postinst is in gnuplot-x11 5.0.5+dfsg1-6+deb9u1.
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 13 | #!/bin/sh
set -e
if [ "$1" = "configure" ] || [ "$1" = "abort-upgrade" ] || [ "$1" != "upgrade"]; then
update-alternatives --remove gnuplot4 /usr/bin/gnuplot4-x11
update-alternatives --install /usr/bin/gnuplot gnuplot /usr/bin/gnuplot-x11 90 \
--slave /usr/share/man/man1/gnuplot.1.gz gnuplot.1.gz \
/usr/share/man/man1/gnuplot-x11.1.gz \
--slave /usr/share/gnuplot/gnuplot.gih gnuplot.gih \
/usr/share/gnuplot/gnuplot-x11.gih
fi
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