Understanding the Problem and Setting Up the Environment

As a R user, you might have encountered issues with the download progress not displaying line breaks for updates from curl. The question at hand is how to set up curl options to improve readability of the progress in R’s download.file().

To solve this problem, we will delve into the details of curl, the underlying mechanism used by R, and provide solutions that cater to both OS X and Linux users.

Setting Up the Environment

For this solution, you’ll need:

• A recent version of R (at least 3.1.0)
• The curl package installed
• The coreutils package for OS X (or equivalent) using homebrew (brew install coreutils)
• Familiarity with bash shell scripting

Ensure you have the necessary packages installed and that your environment is set up correctly.

Exploring Curl Options

The first step in solving this problem is understanding what’s happening behind the scenes. Let’s examine how curl displays its progress bar:

## Progress Display Format
cURL’s standard output format includes a progress bar, which includes several fields such as:
*   Total size of the file to be transferred (in bytes)
*   Total number of bytes received so far
*   Number of bytes that have been skipped over (dowloaded in chunks rather than in complete packets)
*   Current download speed

We want curl to display this information on separate lines, which means adding line breaks. Unfortunately, there is no direct option to achieve this within the standard curl configuration.

However, we can bypass curl directly using shell scripting and R’s underlying mechanism for downloading files (system()).

Creating a Custom Script

To overcome this limitation, let’s create a small bash script called mycurl.sh. This script will take two arguments: the URL to download from and the destination file path.

#!/bin/bash

URL=$1
destfile=$2

gstdbuf -i0 -o0 -e0 curl $URL -o $destfile 2>&1 | gstdbuf -i0 -o0 -e0 tr '\r' '\n'

Make this script executable by running:

chmod +x mycurl.sh

Integrating with R’s download.file()

Now, we’ll modify the download.file() function in R to use our custom script instead of the standard curl. We’ll achieve this by defining a wrapper around the original function that calls our bash script.

Here’s how you can do it:

download_file <- function(url = character(1), destfile = character(1),
                           mode = "wb", quiet = FALSE, extra = character(),
                           method = "auto", type = "binary",
                           ignore.cache = FALSE) {
  # Standard behavior for 'auto' or missing method
  if (method == "auto") {
    method <- "curl"
  }

  if (method == "curl") {

    if (quiet) extra <- c(extra, "-s -S")
    if (!ignore.cache) extra <- c(extra, "-H 'Pragma: no-cache'")
    
    # New behavior for 'curl' using the custom script
    status <- system(paste("/path/to/mycurl.sh", shQuote(url1), shQuote(path.expand(destfile))))
  }
  
  # ... rest of your function ...
}

Replace /path/to/mycurl.sh with the actual path to your mycurl.sh script.

Alternative Solution Using httr Package

Alternatively, you can use the httr package, which provides a more R-like interface for downloading files from URLs. This solution allows you to customize the download progress meter using the progress() function.

library(httr)

download_file <- function(url = character(1), destfile = character(1),
                           mode = "wb", quiet = FALSE, extra = character(),
                           method = "auto", type = "binary",
                           ignore.cache = FALSE) {
  
  # ... rest of your function ...
}

# Usage
status <- GET(url1, write_disk(path.expand(q1f), overwrite=TRUE), progress("down"))

In this solution, you don’t need to manually create a custom script. The httr package will take care of displaying the download progress with new lines.

Conclusion

We have now covered how to set up curl options for improved readability of the progress in R’s download.file(). We’ve explored solutions that cater to both OS X and Linux users, including creating a custom shell script (mycurl.sh) and using the httr package.

Last modified on 2024-06-06