I am building a pdf_book book using bookdown, and successfully (on Mac and Windows).

But I want to do some post-processing on the tex file after it is produced, but before it is compiled by xelatex.

I had ideas for a method, but could not get things to work. (I asked here for help but got no replies… maybe because this new approach is the preferred way, if I could get it to work.)

But then I stumbled across adding a post_processor option into my _output.yml file; it seemed to be just was I was after!

But I cannot get it to work (neither Mac nor Windows). By this, I mean that the build goes OK from start to finish, but whatever I try to do in the post_processing step just does not happen.

I’m not even sure where to find info about doing this; a web-search hasn’t shown much at all (I’ve been getting help from ChatGPT :->). So if anyone knows of where this is explained, that’d be great.

So I have this in my _output.yml file:

bookdown::pdf_book:
  latex_engine: xelatex
  post_processor: !expr source('./post_processor.R')$post_processor

And the file post_processor.R never seems to run; for example, even just print statements don’t appear; for the script below, for instance, the log file is never created.

However, the same function does work OK if I run it from the R console:

post_processor(NULL, "./_book/_main.tex", "output.pdf", clean = TRUE, verbose = TRUE)

I know ChatGPT may be giving my dodgy info, but I can’t find much to guide me.

So: What must I do to get the post_processor.R file to run when called in _output.yml?

Any advice greatly appreciated.

P.

File post_processor.R:

post_processor <- function(metadata, input_file, output_file, clean, verbose) {
  log_file <- "post_processor_log.txt"
  
  # Function to log messages by appending them to the log file
  log_message <- function(message) {
    log_conn <- file(log_file, open = "a")  # Open the log file in append mode
    writeLines(message, log_conn)           # Write the message to the log
    close(log_conn)                         # Close the connection
  }
  
  # Start logging
  log_message("Log started and so it should haven")
  
  # Check if the input .tex file exists
  if (file.exists(input_file)) {
    log_message(paste("Input file exists:", input_file, "n"))
    
    # Read the .tex file
    tex <- readLines(input_file)
      
    # Write the modified .tex content back to the input file
    writeLines(tex, input_file)
  } else {
    log_message("Input file does not exist!n")
  }
  
  # Final log message
  log_message("Post-processing complete.n")
  
  # Return the output file
  return(output_file)
}