This is the first page of a 5 page piece I just finished writing for MacTech. After the last episode of the MacAdmins podcast though, I wanted to go ahead and get some of the information out there. For a much more detailed analysis, check out MacTech
Apple has a number of different logging APIs. For the past few releases, Apple has tried to capture everything possible in logs, creating what many administrators and developers might consider to be a lot of chatter. As such, an entirely new interface needed to be developed to categorize and filter messages sent into system logs.
The logger command is still used to create entries in system logs. However, if you are then using tail to view /var/log/system.log then you will notice that you no longer see your entry being written. This is because as the logs being created in macOS have gotten more complex, the tools to read and write those logs has gotten more complicated as well.
Let’s take a simple log entry. Below, we’ll write the string “Hello Logs” into the system log. To do so, use the –i option to put the process id of the logger process and –s to write to the system log, as well as to stderr. To make the entry easier we’ll tag it with –t followed by the string of the tag. And finally, we’ll quote the entry we want written into the log. This is basically the simplest form of an entry:
logger -is -t krypted "Hello Logs"
Once written, use the log command to read your spiffy new entries. This isn’t terribly different than how things worked previously. If you’re a developer, you will need to note that all of the legacy APIs you might be using, which include asl_log_message, NSLog, and syslog, have been redirected to the new Unified Logging system, provided you build software for 10.12 (you can still build as before for 10.11, iOS 9, tvOS 10, and watchOS 3 and below). These are replaced with the os_log, os_log_info, os_log_debug, os_log_error, os_log_fault, and os_log_create APIs (which correspond to various levels of logs that are written).
Logs are now stored in the tracev3 formatted files in /var/db/diagnostics, which is a compressed binary format. As with all binary files, you’ll need new tools to read the files. Console has been updated with a new hierarchical capability and the ability to watch activities, subsystems, etc.
The log command provides another means of reading those spiffy new logs. To get started, first check out the man page:
That “Hello Logs” string we used earlier is part of a message that you can easily view using the ‘log show’ command. In the below example, we’ll just run a scan of the last 3 minutes, using the –last option, and then providing a –predicate. We’ll explain those a bit later, but think of it as query parameters – here, we’ll specify to look for “Hello Logs” in eventMessage:
log show --predicate 'eventMessage contains "Hello Logs"' --last 3m
Filtering the log data using “eventMessage CONTAINS “Hello Logs”” shows us that our entry appears as follows:
Timestamp Thread Type Activity PID
2017-03-23 23:51:05.236542-0500 0x4b83bb Default 0x0 88294 logger: Hello Logs
Log – Default: 1, Info: 0, Debug: 0, Error: 0, Fault: 0
Activity – Create: 0, Transition: 0, Actions: 0