/usr/share/debhelper/autoscripts/postinst-rebuild-gcj-db is in libgcj-common 1:6.3-4.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 | if which rebuild-gcj-db >/dev/null 2>&1; then
rebuild-gcj-db
fi
|