tar: Create a Tar Archive

Description

Create a tar archive.

Usage

tar(tarfile, files = NULL,
    compression = c("none", "gzip", "bzip2", "xz"),
    compression_level = 6, tar = Sys.getenv("tar"),
    extra_flags = "")

Arguments

tarfile

The pathname of the tar file: tilde expansion (see path.expand) will be performed. Alternatively, a connection that can be used for binary writes.

files

A character vector of filepaths to be archived: the default is to archive all files under the current directory.

compression

character string giving the type of compression to be used (default none). Can be abbreviated.

compression_level

integer: the level of compression. Only used for the internal method.

tar

character string: the path to the command to be used. If the command itself contains spaces it needs to be quoted (e.g., by shQuote) -- but argument tar can also contain flags separated from the command by spaces.

extra_flags

any extra flags for an external tar.

Value

The return code from system or 0 for the internal version, invisibly.

Portability

The ‘tar’ format no longer has an agreed standard! ‘Unix Standard Tar’ was part of POSIX 1003.1:1998 but has been removed in favour of pax, and in any case many common implementations diverged from the former standard. Many R platforms use a version of GNU tar (including Rtools on Windows), but the behaviour seems to be changed with each version. OS X >= 10.6 and FreeBSD use bsdttar from the ‘libarchive’ project, and commercial Unixes will have their own versions.

Known problems arise from

The handling of file paths of more than 100 bytes. These were unsupported in early versions of tar, and supported in one way by POSIX tar and in another by GNU tar and yet another by the POSIX pax command which recent tar programs often support. The internal implementation warns on paths of more than 100 bytes, uses the ‘ustar’ way from the 1998 POSIX standard which supports up to 256 bytes (depending on the path: in particular the final component is limited to 100 bytes) if possible, otherwise the GNU way (which is widely supported, including by untar).

Most formats do not record the encoding of file paths.
(File) links. tar was developed on an OS that used hard links, and physical files that were referred to more than once in the list of files to be included were included only once, the remaining instances being added as links. Later a means to include symbolic links was added. The internal implementation supports symbolic links (on OSes that support them), only. Of course, the question arises as to how links should be unpacked on OSes that do not support them: for regular files file copies can be used.

Names of links in the ‘ustar’ format are restricted to 100 bytes. There is an GNU extension for arbitrarily long link names, but bsdtar ignores it. The internal method uses the GNU extension, with a warning.
Header fields, in particular the padding to be used when fields are not full or not used. POSIX did define the correct behaviour but commonly used implementations did (and still do) not comply.
File sizes. The ‘ustar’ format is restricted to 8GB per (uncompressed) file.

For portability, avoid file paths of more than 100 bytes and all links (especially hard links and symbolic links to directories).

The internal implementation writes only the blocks of 512 bytes required (including trailing blocks of nuls), unlike GNU tar which by default pads with nul to a multiple of 20 blocks (10KB). Implementations which pad differ on whether the block padding should occur before or after compression (or both): padding was designed for improved performance on physical tape drives.

The ‘ustar’ format records file modification times to a resolution of 1 second: on file systems with higher resolution it is conventional to discard fractional seconds.

Details

This is either a wrapper for a tar command or uses an internal implementation in R. The latter is used if tarfile is a connection or if the argument tar is "internal" or "" (the ‘factory-fresh’ default). Note that whereas Unix-alike versions of R set the environment variable TAR, its value is not the default for this function.

Argument extra_flags is passed to an external tar and so is platform-dependent. Possibly useful values include -h (follow symbolic links, also -L on some platforms), --acls, --exclude-backups, --exclude-vcs (and similar) and on Windows --force-local (so drives can be included in filepaths: however, this is the default for the Rtools tar). For GNU tar, --format=ustar forces a more portable format. (The default is set at compilation and will be shown at the end of the output from tar --help: for version 1.28 ‘out-of-the-box’ it is --format=gnu, but the manual says the intention is to change to --format=pax which GNU incorrectly calls ‘POSIX’ -- it was never part of the POSIX standard for tar and should not be used.) For libarchive's bsdtar, --format=ustar is more portable than the default.

One issue which can cause an external command to fail is a command line too long for the system shell: as from R 3.5.0 this is worked around if the external command is detected to be GNU tar or libarchive tar (aka bsdtar).

Note that files = '.' will usually not work with an external tar as that would expand the list of files after tarfile is created. (It does work with the default internal method.)