Chapter 5 Files

R provides many functions to work with files and directories: many of these have been added relatively recently to facilitate scripting in R and in particular the replacement of Perl scripts by R scripts in the management of R itself.

These functions are implemented by standard C/POSIX library calls, except on Windows. That means that filenames must be encoded in the current locale as the OS provides no other means to access the file system: increasingly filenames are stored in UTF-8 and the OS will translate filenames to UTF-8 in other locales. So using a UTF-8 locale gives transparent access to the whole file system.

Windows is another story. There the internal view of filenames is in UTF-16LE (so-called ‘Unicode’), and standard C library calls can only access files whose names can be expressed in the current codepage. To circumvent that restriction, there is a parallel set of Windows-specific calls which take wide-character arguments for filepaths. Much of the file-handling in R has been moved over to using these functions, so filenames can be manipulated in R as UTF-8 encoded character strings, converted to wide characters (which on Windows are UTF-16LE) and passed to the OS. The utilities RC_fopen and filenameToWchar help this process. Currently file.copy to a directory, list.files, list.dirs and path.expand work only with filepaths encoded in the current codepage.

All these functions do tilde expansion, in the same way as path.expand, with the deliberate exception of Sys.glob.

File names may be case sensitive or not: the latter is the norm on Windows and macOS, the former on other Unix-alikes. Note that this is a property of both the OS and the file system: it is often possible to map names to upper or lower case when mounting the file system. This can affect the matching of patterns in list.files and Sys.glob.

File names commonly contain spaces on Windows and macOS but not elsewhere. As file names are handled as character strings by R, spaces are not usually a concern unless file names are passed to other process, e.g. by a system call.

Windows has another couple of peculiarities. Whereas a POSIX file system has a single root directory (and other physical file systems are mounted onto logical directories under that root), Windows has separate roots for each physical or logical file system (‘volume’), organized under drives (with file paths starting D: for an ASCII letter, case-insensitively) and network shares (with paths like \netname\topdir\myfiles\a file). There is a current drive, and path names without a drive part are relative to the current drive. Further, each drive has a current directory, and relative paths are relative to that current directory, on a particular drive if one is specified. So D:dir\file and D: are valid path specifications (the last being the current directory on drive D:).

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