dir() function lists all the files in a directory.

dir(path = ".", pattern = NULL, all.files = FALSE,
full.names = FALSE, recursive = FALSE,
ignore.case = FALSE, include.dirs = FALSE)

path: a character vector of full path names; the default corresponds to the working directory, getwd(). Tilde expansion (see path.expand) is performed. Missing values will be ignored.
pattern: an optional regular expression. Only file names which match the regular expression will be returned
all.files: a logical value. If FALSE, only the names of visible files are returned. If TRUE, all file names will be returned
full.names: a logical value. If TRUE, the directory path is prepended to the file names to give a relative file path. If FALSE, the file names (rather than paths) are returned
recursive: logical. Should the listing recurse into directories?
ignore.case: logical. Should pattern-matching be case-insensitive?
include.dirs: logical. Should subdirectory names be included in recursive listings? (There always are in non-recursive ones)