postinst is in binutils-alpha-linux-gnu 2.26-8ubuntu2.
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 | #! /bin/sh
set -e
if [ "$1" = "configure" ]; then
ldconfig
fi
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