grep by default uses Basic Regular Expressions. This means the ( and ) lose their special meaning and are matched literally. Either use a backslash version \( to have a group, or use Extended Regular Expressions with -E "(IN|OUT)" . In man grep under REGULAR EXPRESSIONS are some differences noted.

It’s marked solved, but since OP didn’t post the solution:

-e uses basic regular expressions, where you need to escape the meta-characters ((|)) with a backslash. Alternatively, use extended regex with -E

$ echo a | grep -E "(a|b)"
a
$ echo a | grep -e "\(a\|b\)"
a
$ echo a | grep -e "(a|b)"
$ echo a | grep -E "\(a\|b\)"

Do you get output if you use that exact tail command without the grep pipe?