SIGUSR2
The second free bell — the one that usually remains without an agreed ring.
SIGUSR1's sibling, with the same freedom and less fame. When a program has already spent the first signal on the obvious use, the second inherits the exotic one: in nginx, USR2 triggers the binary upgrade in mid flight, without dropping connections.
In scripts, the USR1/USR2 pair becomes a homemade two-button protocol — raise and lower verbosity, toggle a mode — for the price of two traps. The sibling's warning applies here too: with no handler, the default is to terminate.
$ cat vigia.sh
trap 'verbo=1' USR1
trap 'verbo=0' USR2
...
$ kill -USR2 $(pidof -s vigia.sh)
(the watcher goes back to silent mode)