postinst is in apache2-doc 2.2.22-1ubuntu1.
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 | #!/bin/sh -e
if [ -x "/etc/init.d/apache2" ]; then
        if [ -x /usr/sbin/invoke-rc.d ]; then
                invoke-rc.d apache2 reload ||true
        else
                /etc/init.d/apache2 reload ||true
        fi
fi
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