/usr/share/fish/completions/ifup.fish is in fish-common 2.4.0-1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 | complete -x -c ifup -a "(__fish_print_interfaces)" --description "Network interface"
complete -c ifup -l force --description "force"
|