Shortest song ?

It must be Napalm Death - You Suffer.

Shortest actual song that comes to mind is They Might Be Giants with Minimum Wage, 46 seconds.

Even within the same band there's Fingertips from the album Apollo 18, which is a something like 20 tracks that total a bit over 4 minutes. That calls into question what a song truly is, though, as these would qualify as snippets but not songs in my mind.

If a snippet isn't and a jingle is, where's the line in the sand?

Supernova’s album “Ages 3 & Up” from 1995 is full of short songs.

This one may not be a 'serious' answer, but "a world on fire" by Bo Burnham is quite short. https://youtu.be/1ws33f6qys4

I was gonna say the intros and interludes in the Modest Mouse Good News album are very short, but hard to call them songs per se. Dig Your Grave is 13 seconds of mumbling 'something about diggin' and 'i hope that youre dead'...

The Beatles song Her Majesty is ~23 seconds and feels like a complete song. Crazy stuff.

"Yeah" by Kyuss clocks in at 4 seconds. I keep it in regular shuffle for a laugh.

Comma comma comma full stop by Brakes is 8 seconds long. It’s an album track and part of the track listing but opinions may differ on whether it’s truly a song or not.

Short Songs - Dead Kennedys is like 30 seconds