/usr/share/bash-completion/completions/ifup is in ifupdown2 1.0~git20170314-1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 | _python_argcomplete() {
local IFS=' '
COMPREPLY=( $(IFS="$IFS" COMP_LINE="$COMP_LINE" COMP_POINT="$COMP_POINT" _ARGCOMPLETE_COMP_WORDBREAKS="$COMP_WORDBREAKS" _ARGCOMPLETE=1 "$1" 8>&1 9>&2 1>/dev/null 2>/dev/null) )
if [[ $? != 0 ]]; then
unset COMPREPLY
fi
}
complete -F _python_argcomplete ifup ifdown ifquery ifreload
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