/usr/share/fish/functions/grep.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 6 7 8 9 | #
# Match colors for grep, if supported
#
if echo | command grep --color=auto "" >/dev/null 2>&1
function grep
command grep --color=auto $argv
end
end
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