[kps]

In this particular example, ‘unnecessary use of cat’ is accompanied by ‘unnecessary use of grep’.

    cat input | grep '^x' | sed 's/foo/bar/g'

→

    sed '/^x/s/foo/bar/g' <input

[irishsultan]

That's not the same thing. The sed output will still keep lines not starting with x (just not replacing foo with bar in those) where grep will filter those out.

[kps]

Yeah, Muphry's law at work. Corrected version:

   sed -n '/^x/s/foo/bar/gp' <input

This may be an inadvertent argument for the ‘connect simpler tools’ philosophy.