/usr/share/fish/completions/nice.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 3 4 5 | complete -c nice -a "(__fish_complete_subcommand -- -n --adjustment)" --description "Command"
complete -c nice -s n -l adjustment -n "__fish_no_arguments" --description "Add specified amount to niceness value" -x
complete -c nice -l help -n "__fish_no_arguments" --description "Display help and exit"
complete -c nice -l version -n "__fish_no_arguments" --description "Display version and exit"
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