R: Create a Tar Archive

tar {utils} | R Documentation

## 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`.

### 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 current default in version 1.26 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`). For libarchive `tar`, --format=ustar is more portable than the default.

### 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. Most R platforms use a version of GNU `tar` (including `Rtools` on Windows, but the behaviour seems to be changed with each version), Mac OS >= 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, or the GNU way (which is widely supported).

  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, at least, 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` does not read. 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.

### See Also

http://en.wikipedia.org/wiki/Tar_(file_format), http://pubs.opengroup.org/onlinepubs/9699919799/utilities/pax.html#tag_20_92_13_06 for the way the POSIX utility `pax` handles `tar` formats.

https://github.com/libarchive/libarchive/wiki/FormatTar.

`untar`.

[Package utils version 3.1.2 Index]