I'd like to use both multiple file paths in the input argument to render_book() (in order to supply multiple dependencies for drake::knitr_in() in my drake plan), as well as specify output_format = "all". However, just doing the former gives me warnings that suggest that only the first file path is being used, and supplying both gives me an error. I struggled making a minimal reprex, so instead cloned the bookdown-demo repo and executed the below. I also found an rmarkdown issue that presents the same kind of error (rstudio/rmarkdown#1722), but I couldn't easily work out how it could apply here.

Works

bookdown::render_book('index.Rmd')

bookdown::render_book('index.Rmd', output_format = "all")

Produces warnings

bookdown::render_book(c('index.Rmd', '01-intro.Rmd'))

Warning messages:
1: In if ((n1 <- nchar(p)) == 0) return(x) :
  the condition has length > 1 and only the first element will be used
2: In if (is_sub_path(p, d, n2)) { :
  the condition has length > 1 and only the first element will be used
3: In if (p2 == "") p2 = "." :
  the condition has length > 1 and only the first element will be used

Produces error

bookdown::render_book(c('index.Rmd', '01-intro.Rmd'), output_format = "all")

Error in base::file(file, encoding = "UTF-8") : 
  invalid 'description' argument

Session info

R version 4.0.2 (2020-06-22)
Platform: x86_64-apple-darwin17.0 (64-bit)
Running under: macOS Mojave 10.14.6

Locale: en_AU.UTF-8 / en_AU.UTF-8 / en_AU.UTF-8 / C / en_AU.UTF-8 / en_AU.UTF-8

Package version:
  base64enc_0.1.3   bookdown_0.21.6   digest_0.6.27     evaluate_0.14    
  glue_1.4.2        graphics_4.0.2    grDevices_4.0.2   highr_0.8        
  htmltools_0.5.1.1 jsonlite_1.7.2    knitr_1.31        magrittr_2.0.1   
  markdown_1.1      methods_4.0.2     mime_0.10         rlang_0.4.10     
  rmarkdown_2.7.2   stats_4.0.2       stringi_1.5.3     stringr_1.4.0    
  tinytex_0.29      tools_4.0.2       utils_4.0.2       xfun_0.21        
  yaml_2.2.1       

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