Why pipes sometimes get "stuck": buffering

Here’s a niche terminal problem that has bothered me for years but that I never really understood until a few weeks ago. Let’s say you’re running this command to watch for some specific output in a log file: tail -f /some/log/file | grep thing1 | grep thing2 If log lines are being added to the file relatively slowly, the result I’d see is… nothing! It doesn’t matter if there were matches in the log file or not, there just wouldn’t be any output. I internalized this as “uh, I guess pipes just get...