Options For Viewing Logs

cat

There are a couple quick options for looking at logs.

$ dmesg (contents of the kernel ring buffer. Also sent to syslog.)
$ cat /var/log/syslog (can always $ cat /var/log/syslog | grep 'keyword')
on Rpi
$ cat /var/log/messages Another option with more functionality is journald

journalctl

Some quick journalctl commands to get started

$ journalctl -b (current boot)
$ journalctl -b -1 (previous boot)
To list the boots of the system
$ journalctl --list-boots
$ journalctl --since "1 hour ago"
$ journalctl --since "7 days ago"
$ journalctl --since "2019-01-01 00:00:00" --until "2019-02-01 00:00:00"

By services (can add options at the end. “-r” to reverse sort, “-o json-pretty” format)

$ journalctl -u ssh.service -u bluetooth.service

To follow add ‘f

$ journalctl -u bluetooth.service -f

To send to a file

$ journalctl > dump.log

Examples

$ journalctl --unit=systemd-networkd
$ journalctl --unit=wpa_supplicant

Python Logging (including journal)

Logging libraries give useful functions in your python script for logging (info, debugging, errors). And you can add a journal option to output to the journal.

Couple approaches I use

basicConfig(). Quick, simple logging with basicConfig(). With basicConfig() you can configure the root logger but you can not configure custom loggers. Have to use handlers and formatters.
RotatingFileHandler/JournalHandler. When you want to output to multiple places (console/standard output stream, a log file, and the journal) it is better to use Handlers. RotatingFileHandler lets you specify the max size of the log and enables rolling log files.

RotatingFileHandler

Using RotatingFileHandler for logging will create 3 loggers in this example for three outputs

console
regular log
error log

#!/usr/bin/env python3
import logging, sys, os
from logging.handlers import RotatingFileHandler

def setup_logging(log_dir):
    # Main logger
    #main_logger = logging.getLogger()
    main_logger = logging.getLogger(__name__)
    main_logger.setLevel(logging.INFO)
    log_file_format = logging.Formatter("[%(levelname)s] - %(asctime)s - %(name)s - : %(message)s in %(pathname)s:%(lineno)d")
    log_console_format = logging.Formatter("[%(levelname)s]: %(message)s")

    console_handler = logging.StreamHandler()
    console_handler.setLevel(logging.INFO)
    console_handler.setFormatter(log_console_format)

    exp_file_handler = RotatingFileHandler('{}/exp_debug.log'.format(log_dir), maxBytes=10**6, backupCount=5) # 10MB file
    exp_file_handler.setLevel(logging.DEBUG)
    exp_file_handler.setFormatter(log_file_format)

    exp_errors_file_handler = RotatingFileHandler('{}/exp_error.log'.format(log_dir), maxBytes=10**6, backupCount=5)
    exp_errors_file_handler.setLevel(logging.WARNING)
    exp_errors_file_handler.setFormatter(log_file_format)

    main_logger.addHandler(console_handler)
    main_logger.addHandler(exp_file_handler)
    main_logger.addHandler(exp_errors_file_handler)
    return main_logger

if __name__ == "__main__":
    main_logger = setup_logging(os.path.dirname(os.path.abspath(__file__)))
    main_logger.info("setup logging module")
    mylist = [1, 2]
    try:
        print(mylist[4])
    except Exception as e:
        main_logger.error("exception error", exc_info=True)

Add JournalHandler

Add JournalHandler for outputting logs to journal (useful with boot)
$ sudo apt install python3-systemd

#!/usr/bin/env python3
import logging, sys, os, systemd.daemon
from systemd.journal import JournalHandler
from logging.handlers import RotatingFileHandler

def setup_logging(log_dir):
    # Main logger
    # If main/journal=INFO    then ALL (both loggers) info's get journal'd and logged
    # If main=CRIT,journ=INFO then only journal.info's get journal'd
    # If main=INFO,journ=CRIT then nothing journal'd but main info is logged
    # Use $ journalctl --since "1 hour ago" to check

    main_logger = logging.getLogger(__name__)
    journal_logger = logging.getLogger('journal')

    main_logger.setLevel(logging.CRITICAL)
    journal_logger.setLevel(logging.INFO)  # use logging.CRITICAL to turn logging off

    log_file_format = logging.Formatter("[%(levelname)s] - %(asctime)s - %(name)s - : %(message)s in %(pathname)s:%(lineno)d")
    log_console_format = logging.Formatter("[%(levelname)s]: %(message)s")
    journal_format = logging.Formatter("[%(levelname)s] - %(name)s - : %(message)s in %(pathname)s:%(lineno)d")

    journal_logger.addHandler(JournalHandler())

    console_handler = logging.StreamHandler()
    console_handler.setLevel(logging.INFO)
    console_handler.setFormatter(log_console_format)

    exp_file_handler = RotatingFileHandler('{}/exp_debug.log'.format(log_dir), maxBytes=10**6, backupCount=5) # 10MB file
    exp_file_handler.setLevel(logging.DEBUG)
    exp_file_handler.setFormatter(log_file_format)

    exp_errors_file_handler = RotatingFileHandler('{}/exp_error.log'.format(log_dir), maxBytes=10**6, backupCount=5)
    exp_errors_file_handler.setLevel(logging.WARNING)
    exp_errors_file_handler.setFormatter(log_file_format)

    main_logger.addHandler(console_handler)
    main_logger.addHandler(exp_file_handler)
    main_logger.addHandler(exp_errors_file_handler)
    return main_logger, journal_logger

if __name__ == "__main__":
    main_logger, journal_logger = setup_logging(os.path.dirname(os.path.abspath(__file__)))   
    main_logger.info("Testing logger output")
    journal_logger.info(__file__ + ' Marking journal')

    mylist = [1, 2]
    try:
        print(mylist[4])
    except Exception as e:
        main_logger.error("exception error", exc_info=True)
        journal_logger.error("exception error", exc_info=True)

Short method

from systemd import journal
journal.write("Test outputting to journal")