# All logs with error levels emerg(0) to warning(4)
$ journalctl -p "0..warning" -b

# Log levels are:
# 0: emerg
# 1: alert
# 2: crit
# 3: err
# 4: warning
# 5: notice
# 6: info
# 7: debug

# All units with logs
$ journalctl --field _SYSTEMD_UNIT | grep -P '.service$'

# No, there is no file per unit!
$ find /var/log/journal/ -type f -mtime -1
/var/log/journal/05cb8c7b39fe0f70e3ce97e5beab809d/user-1020.journal
/var/log/journal/05cb8c7b39fe0f70e3ce97e5beab809d/system.journal
/var/log/journal/05cb8c7b39fe0f70e3ce97e5beab809d/system@db6f7e062ad14bdb9cf5b4780c2b5b58-000000000ef6569b-000631e80e16ec34.journal
/var/log/journal/05cb8c7b39fe0f70e3ce97e5beab809d/system@db6f7e062ad14bdb9cf5b4780c2b5b58-000000000ef544e6-000631d8f795f057.journal
/var/log/journal/05cb8c7b39fe0f70e3ce97e5beab809d/user-1020@331b72423f2f4550a32bf14557cab2de-000000000ef59483-000631ddc5b458a6.journal
/var/log/journal/05cb8c7b39fe0f70e3ce97e5beab809d/system@db6f7e062ad14bdb9cf5b4780c2b5b58-000000000ef59d1a-000631ddff15e614.journal
/var/log/journal/05cb8c7b39fe0f70e3ce97e5beab809d/system@db6f7e062ad14bdb9cf5b4780c2b5b58-000000000ef5fd9b-000631e306955d6e.journal

# follow logs
journalctl  -f -u wildfly.service 

# logs in reverse order(newest on top)
journalctl -r  -u wildfly.service 

# all logs fromon 5 days ago
journalctl -u wildfly.service --since '5 days ago'

# Verfiy journals
journalctl --verify

# Get logs as JSON
journalctl --boot -o json