Using Shiny fileInput to get path only

Question

I have a very large fixed width file I need to read in using my Shiny application. The way my program is currently structured is the ui.R contains a fileInput allowing the user to locate the file using the browser.

On the server side, I only capture the path to the file such as the following:

path2file <- reactive({
    infile <- input$path2file
    if (is.null(infile)) return(NULL)    
    infile$datapath  
}) 

A subsequent function takes that path as input and the proceeds to read in the file according to its layout specifications. This all works just fine; however when dealing with extremely large fwf files my program slows down tremendously and takes hours to get the path name of the file read in using fileInput

My suspicion is that fileInput is actually reading in the entire file and then my function only returns the datapath even though I am not explicitly reading in any file format type within the function.

My aim is to continue using the program as I have it structured now and obtain only the path to this file using my fileInput. I have found this topic on SO, and see it is a possible option.

Getting file path from Shiny UI (Not just directory) using browse button without uploading the file

However, I also aim to minimize the number of package dependencies I have; this has become a big issue and so if I MUST use an additional package I will, but I'd like to avoid that at all costs.

I experimented with this cheap trick:

path2file <- reactive({
    infile <- input$path2file
    if (is.null(infile)) return(NULL)    
    scan(infile$datapath, n = 1)
    infile$datapath  
}) 

Thinking that it would be a fast workaround, but it too is extremely slow so I suspect it too is not reading in only n = 1. So, my question is can anyone identify a way to use fileInput to allow a user to locate a file and have the server side function capture only the path and NOT read in the file or try and parse it in any way? More importantly, can this be done using functions in base R and Shiny alone without having to grab functions from other extended packages?

The above is the relevant portion of code in the server.R file and the relevant portion of code in the ui.R file is

fileInput('path2dor', 'Choose the DOR .txt file to format',
        accept=c('text/csv', 
        'text/comma-separated-values,text/plain', '.csv')), 

Thank you for you advice.

Answer 1

This functionality is not possible with fileInput. The reason is because 'fileInput' do not provide local path information to the server for security reasons. With fileInput the user navigates through the browser on his local machine, the resulting file on the server side is the uploaded copy of the selected local once.

As an alternative you can use the shinyFiles package, which do navigate through the server side. This means, that you get all the paths on your local machine.

A second alternative could be a simple text input, which lets the user add a path by hand (make sure to check the path on the server side to not run into any troubles).