The reference element is used to specify the URL of the reference archive to download if the local copy is not available.
<unpack> <reference>ftp://ftp.gnu.org/gnu/gzip/gzip.1.2.4a.tar.gz</reference> <archive>/usr/src/lfs-packages/gzip.1.2.4a</archive> <destination>/usr/src</destination> </unpack>
The equivalent bash script is :
echo Unpacking /usr/src/lfs-packages/gzip.1.2.4a.tar.gz into /usr/src
[[ ! -f /usr/src/lfs-packages/gzip.1.2.4a.tar.gz ]] &&
echo Retrieving ftp://ftp.gnu.org/gnu/gzip/gzip.1.2.4a.tar.gz
[[ ! -f /usr/src/lfs-packages/gzip.1.2.4a.tar.gz ]] &&
wget -O /usr/src/lfs-packages/gzip.1.2.4a.tar.gz ftp://ftp.gnu.org/gnu/gzip/gzip.1.2.4a.tar.gz
[[ ! -f /usr/src/lfs-packages/gzip.1.2.4a.tar.gz ]] &&
echo Archive does not exist && exit 3
cd /usr/src
tar xzvf /usr/src/lfs-packages/gzip.1.2.4a.tar.gz