Comment by _pvxk

Oh and another one I use all the time is as an "identity transform" when selecting between filters, e.g.

    dostuff () { 
        if [[ $1 = clean ]]; then 
            grep -v dirt
        else 
            cat
        fi | do_other_stuff
    }

See also https://www.in-ulm.de/~mascheck/various/uuoc/ (via https://lobste.rs/s/rtvp2u/useful_uses_cat#c_0xpqkr )