Swift Debugging

Debug Swift applications, especially unsigned debug builds where standard logging doesn't capture output.

OSLog Limitation: Unsigned Debug Builds

Swift's Logger (OSLog) writes to the unified logging system, but for unsigned debug builds run via swift run, logs are NOT captured by log show or log stream.

Symptoms

• •logger.info() calls produce no output in Console.app
• •log stream --predicate 'subsystem == "com.myapp"' shows nothing
• •App runs fine, but no log output visible

Why This Happens

Unsigned binaries lack entitlements for the unified logging system. Affects:

• •SPM packages run with swift run
• •Debug builds without development certificate signing
• •CLI tools during development

Workaround: Stderr Telemetry

Write directly to stderr for debugging sessions:

private func telemetry(_ message: String) {
    FileHandle.standardError.write(Data("[TELEMETRY] \(message)\n".utf8))
}

Capture output:

./MyApp 2> /tmp/telemetry.log &
tail -f /tmp/telemetry.log

For backgrounded apps:

nohup ./MyApp > /tmp/app.stdout 2> /tmp/app.stderr &

When to Use Each Approach

Conditional Telemetry

Add during debugging, remove before committing:

#if DEBUG
private func telemetry(_ message: String) {
    FileHandle.standardError.write(Data("[TELEMETRY] \(message)\n".utf8))
}
#endif

Or use compile flag:

#if TELEMETRY_ENABLED
telemetry("Debug info: \(value)")
#endif

Log File Patterns

# Combined stdout/stderr
./MyApp > /tmp/app.log 2>&1

# Separate files
./MyApp > /tmp/stdout.log 2> /tmp/stderr.log

# Watch updates
tail -f /tmp/app.log

OSLog for Signed Builds

When OSLog works (signed builds):

import OSLog

private let logger = Logger(subsystem: "com.company.app", category: "networking")

logger.debug("Request started")   // Debug builds only
logger.info("User logged in")     // Informational
logger.error("Failed: \(error.localizedDescription)")