. So long as I'm in the File.open block, the file is open, and its automatically closed upon the end of the block.
Cutting and pasting the line of code down below the end of the block made it work just like I wanted. However, for the sake of clarity and the rare case where someone else has this problem, its actually better to open a separate system shell, navigate to the directory where the latex document is and do the compiling there. So, my updated code looks like:
fout.write("\\end{document}")
end
system 'runlatex.bat'
where that batch file is as follows:
cd app/assets/tex
pdflatex Questions.tex
That way any additional files in the tex directory are found, the log file is created there, etc.
Reason why I never got a log file? The pdflatex never executed - the OS stopped it with a permissions error before it ever ran.
Hope this helps!

Answer 2

Backticks (and %x{}) provide the same parsing context as a double quoted string. That means that the usual backslashed escape sequences are interpreted inside backticks; in particular, \t is a tab so this:
puts `pdflatex app\assets\tex\Questions.tex --output-directory=app\assets\tex`
will end up with two tabs in and that breaks everything. You can start escaping your backslashes (I think you'll need two or three backslashes to get one down to the shell) or switch to normal slashes (which Windows usually accepts in paths):
puts `pdflatex app/assets/tex/Questions.tex --output-directory=app/assets/tex`
Alternatively, you could switch to open3 to avoid the escaping and quoting issues and also get better error handling capabilities.