setwd which allows for path containing Unix-like environment variables

setwd.R

#' Set Working Directory
#'
#' \code{setwd(dir)} is used to set the working directory to \code{dir}.
#' 
#' This version of \code{setwd} is different from the one in \code{base} in that it allows for paths containing Unix-like
#' environment variables. For example, if HOME is set (globally) to your home directory, then \code{base::setwd($HOME)} 
#' will not work. However, the version below will work.
#' 
#' @param dir	A character string: tilde expansion will be done.

setwd <- function(dir) {
  if (!grepl("$", dir, fixed = TRUE)) {
    # If there is no environment variable, use the usual setwd function
    base::setwd(dir)
  } else {
    # Separate dir into its components, those separated by /
    comp <- strsplit(path.expand(dir), split = "/", fixed = TRUE)[[1]]
    # Focus only on those containing a $
    var <- comp[grep("$", comp, fixed = TRUE)]
    # Retrieve value for variables; need to remove $ before the call
    var <- Sys.getenv(substr(var, start = 2, stop = nchar(var)))
    # Substitute results in place of original variable
    comp[grep("$", comp, fixed = TRUE)] <- var
    base::setwd(paste(comp, collapse = "/"))
  }
}

There may be some problems left with the above code. For example, this may fail in a non-obvious way on Windows and non Unix-like machines. Moreover, it implicitly assumes (on line 21) that the dollar sign $ appears at the beginning of the string (is there a realistic scenario where this could cause some problems?).

I'm open to suggestions!