journalctl may be used to query the contents of the systemd journal as written by systemd-journald.service.

If called without parameters, it will show the full contents of the journal, starting with the oldest entry collected.

The following can be used to narrow the displayed log data.

# Reverse order
journalctl -r

# No paging
journalctl --no-pager

# Time range (can be used with other filters below)
journalctl --since "2018-08-30 14:10:10"
journalctl --since today 

# Monitor new messages live (similar to tail -f)
journalctl -f

# Show last boot
journalctl -b

# List Boots
journalctl --list-boots

# Show specific boot
journalctl -b 73d7f65a5b3d4301abcd529ff8601f5f

# Show logs for a specific service
journalctl -u ssh

# Show Kernal messages (can be combined with -b)
journalctl -k