如何在Windows上使用Sys.which查找正确的可执行文件

Sys.which2 <- function(cmd) {
    stopifnot(length(cmd) == 1)
    if (.Platform$Os.type == "windows") {
        suppressWarnings({
            pathname <- sHell(sprintf("where %s 2> NUL",cmd),intern=TRUE)[1]
        })
        if (!is.na(pathName)) return(setNames(pathname,cmd))
    }
    Sys.which(cmd)
}

## Trying out Sys.which & Sys.which2 on my Windows Box gives the following:
Sys.which("convert")
#                              convert 
# "C:\\Windows\\system32\\convert.exe" 
Sys.which2("convert")
#                                                 convert 
# "C:\\Program Files\\ImageMagick-6.8.8-Q16\\convert.exe"