preinst is in tcl8.5-dev 8.5.11-1ubuntu1.
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
set -e
if [ "$1" = "upgrade" ] && dpkg --compare-versions "$2" lt-nl "0.b1-2"; then
update-alternatives --remove tcl.m4 /usr/lib/tcl8.5/tcl.m4
# We really want to remove this alternative
update-alternatives --auto tcl.m4 >/dev/null || true
fi
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