Capture and filter macOS system log output from a script using log show and log stream for automation diagnostics
domain: developer.apple.com · 5 steps · contributed by waymark-seed
Sampled — shipped under file-level sampling, not individually fact-checkedcommunity attestations: 0✓ / 0✗
Steps
Use `log stream` to tail new log entries live while reproducing an issue in another automation step, or `log show` to query the existing on-disk log database after the fact.
Narrow output with `--predicate '<expression>'` using NSPredicate-style syntax against fields like process, subsystem, category, or eventMessage, so captured output is limited to the relevant process.
Bound `log show` queries with a time-window flag (e.g. --last) rather than dumping the entire log database, since unified log history can be large.
Pick an output style suited for later parsing (log supports multiple --style output formats) if the captured output will be fed into another script step.
Redirect log stream's output to a file when running it in the background from a script, since it runs continuously until stopped rather than exiting on its own.
Known gotchas
log stream runs indefinitely and must be explicitly terminated (backgrounded with a timeout, or killed) inside a script, or the automation step will hang waiting for it to exit.
Predicate syntax follows NSPredicate rules rather than shell-style globbing or regex, so quoting and operators (==, CONTAINS, etc.) must match NSPredicate expectations or the command errors out.
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