This is a fun bass line. Bud changes it up a lot but what
I have here are the main licks he uses. Just be creative and improvise
a lot. Also Bud is playing an upright bass but to imitate the sound
just use a fretless.
The chord progression is G C A
Main (G)(Repeated until Bridge)
G:-----------------!
D:-----------------!
A:-------5-3-6-5-3-!
E:-3--6---6--------!
[ Tab from: https://www.guitartabs.cc/tabs/s/sublime/summertime_btab.html ]
Bridge 1 (C)(played once right before bridge 2)
G:------0-0------0-!
D:----1--------1---!
A:---3--------3----!
E:-----------------!
Bridge 2 (A)(played once right after bridge 1)
G:----------------------!
D:----------------------!
A:---5/(9)--5-0---------!
E:-5------------3-3-5-6-!
This is the trickiest part of the song. Listen carefully to the song
to hear the rhythm here. Also I am not 100% positive on the last 4
notes but it's the best I could come up with