Streaming Logs with the log Tool

Episode #346 | 5 minutes | published on July 12, 2018 | Uses Xcode-10.0-beta2

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logging

Console.app is a nice way of viewing logs, but most of this power (and more) is builtin to the log utility from the command line. In this episode we will see how to stream debug logs from the simulator, as well as how to retrieve logs from customer or tester devices out in the field by triggering a sysdiagnose.

This episode is part of a series: Unified Logging and Activity Tracing.

1. Introduction to the Unified Logging Framework 10 min

2. Using os_log to Log Messages 12 min

3. Private Data and the log Utility 6 min

4. Streaming Logs with the log Tool 5 min

Episode Links

simctl recipes

Streaming Logs from your Mac

To stream logs from the terminal, use the log stream command in Terminal. You can influence the level of logs you get back by using

log stream --debug

log stream --info

Streaming Logs from the Simulator

Doing the above will not include logs in the iOS Simulator. To stream these logs, you have to run the log utility within the booted simulator.

We can use simctl to do this:

xcrun simctl spawn booted log ...

Filtering Based on Subsystem

If we want to limit the output just to one subsystem, we can filter using the --predicate flag:

xcrun simctl spawn booted log stream --debug --predicate 'subsystem == "com.ficklebits.UnifiedLoggingDemo"'

Note that this takes an NSPredicate type string, so you can do things like "subsystem CONTAINS[cd] 'foo'", filter on category, or any of the other fields available to you.

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