preinst is in gnuplot 4.6.4-2.
This file is a maintainer script. It is executed when installing (*inst) or removing (*rm) the package.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 9 10 11 12  | #!/bin/sh
set -e
if [ -L "/usr/share/doc/gnuplot" ]; then
    echo "Removing obsolete \"/usr/share/doc/gnuplot\" symlink" >&2
    rm -f /usr/share/doc/gnuplot
fi
exit 0
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