Introduction

The purpose of create_pax.sh is to create a tar archive with VENDOR specific keywords in the PAX extended header files using the GNU tar utility.

Some information about PAX could be also found in the FreeBSD documentation.

Problem

The GNU tar is not directly able to create a tar archive with PAX VENDOR options specific to a file. For example, if we want to create an archive with two file, each file with a vendor specific option to a different value, we would try this:

$ tar -c -v -f archive.tar --format=pax --pax-option "VENDOR.opt:=1" file1 --pax-option "VENDOR.opt:=2" file2

But the extended record headers of the two files will contain the both options "VENDOR.opt=1" and "VENDOR.opt=2". It is not possible to write an option just for the next file of the archive.

Addtionally the pax utility in FreeBSD 12.0 refuses to create vendor specifc options.

Solution

create_pax.sh uses a sequence of calls to tar utility. First we create an empty archive with the global keywords. These options will be written in the first "g" record of the archive.

$ tar -c -v -f archive.tar --format=pax --pax-option "VENDOR.global=1" -T /dev/null

Now, we can add the files in the archive with the corresponding option keywords. These options will be written in the "x" record preceding each file record:

$ tar -r -v -f archive.tar --pax-option "VENDOR.opt:=1" file1
$ tar -r -v -f archive.tar --pax-option "VENDOR.opt:=2" file2

Note that these extended header keywords are unknown by GNU tar. Therefore their are ignored during the extraction.

$ tar -x -v -f archive.tar
tar: Ignoring unknown extended header keyword 'VENDOR.opt'
file1
tar: Ignoring unknown extended header keyword 'VENDOR.opt'
file2

Usage

Usage: ./create_pax.sh ARCHIVE VENDOR_IDENTIFIER < INI_FILE
Example: ./create_pax.sh archive.tar VENDOR < conf.ini

The INI file taken on standard input is a INI configuration wile with section [global] describing the global options. And for each file a [file] section with a mandatory pathname property and the corresponding file options.

$ create_pax.sh archive.tar "VENDOR" <<EOF
[global]
global=1

[file]
pathname=file1
opt=1

[file]
pathname=file2
opt=2
EOF