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extracting-logs-with-logcat

Testing & quality

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Updated 10-16 01:27

About this skill

Extracting Logs With logcat — Reading Device Logs

This skill covers adb logcat end-to-end: streaming vs dumping, buffer selection, filter expressions, format flags, PID and time filters, file rotation, and the R8 rule that strips Log.d from release builds. The companion CI capture-on-failure pattern lives in ../../automation/scripting-adb-for-ci/SKILL.md.

When to use this skill

  • A test fails on CI; the developer needs to dump the device log into the artefact archive.
  • The developer's adb logcat | grep MyApp is overwhelming the SSH session — the logcat-side filter is the fix.
  • A scenario only reproduces inside system_server or crash buffers, not the default main.
  • The developer wants structured logs (-v json) for machine ingestion.
  • A Log.d("Sensitive", "...") call appears to leak in release — the R8 -assumenosideeffects rule is missing.
  • Logs disappear after the app is killed by Doze/ANR — --pid plus a re-resolution loop is needed.

When NOT to use this skill

  • Capturing a screenshot or video. Use ../../capture/capturing-screenshots-and-screenrecord/SKILL.md.
  • Pulling a generic file from the device. Use ../../transfer/extracting-test-artifacts/SKILL.md.
  • The whole script — retries, port forwarding, parallel device fan-out. Use ../../automation/scripting-adb-for-ci/SKILL.md.
  • Driving gestures or settings changes. Use ../../control/injecting-input-and-state/SKILL.md.

Prerequisites

  • adb get-state returns device. See ../../devices/connecting-to-devices/SKILL.md.
  • For -v json: device API 30+ (Android 11+). Stack with -v UTC -v year for unambiguous timestamps.
  • For per-PID filtering: adb shell pidof -s <pkg> requires API 24+ (toybox pidof -s); on older releases use adb shell ps -A | grep <pkg>.

Three fundamental operations

Command Behavior
adb logcat Stream the device log to stdout; runs until interrupted.
adb logcat -d "Dumps the log and exits." Snapshot mode — perfect for CI.
adb logcat -c "Clears the log buffer." Run this before reproducing a bug.

(Verbatim quotes from developer.android.com/tools/logcat.)

Idiomatic CI capture pattern:

adb logcat -c                           # clear before scenario
./run-scenario.sh                       # reproduce
adb logcat -d > artifacts/log.txt       # dump after

Buffer selection

-b <name> selects which kernel/userspace ring buffer is read. Buffers (developer.android.com/tools/logcat):

Buffer What it contains
main Default app-side buffer. Does NOT contain system/crash.
system Framework / system_server messages.
crash Tombstones + unhandled-exception output.
events Structured/binary system event buffer. Pair with -v descriptive to decode tag names.
radio Radio/telephony related messages.
kernel Kernel buffer.
all Every buffer.
default Implicit set: main, system, crash.

Multiple -b flags or comma-separated lists both work:

adb logcat -b radio
adb logcat -b main -b radio -b events
adb logcat -b main,radio,events

The events buffer is where to look for am_*, wm_*, input_focus, notification_* — emitted by the framework for instrumentation, not for human reading. Decode tag names with:

adb logcat -b events -v descriptive

Priority ladder + filter expressions

Filter specs are space-separated tag:priority pairs. * matches every tag.

Letter Meaning
V Verbose
D Debug
I Info
W Warning
E Error
F Fatal
S "Silent (highest priority, nothing is printed)"

Setting *:S silences everything, then any preceding tag:P re-enables that tag at priority P or above. The canonical "show only my app's logs" idiom:

adb logcat ActivityManager:I MyApp:D *:S

"Suppress all logs except ActivityManager (Info+) and MyApp (Debug+)" — developer.android.com/tools/logcat.

Other staples:

adb logcat *:W                          # warnings and above, all tags
adb logcat *:E                          # errors and above (very common in CI)

zsh/bash will glob-expand *:S outside of quotes when there is a file named S in cwd — quote when scripting:

adb logcat "ActivityManager:I MyApp:D *:S"

The same filter can be the host default via env var:

export ANDROID_LOG_TAGS="ActivityManager:I MyApp:D *:S"

Format flags — -v <format>

Format What you get
brief "Displays priority, tag, and PID"
long "All metadata fields with blank lines between messages"
process "PID only"
raw "Raw log message with no metadata"
tag "Priority and tag only"
thread "Legacy format showing priority, PID, and TID"
threadtime DEFAULT. "Date, time, priority, tag, PID, and TID"
time "Date, time, priority, tag, and PID"

Format modifiers stack with -v (comma-combinable or repeatable):

Modifier Effect
color Per-priority colour.
descriptive Decode event log tag names.
epoch Time in seconds since 1970-01-01.
monotonic CPU seconds from last boot.
printable Escape binary content.
uid UID or Android ID of logged process.
usec Time with microsecond precision.
UTC Time as UTC.
year Add year to displayed time.
zone Add local time zone. 
adb logcat -v json -v UTC -v year       # Android 11+; structured stream, unambiguous time
adb logcat -b all -v color -d           # color dump, all buffers

Default-line example (developer.android.com/tools/logcat):

I/ActivityManager(  585): Starting activity: Intent { action=android.intent.action.MAIN ... }

Schema for brief: <priority>/<tag>(<PID>): <message>. threadtime adds date/time/TID.

Time and PID filters

  • -T '<date> <time>' — start at the first entry on or after the given timestamp. Format matches threadtime, e.g. '01-01 12:00:00.000'.
  • -T <count> — print the last N entries from the buffer, then continue streaming.
  • -t <count> — print the last N entries, then exit (i.e. -t is -T with auto-quit).
  • --pid <pid> — emit only entries from a single process. Combine with pidof:
adb logcat --pid=$(adb shell pidof -s com.example.app)

pidof -s returns a single pid; without -s you get a space-separated list which --pid will not accept directly. Wrap a re-resolution loop if the app may be killed and restarted (Doze, ANR, crash):

while true; do
  PID=$(adb shell pidof -s com.example.app 2>/dev/null)
  [ -n "$PID" ] && adb logcat --pid="$PID"
  sleep 1
done

Buffer rotation and persistence

For long captures, persist to a file with rotation:

Flag Meaning
-f <file> Write to <file> instead of stdout. Subtle: with adb logcat -f, the file is host-side; with adb shell logcat -f, the path is device-side.
-r <kbytes> Rotate output every N kilobytes. Requires -f.
-n <count> Keep at most <count> rotated files (logfile, logfile.1, ...).
-G <size> Resize the kernel log buffer for the selected -b ring (e.g. 1M, 16M). Persists for the life of logd.
-g Print buffer sizes and current usages, then exit. Cheap diagnostics for "why am I missing logs?". 
adb logcat -G 16M                                     # grow the main buffer
adb logcat -g                                         # inspect sizes
adb logcat -b main -f /data/local/tmp/app.log \
           -r 1024 -n 8                               # 8x ~1MB rotated files (device-side)
adb shell logcat -b main -f /data/local/tmp/app.log \
           -r 1024 -n 8                               # same, but the device runs it

The -G resize is global to that buffer and persists across logd restarts (until reboot). Don't set 16 MB on a memory-constrained device and forget — release memory back with -G 256K after the suite if your CI farm is shared.

Stripping Log.d in release — R8 rule

android.util.Log calls remain in release APKs unless code-shrinking removes them. The standard idiom in proguard-rules.pro (R8/ProGuard):

-assumenosideeffects class android.util.Log {
    public static *** v(...);
    public static *** d(...);
    public static *** i(...);
    public static boolean isLoggable(java.lang.String, int);
}

-assumenosideeffects lets R8 treat the listed methods as pure no-ops, so all calls (and their string-builder argument expressions, when constants) are eliminated. Keep w, e, wtf for production diagnostics. With Timber: strip the BuildConfig.DEBUG-gated DebugTree plant and rely on a ReleaseTree that forwards to Crashlytics for warnings/errors.

For Compose specifically, do not -assumenosideeffects kotlin.jvm.internal.Intrinsics — it removes null-checks the compiler relies on.

Patterns

Pattern: WRONG — adb logcat | grep MyApp for filtering

# WRONG
adb logcat | grep MyApp
# WRONG because: the device sends every log line over the USB/TCP transport, then the host
# discards 99% via grep. On a busy device this saturates the transport and the grep can lag
# behind real time. Worse, `grep` runs without -F, so a tag that contains regex metacharacters
# (`.`, `(`, `+`) silently mismatches.

# RIGHT — filter at the logcat level
adb logcat MyApp:D *:S

The device only emits matching lines; the transport is uncongested.

Pattern: WRONG — assuming -T <count> exits

# WRONG
adb logcat -T 100 > tail.txt
# Hangs forever — -T <count> prints the last 100 lines THEN CONTINUES STREAMING.

# RIGHT — `-t` is `-T` with auto-exit
adb logcat -t 100 > tail.txt

Pattern: structured logs for machine ingestion

adb logcat -v json -v UTC -v year -d -b all > run.jsonl

Each line is a single JSON object — pipe through jq:

jq -c 'select(.priority == "ERROR")' run.jsonl

(-v json requires Android 11+ on the device.)

Pattern: capture-on-failure — clear, run, dump

adb logcat -c
./run-scenario.sh
RC=$?
if [ "$RC" -ne 0 ]; then
  adb logcat -d -v threadtime               > artifacts/logcat.txt
  adb logcat -d -b crash    -v threadtime   > artifacts/crash.txt
  adb logcat -d -b events   -v descriptive  > artifacts/events.txt
fi
exit "$RC"

-c clears before the run so the dump captures only the scenario's logs.

Pattern: PID-bound stream that survives a process restart

while true; do
  PID=$(adb shell pidof -s com.example.app 2>/dev/null)
  if [ -n "$PID" ]; then
    adb logcat --pid="$PID" -v threadtime
  fi
  sleep 1
done

The outer loop re-resolves the PID after Doze/ANR/crash kills and restarts the app.

Pattern: -f host vs device side

# host-side rotation (file lives on the developer machine)
adb logcat -b main -f ~/logs/app.log -r 1024 -n 8

# device-side rotation (file lives in /data/local/tmp on the device)
adb shell logcat -b main -f /data/local/tmp/app.log -r 1024 -n 8
adb pull /data/local/tmp/app.log ~/logs/app.log     # later

Mandatory rules

  • MUST filter at the logcat level (e.g. MyApp:D *:S), not via host-side grep. Filtering on the device avoids saturating the transport.
  • MUST quote filter strings containing * in zsh/bash (e.g. "MyApp:D *:S") — globbing eats the asterisk if a file named S exists in cwd.
  • MUST use -t <count> for "tail and exit". -T <count> continues streaming after the initial dump.
  • MUST use -d for snapshot dumps in CI. Streaming adb logcat requires explicit termination.
  • MUST call adb logcat -c before reproducing a bug whose log dump you want to attach. Otherwise the dump includes irrelevant prior history.
  • MUST distinguish adb logcat -f <file> (host-side path) from adb shell logcat -f <file> (device-side path). The shell forms diverge.
  • MUST add the R8 -assumenosideeffects rule for android.util.Log v/d/i (and isLoggable) when shipping release APKs that previously called Log.d with sensitive data. Keep w/e/wtf.
  • MUST NOT -assumenosideeffects kotlin.jvm.internal.Intrinsics — that removes null-check semantics the Kotlin compiler relies on.
  • MUST NOT rely on -v json on devices below Android 11 — silently emits an empty stream.
  • PREFERRED: stack -v threadtime,year,UTC (or -v threadtime -v year -v UTC) for unambiguous absolute timestamps in CI artefacts.
  • PREFERRED: run with -G 16M for the main buffer when capturing slow-burning bugs, and reset to default in teardown.

Verification

  • No script in scripts/ or .github/workflows/ pipes adb logcat directly into grep for filtering. All filters are passed as logcat args.
  • CI capture-on-failure dumps include at least main, crash, and events buffers.
  • Release APKs do not emit Log.d calls (verify via apkanalyzer dex packages app-release.apk | grep 'android.util.Log' returning only e/w/wtf).
  • Filter strings containing *:S in shell scripts are quoted.
  • All --pid invocations use pidof -s <pkg> (singular) and re-resolve in a loop if the app may restart.
  • Every long-running adb logcat session in CI has a wall-clock budget (timeout 600s adb logcat ...) so a stuck session doesn't fill the disk.

References

  • developer.android.com/tools/logcat — buffer table, priority ladder, filter expressions, -v threadtime/json/descriptive, format modifiers, -T/-t/--pid/-r/-n/-G/-g flags, line-format schema.
  • developer.android.com/tools/adb — adb logcat host-side shorthand vs adb shell logcat.
  • developer.android.com/studio/build/shrink-code — R8 -assumenosideeffects and the android.util.Log strip rule.
  • developer.android.com/reference/android/util/Log — Log.v/d/i/w/e/wtf/isLoggable signatures.
  • Research note tasks/research/A3-adb-observability-automation.md — full logcat surface, -f host-vs-device subtlety, R8 keep-rule rationale, capture-on-failure CI pattern.
  • Sibling skill: ../../architecture/understanding-adb-architecture/SKILL.md — server / daemon, transport.
  • Sibling skill: ../../devices/connecting-to-devices/SKILL.md — device states, wait-for-device.
  • Sibling skill: ../../devices/connecting-over-wifi/SKILL.md — adb pair / adb connect.
  • Sibling skill: ../../apps/installing-and-managing-apps/SKILL.md — installing the APK whose logs are being captured.
  • Sibling skill: ../../tests/running-instrumented-tests-via-adb/SKILL.md — am instrument -w -r interplay with logcat.
  • Sibling skill: ../../control/injecting-input-and-state/SKILL.md — driving gestures whose effects show up in logcat.
  • Sibling skill: ../../capture/capturing-screenshots-and-screenrecord/SKILL.md — companion screenshot capture.
  • Sibling skill: ../../transfer/extracting-test-artifacts/SKILL.md — pulling rotated log files from the device.
  • Sibling skill: ../../automation/scripting-adb-for-ci/SKILL.md — full CI capture-on-failure / retry / cleanup patterns.
  • Cross-set: ../../../instrumentation/runner/running-instrumented-tests-with-androidjunit4/SKILL.md — instrumentation logs interleaved with logcat.
  • Cross-set: ../../../instrumentation/scenarios/launching-activities-with-activityscenario/SKILL.md — Activity lifecycle events surfaced in events buffer.
  • Cross-set: ../../../fundamentals/strategies/applying-testing-strategies/SKILL.md — when log capture is the appropriate observability layer.