对于项目,我需要定期下载来自不同网站的数据文件,以根据这些文件创建指标。
由于这些文件的更新频率差异很大,我正在寻找一种有效的方法来检测远程文件是否已更新。
建议使用curl的-I选项。如何使用curl包转换?
https://superuser.com/questions/619592/get-modification-time-of-remote-file-over-http-in-bash-script
备用解决方案似乎解析了文件大小或修改日期的标题:
类似于:
PHP: Remote file size without downloading file
然而,我在下面的尝试(使用小文件)会下载完整文件。
library(curl)
req <- curl_fetch_memory("http://www.pcr.uu.se/digitalAssets/124/124932_1ucdponesided2015.rdata")
str(req)
object.size(req)
parse_headers(req$headers)
是否可以只使用curl软件包下载标头,或指定一个选项以避免冗余下载?
答案 0 :(得分:1)
您必须保留文件的最后修改日期的历史记录(假设Web服务器在报告时是一致的)并在下载之前检查httr::HEAD()
(即您有一些工作要做在某个地方存储最后修改过的值的vis,可能是在带有URL的数据框中:
library(httr)
URL <- "http://www.pcr.uu.se/digitalAssets/124/124932_1ucdponesided2015.rdata"
#' Download a file only if it hasn't changed since \code{last_modified}
#'
#' @param URL url of file
#' @param fil path to write file
#' @param last_modified \code{POSIXct}. Ideally, the output from the first
#' successful run of \code{get_file()}
#' @param overwrite overwrite the file if it exists?
#' @param .verbose output a message if the file was unchanged?
get_file <- function(URL, fil, last_modified=NULL, overwrite=TRUE, .verbose=TRUE) {
if ((!file.exists(fil)) || is.null(last_modified)) {
res <- GET(URL, write_disk(fil, overwrite))
return(httr::parse_http_date(res$headers$`last-modified`))
} else if (inherits(last_modified, "POSIXct")) {
res <- HEAD(URL)
cur_last_mod <- httr::parse_http_date(res$headers$`last-modified`)
if (cur_last_mod != last_modified) {
res <- GET(URL, write_disk(fil, overwrite))
return(httr::parse_http_date(res$headers$`last-modified`))
}
if (.verbose) message(sprintf("'%s' unchanged since %s", URL, last_modified))
return(last_modified)
}
}
# first run == you don't know the last-modified date.
# you need to pair this with the URL in some data structure for later use.
last_mod <- get_file(URL, basename(URL))
class(last_mod)
## [1] "POSIXct" "POSIXt"
last_mod
## [1] "2015-11-16 17:34:06 GMT"
last_mod <- get_file(URL, basename(URL), last_mod)
#> 'http://www.pcr.uu.se/digitalAssets/124/124932_1ucdponesided2015.rdata' unchanged since 2015-11-16 17:34:06
答案 1 :(得分:0)
httr
包的替代方法是 base
函数 base::curlGetHeaders(url)
,但您仍需要自己解析上次修改日期!