This log is taken from starting jack, playing a short audio snippet, and stopping jack.

00:38:32.975 Statistics reset.
00:38:32.980 ALSA connection change.
Cannot connect to server socket err = No such file or directory
Cannot connect to server request channel
jack server is not running or cannot be started
JackShmReadWritePtr::~JackShmReadWritePtr - Init not done for -1, skipping unlock
JackShmReadWritePtr::~JackShmReadWritePtr - Init not done for -1, skipping unlock
00:38:33.022 ALSA connection graph change.
00:38:37.295 JACK is starting...
00:38:37.296 /bin/jackd -dalsa -dhw:USB,0 -r88200 -p512 -n4 -Xseq
Cannot connect to server socket err = No such file or directory
Cannot connect to server request channel
jack server is not running or cannot be started
JackShmReadWritePtr::~JackShmReadWritePtr - Init not done for -1, skipping unlock
JackShmReadWritePtr::~JackShmReadWritePtr - Init not done for -1, skipping unlock
00:38:37.312 JACK was started with PID=11234.
no message buffer overruns
no message buffer overruns
no message buffer overruns
jackdmp 1.9.12
Copyright 2001-2005 Paul Davis and others.
Copyright 2004-2016 Grame.
Copyright 2016-2017 Filipe Coelho.
jackdmp comes with ABSOLUTELY NO WARRANTY
This is free software, and you are welcome to redistribute it
under certain conditions; see the file COPYING for details
JACK server starting in realtime mode with priority 10
self-connect-mode is "Don't restrict self connect requests"
audio_reservation_init
Acquire audio card Audio1
creating alsa driver ... hw:USB,0|hw:USB,0|512|4|88200|0|0|nomon|swmeter|-|32bit
configuring for 88200Hz, period = 512 frames (5.8 ms), buffer = 4 periods
ALSA: final selected sample format for capture: 32bit integer little-endian
ALSA: use 4 periods for capture
ALSA: final selected sample format for playback: 32bit integer little-endian
ALSA: use 4 periods for playback
port created: Midi-Through:midi/playback_1
port created: Midi-Through:midi/capture_1
port created: Scarlett-18i20-USB:midi/playback_1
port created: Scarlett-18i20-USB:midi/capture_1
00:38:37.508 ALSA connection graph change.
00:38:39.443 JACK connection change.
00:38:39.444 Server configuration saved to "/home/jorin/.jackdrc".
00:38:39.444 Statistics reset.
00:38:39.450 Client activated.
00:38:39.451 Patchbay deactivated.
00:38:39.477 JACK connection graph change.
00:38:43.025 JACK connection graph change.
00:38:43.090 JACK connection change.
00:38:46.223 JACK connection graph change.
00:38:46.314 JACK connection change.
00:38:50.553 Client deactivated.
00:38:50.554 JACK is stopping...
Jack main caught signal 15
port deleted: Scarlett-18i20-USB:midi/playback_1
port deleted: Midi-Through:midi/playback_1
port deleted: Scarlett-18i20-USB:midi/capture_1
port deleted: Midi-Through:midi/capture_1
Released audio card Audio1
audio_reservation_finish
00:38:50.579 ALSA connection graph change.
00:38:50.615 JACK was stopped