untar Extract or List Tar Archives

Description

Extract files from or list the contents of a tar archive.

Usage

untar(tarfile, files = NULL, list = FALSE, exdir = ".",
      compressed = NA, extras = NULL, verbose = FALSE,
      restore_times =  TRUE,
      support_old_tars = Sys.getenv("R_SUPPORT_OLD_TARS", FALSE),
      tar = Sys.getenv("TAR"))

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 reads. For a compressed tarfile, and if a connection is to be used, that should be created by gzfile(.) (or gzcon(.) which currently only works for "gzip", whereas gzfile() works for all compressions available in tar()).
files	A character vector of recorded filepaths to be extracted: the default is to extract all files.
list	If TRUE, list the files (the equivalent of tar -tf). Otherwise extract the files (the equivalent of tar -xf).
exdir	The directory to extract files to (the equivalent of tar -C). It will be created if necessary.
compressed	(Deprecated in favour of auto-detection, used only for an external tar command.) Logical or character string. Values "gzip", "bzip2" and "xz" select that form of compression (and may be abbreviated to the first letter). TRUE indicates gzip compression, FALSE no known compression, and NA (the default) indicates that the type is to be inferred from the file header. The external command may ignore the selected compression type but detect a type automagically.
extras	NULL or a character string: further command-line flags such as -p to be passed to an external tar program.
verbose	logical: if true echo the command used for an external tar program.
restore_times	logical. If true (default) restore file modification times. If false, the equivalent of the -m flag. Times in tarballs are supposed to be in UTC, but tarballs have been submitted to CRAN with times in the future or far past: this argument allows such times to be discarded. Note that file times in a tarball are stored with a resolution of 1 second, and can only be restored to the resolution supported by the file system (which on a FAT system is 2 seconds).

support_old_tars

logical. If false (the default), the external tar command is assumed to be able handle compressed tarfiles and if compressed does not specify it, to automagically detect the type of compression. (The major implementations have done so since 2009; for GNU tar since version 1.22.) If true, the R code calls an appropriate decompressor and pipes the output to tar, for compressed = NA examining the tarfile header to determine the type of compression.

tar

character string: the path to the command to be used or "internal". If the command itself contains spaces it needs to be quoted – but tar can also contain flags separated from the command by spaces.

Details

This is either a wrapper for a tar command or for an internal implementation written in R. The latter is used if tarfile is a connection or if the argument tar is "internal" or "" (except on Windows, when tar.exe is tried first).

Unless otherwise stated three types of compression of the tar file are supported: gzip, bzip2 and xz.

What options are supported will depend on the tar implementation used: the "internal" one is intended to provide support for most in a platform-independent way.

GNU tar:

Modern GNU tar versions support compressed archives and since 1.15 are able to detect the type of compression automatically: version 1.22 added support for xz compression.

On a Unix-alike, configure will set environment variable TAR, preferring GNU tar if found.

bsdtar:

macOS 10.6 and later (and FreeBSD and some other OSes) have a tar from the libarchive project which detects all three forms of compression automagically (even if undocumented in macOS).

NetBSD:

It is undocumented if NetBSD's tar can detect compression automagically: for versions before 8 the flag for xz compression was --xz not -J. So support_old_tars = TRUE is recommended (or use bsdtar if installed).

OpenBSD:

OpenBSD's tar does not detect compression automagically. It has no support for xz beyond reporting that the file is xz-compressed. So support_old_tars = TRUE is recommended.

Heirloom Toolchest:

This tar does automagically detect gzip and bzip2 compression (undocumented) but has no support for xz compression.

Older support:

Environment variable R_GZIPCMD gives the command to decompress gzip files, and R_BZIPCMD for bzip2 files. (On Unix-alikes these are set at installation if found.) xz is used if available: if not decompression is expected to fail.

Arguments compressed, extras and verbose are only used when an external tar is used.

Some external tar commands will detect some of lrzip, lzma, lz4, lzop and zstd compression in addition to gzip, bzip2 and xz. (For some external tar commands, compressed tarfiles can only be read if the appropriate utility program is available.) For GNU tar, further (de)compression programs can be specified by e.g. extras = "-I lz4". For bsdtar this could be extras = "--use-compress-program lz4". Most commands will detect (the nowadays rarely seen) ‘.tar.Z’ archives compressed by compress.

The internal implementation restores symbolic links as links on a Unix-alike, and as file copies on Windows (which works only for existing files, not for directories), and hard links as links. If the linking operation fails (as it may on a FAT file system), a file copy is tried. Since it uses gzfile to read a file it can handle files compressed by any of the methods that function can handle: at least compress, gzip, bzip2 and xz compression, and some types of lzma compression. It does not guard against restoring absolute file paths, as some tar implementations do. It will create the parent directories for directories or files in the archive if necessary. It handles the USTAR/POSIX, GNU and pax ways of handling file paths of more than 100 bytes, and the GNU way of handling link targets of more than 100 bytes.

You may see warnings from the internal implementation such as

    unsupported entry type 'x'

This often indicates an invalid archive: entry types "A-Z" are allowed as extensions, but other types are reserved. The only thing you can do with such an archive is to find a tar program that handles it, and look carefully at the resulting files. There may also be the warning

    using pax extended headers

This indicates that additional information may have been discarded, such as ACLs, encodings ....

The former standards only supported ASCII filenames (indeed, only alphanumeric plus period, underscore and hyphen). untar makes no attempt to map filenames to those acceptable on the current system, and treats the filenames in the archive as applicable without any re-encoding in the current locale.

The internal implementation does not special-case ‘resource forks’ in macOS: that system's tar command does. This may lead to unexpected files with names with prefix ‘._’.

Value

If list = TRUE, a character vector of (relative or absolute) paths of files contained in the tar archive.

Otherwise the return code from system with an external tar or 0L, invisibly.

See Also

tar, unzip.